Archive for category technical

A frame project

I decided I needed a sporty bike with wider tires than will fit on the Early, so I built a new one.  I equipped it with a few new parts, but mostly items scrounged from the Early and the brown bike.

It is pretty much full Heine Kool-Aid.  Toptube is .7/.4/.7, downtube is .8/.5/.8 (True Temper Oxplatinum), and it uses the Kaisei fork blades and Grand Bois fork crown (and braze-on centerpull brakes) sold by Compass.  I couldn’t find a True Temper chainstay with a curve I liked, so I went for Columbus Life cyclocross units.  The frame design is conventional sport/touring 73° parallel with 17 inch chainstays.  But the wide fork crown and curved chainstays, along with appropriate blade length and bridge locations, means that it will fit 42 mm 650b tires and fenders.  It has a low bottom bracket and low trail.

The best analogy for its ride is a like a candy that is soft on the outside with a firm caramel center.  The initial ride impression is cushiness, but it feels fast, handles very well, inspires confidence on descents, and there is no feeling of excessive  flex, even under as much power as I can muster.

I will continue tweaking the components.  The Turbo saddle in the picture is already gone, since it causes significant pain after 40 miles or so, and I borrowed the Early’s B17, which is much more comfortable on an extended ride.  It now has a front rack and a new Berthoud bag.

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RRDropout

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FkCrown

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More thoughts on tubing specifications and ride qualities

In a previous post, I noted that some highly respected people in the bike world assert that the frame tubing specifications make little or no difference to the ride.  However, in my experience, there are some circumstances in which tube choices make a huge difference.

For example, there was a Monark Siver King at Stu’s for repair.  It was an American balloon-tire kid’s bike, the frame of which was made from 1” round aluminum tubing.  If I remember correctly, the bottom bracket deflection could be measured in inches as you pedaled, yet it had a surprisingly lively ride.  My time with it was short, and I did not try it with a load, nor did I dive into any tricky corners; I suspect that with that much flex the handling could have been challenging.

Close to the same time (early 1980’s), I was at a Portland cyclocross race.  Jim Merz, then a Portland frame builder, brought a bike to the race that appeared to be nearly as flexy as the Monark.  The frame was steel (I think), and I don’t know anything about the specifications, but it must have had very thin walls.  He was using the flexibility as a substitute for suspension and he was riding over some of the obstacles that made the rest of us dismount.  He said something at the time about a good competition bike being “on the hairy edge of breakdown”.  I did not ride the bike, and I don’t know how long that frame survived.

I have ridden two tandems that had frames that were clearly too flexible.  Tandems are an extreme example because of the long tubes, double power input, and double human payload compared to a single bike.

One tandem was an American coaster-brake balloon-tire bike with 1” steel tubing, which usually means low strength steel and thick walls.  While riding placidly it was pleasant enough, but anything other than a gentle change in direction would cause the frame to twist, bend, and buck and generally become a challenge to keep under control.  Another tandem, a Gitane of traditional-diameter (unknown wall thickness but possibly tandem gauge) Reynolds 531 was better, but still could be a handful at low speeds and when cornered hard, and there was a lot of flex when accelerating or climbing.  On the other hand, I have ridden tandems built with oversize and presumably heavier gauge tubing that handle just like single bikes.

In this vein, Mr. Weiss, over at  Bike Snob NYC posted a  link  to a magazine article published in 1996 (back when steel was still taken seriously). The article is an account of a blind test of seven bikes with different frame tube specifications but otherwise identical design. It is remarkable that they assembled this group of bikes, but they could have made more of the opportunity.

In fact, they blew it.  This group of bikes could have been ridden by a group of testers whose pooled observations might have yielded some real insights.  And they could have performed some objective testing, like keeping track of lap times and measuring frame deflection.  Instead, one rider/writer recorded his hurried impressions.

The writer, Alan Cote, had a great deal of trouble telling the tube sets apart by their riding properties. With only one observer (contrary to the impression given by the graphics accompanying the article), this could either mean that differences were truly too subtle to distinguish, or there were too many bikes and too little time to sort out all the sensory information, or maybe that Mr. Cote was not a good observer.

Cote reaches some preliminary conclusions, then appears to second guess himself when he is informed of the construction material of each bike. “I think my ride impressions were essentially random”, he says after he discovers that the tubing specifications do not necessarily correlate with his initial ride impressions.

All of these tube sets are steel, so the specific steel alloy (or associated tensile strength) makes essentially no difference in tube rigidity, and the alloy, by itself, should make no difference in ride qualities.  Ride characteristics are determined by a combination of tube diameter and wall thickness.  Of course a higher-strength alloy does allow thinner walls and lighter weight without reducing the total strength, so the alloy characteristics can influence the long-term usefulness of a frame.

Using my  previous analysis  and a subjective analysis of the other frame specifications, most of these frames should have similar characteristics, but there is enough of a range that differences should have been perceptible, at least in the frames at the extremes.

The most flexible frame should have been the one constructed from SLX tubing.  It has the most flexible main triangle and the lightest chain stays, seat stays, and fork blades.  Cote identified this bike as the “softest”, apparently only for power transmission.  I don’t see any reason it would not also be the softest in its over-the-bumps ride.

I am surprised that he could not distinguish the straight-gauge (Aelle) frame.  Unless we have been defrauded about the value of butted tubing over the last 120 years (a possibility I cannot entirely eliminate) there should be a noticeable difference between this and the other bikes.  I found my Schwinn Sports Tourer, with similar tubing, harsher and less lively than comparable butted frames (though I recognize that that perception could have been because I knew it was straight gauge).

The stiffest frame should have been the Thron, with stout oversize tubing and heavier chain stays and seat stays.  This frame should be 30% or more stiffer than the SLX, a difference I would expect a rider to feel both in lateral flex during pedaling and over the bumps.  Cote called this the best at shock absorption, an impression that is surprising.

Cote’s favorite (which he called the “stiffest”) was the Neuron frame, made from tubing that used a complicated elliptical butting pattern.  Based on its specs, I would expect that it should be only incrementally stiffer than the SLX.  It is possible that the unique butting patterns provide a noticeably better ride, but if so, that fact did not save Neuron tubing from being eliminated from the Columbus catalog.

For a wine drinker who is happy with generic white wine, the difference between chardonnay and pinot gris is too subtle to bother with.  But a wine journalist worthy of the name would easily identify the two, wax poetic about the contributing tastes and mouth feel, and maybe tell you what vineyard the grapes came from.  A bike journalist should have a similarly well-calibrated sense of the contribution of frame construction to ride characteristics.  Maybe the differences between these frames were subtle, and maybe any differences really don’t matter when it comes down to the overall riding experience, but I have a hard time believing that the differences were indiscernible.  Bike journalism is driven too much by manufacturers’ (advertisers’) claims rather than educated, calibrated experience.  This was a unique opportunity to improve the writer’s ability to judge ride characteristics (maybe bike magazines should have a stable of test bikes with known and validated characteristics for training aspiring writers).

Or, if a panel reached consensus, this test would have reached a defensible conclusion that frame material really doesn’t matter (at least within this range).

Instead, it was a lost opportunity.

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A little more about tires and rolling resistance

Bicycle Rolling Resistance is a website run by Jarno Bierman in the Netherlands.  Jarno* uses a roller to measure tire rolling resistance.  The attraction of Jarnos’s work is that he uses a consistent method for a large number of different tires, and he uses a textured roller in an effort to account for energy loss from vibration as well as tire flex.  The consistent test procedure gives me more confidence in his results than I have in roll-down test of just a few tire models, and the textured roller presumably approximates a real-world road surface better than a smooth roller.

However, he focuses on a narrow range of road tire sizes, staying close to 25 mm width. He treats stouter touring tires as a separate class, and he sticks to tires of around 37 mm for that group.  Road and touring tires are all 700c.  He has not yet directly explored the characteristics of wide light and supple tires.

He has, however, compared the rolling resistance of a particular model of high-performance road tires in different widths and concluded that the widest (28 mm) had the lowest rolling resistance (conti GP link ) .  And he performed a similar test with a touring tire, showing that the 37 mm tire had lower rolling resistance than the 40 mm or 47 mm tire, while the 32 mm version had the highest rolling resistance for this tire model (Marathon link ).

However, the real outlier is in the mountain bike category.  I am a little dubious of the usefulness of this test on knobby mountain bike and fat bike tires (since the knobs are larger than the roughness elements on the test roller — it seems to me that a more pertinent test would involve a rougher roller**), but the relatively smooth tires in this category should behave like road tires.

The best-performing tire in the mountain bike class  has a rolling resistance that is competitive with the best 25 mm road tires, even with each class at inflation pressures appropriate to their widths.  As one might expect, it has a very light tread and casing but measures 47 mm wide.

These results seem to show that wide/light/supple tires can be as fast as their proponents claim.  Jarno has a long list of tires that he wants to test and not a lot of capacity to do the testing, but I would love to see him test some performance-oriented tires in the 38 mm range.

crr_chart

 

*I understand that you go straight to first names in the Netherlands rather than referring to someone with an honorific (e.g. Mijnheer Bierman) as you might in English.  I prefer that informality, but I don’t want to sound disrespectful, and I apologize if I am incorrect.

**I think it might be useful to test tires on a series of rollers with different size roughness elements, comparable to Nikuradse’s experiments with pipes.  This additional experimental data might provide some insight about optimum tires, width, and pressure for different surfaces.

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A list of 650b road tires and some general tire observations

As I am sure all of you bike geeks out there know by now, there are two mechanisms by which rolling is resistance is generated. One is the energy consumed by flexing of the tire as it rolls (hysteresis loss). The other is vibration, which converts forward motion to vertical motion and consumes energy within the body of the rider.

Hysteresis loss can be minimized by using a hard tire. As an extreme example, rolling resistance is very low for steel train wheels rolling on smooth steel track. High air pressure in a bicycle tire can minimize the amount of flex.

Two other factors influence hysteresis loss.  A tire with a light and supple construction tends to lose less energy to flex at any pressure.  Thick tread, heavy casing construction, and puncture-resistant belts all tend to increase energy losses.

A wider tire can lose less energy to hysteresis at the same pressure as a narrower tire because of the shape of the contact patch. However, the maximum pressure a tire can handle is limited by the tire’s width.

Historically, many of the measurements of bike tire rolling resistance have been made on smooth rollers. These measurements ignore energy loss due to vibration and have helped encourage an emphasis on narrow, high-pressure tires for most applications. Recent research has used more real-world conditions and has shown that, because of energy losses to vibration, wider tires at lower pressures can be faster on the less-than-perfect surfaces that we are likely to experience in our daily riding lives. In addition, wider tires provide greater comfort and control, especially on rough surfaces and with heavy loads.

It is worth noting that light tire construction, including both thin tread and a light-weight fabric casing, reduce both hysteresis loss and vibration loss. Thus the fastest tires tend to be the lightest tires, and they also provide the most comfortable ride.

There are circumstances in which the best tire is narrow and hard, but they are restricted mostly to competition on good pavement or at the velodrome. Outside of those conditions, it makes little sense to be stuck with skinny tires that ride harshly, require frequent inflation, and can be steered and bounced by surface irregularities. For riders not involved in competition (and many who are), a 28mm tire is the narrowest that is needed, and tire widths of 42 mm and wider are very practical, and can be as fast as narrower tires.

For those of us who are low-performance riders, low rolling resistance still matters. In fact, because we cruise at a lower speed, wind resistance is lower than it is for faster riders, so rolling resistance becomes a higher proportion of total resistance. The difference between a very fast tire and a slow tire result in as much as 2 mph cruising speed on a flat road for a 100-watt rider.

On the down side, light high-performance tires tend to be relatively easy to puncture or cut and are more prone to stone bruises (broken cords with no external damage). And, as one might expect, they wear out relatively quickly. I get between 1500 and 2000 miles from light tires on the back wheel (3000 to 4000 miles for a pair that is judiciously rotated), and wider tires do not seem to last appreciably longer than similar narrower tires (at least in the range of 28 mm to 42 mm). I don’t have a lot of problems with punctures on the roads that I ride on my weekend peregrinations, and I have the skills to fix them easily when I do, so the greater speed (and therefore greater range) and comfort are worth it to me under these circumstances. However, when I do have flats, some are caused by very small objects, such as tiny thorns or miniscule slivers of glass that would not penetrate thicker tread and casing.

I use stouter tires for commuting and utility riding and accept any loss of speed in exchange for the practicality and reliability. City streets are littered with more broken glass and other sharp things than are country roads. When I have to stick to a schedule and when I am wearing office clothes, I really don’t want to repair a flat, and I seldom need to do so when using heavier tires with protective belts. Most of these tires last significantly longer than performance tires, on the order of 2500-3500 miles, even while hauling groceries and other loads. The Schwalbe Marathon currently on the back of my utility bike has about 2000 miles on it and looks like it just getting warmed up. From an economic perspective, a good quality tire that is heavier and more robust can cost half as much as a light tire and last twice as many miles.

For me, touring is more similar to utility riding than it is to sportier weekend jaunts.  Tires wear faster and are more likely to puncture with the dead weight of a load of camping gear.  Light tires are okay for a fast weekend trip, but a journey to the opposite coast would require tire replacements (maybe more than one) along the way, perhaps at an inopportune time, and the risk of tire failure that would leave the traveler stranded would be increased. And repairing flats on a loaded bike is considerably more trouble. I am still not sure how much priority durability should get compared to rolling resistance.

Although the general tendency is for lighter tires to be faster but more fragile, manufacturers seem to be able to do some tricks with rubber compounds and casing design that make some tires behave differently than expected looking at only weight, or at least mitigating the performance impacts of added weight.  Some of the tricks seem to be expensive (tire costs range by a factor of four or more), and it appears that you get what you pay for to some extent if you are discerning about ride quality.  I would recommend doing further research about any tires you are considering.

The renaissance of the 650b tire has paralleled the rediscovery of the advantages of wider tires. The 650b size allows a wider tire to work with a traditional road bike design, whether as a retrofit on an existing frame or on a frame designed for that size. Under the marketing designation of 27.5”, the size has become quite popular in the mountain bike world (and now on fat bikes), but is still not entirely mainstream on road bikes. However, there is now a wide range of 650b tires available, from extremely light tires that roll fast and float over rough surfaces to massive and possibly bullet-proof trekking tires. There is also some bleed-through from the off-roaders, providing road tires that fit those 650b mountain bikes.

I have assembled a list of  650b road tires available as of late November 2016, posted below. I expect that assembling a list like this in the future will become an impractical undertaking because  the number of options will continue to increase. I probably missed a few in my searches, but I hit the major tire manufacturers and marketers of this tire size. I cut off the maximum width for the list at 50 mm because there are few road frames that accept a tire any wider than that and because there are very few road tires that are wider. I did include some all-surface knobbies in the list, up to the 50 mm Continentals.

Finding all of these tires on the manufacturers’ web sites illustrated the lack of standardization for tire size nomenclature. Tires were listed as 27.5 (inches) x width in inches, 650b, 650 x zzb) (where “zz” is the width in mm), zz-584, or 26 x 1 ½ (with some other number to designate the actual width). Even in the same web site, it could take multiple searches using these different designations to locate all of the 650b tires.

Because I am too lazy to add references to the text of this blog, I will leave it to the reader to search out the details.  Here are few links to get you started.

Myths

Roller data

Roues Artisinales

Rolling resistance tests

Velonews

Heine 1

Heine 2

Peterson on 650b

Yet another useless marketing term: Road Plus

 

And here is the table of 650b road and all-surface tires

Manufacturer/
marketer
Model Width (mm) Series Advertised Weight (g) Comment
Schwalbe One H 462a 25 Evo 215
Schwalbe Pro One HS 462 25 Evo 225
Schwalbe Durano 28 performance 405
Panaracer/ Compass Cypres 32 Extra Leger 261
Panaracer/ Compass Cypres 32 standard 285
Panaracer/ Rivendell Nifty Swifty 34 406
Schwalbe Kojak 35 performance 410
Schwalbe HS 159 37 active 580
Continental Tour Ride 37 580
Panaracer/ Pacenti Pari-Moto 38 300
Panaracer Gravelking 38 310
Panaracer/ Compass Loup Loup Pass 38 Extra Leger 333
Panaracer/ Compass Loup Loup Pass 38 standard 354
Panaracer/ Soma B-line 38 380
Maxxis Detonator 38 480
Panaracer/ Soma Xpress 38 410
Panaracer Col de la Vie 38 500
Panaracer Ribmo 38 520
Schwalbe G-One HS 473 40 evo 420
Schwalbe Marathon plus HS 440 40 performance 920
Surly Knard 41 465 knob
Panaracer/ Rivendell Fatty Rumpkin 41 green label 480
Panaracer/ Rivendell Fatty Rumpkin 41 force field 620
Panaracer/ Soma Grand Rando 42 SL 300
Panaracer/ Pacenti Pari-Moto 42 330
Panaracer Gravelking 42 350
Panaracer/ Compass Babyshoe Pass 42 Extra Leger 362
Panaracer/ Compass Babyshoe Pass 42 standard 390
Panaracer/ Soma Grand Rando 42 EX 390
Schwalbe Marathon supreme HS 469 42 evo 470
Panaracer/ Soma Cazadero 42 470 knob
Panaracer Pasela 42 fold 480
Panaracer Pasela 42 wire 520
Maxis Overdrive 42 535
Panaracer/ Soma Grand Rando 42 Blue label 610
Continental Tour Ride 42 650
Panaracer/ Bruce Gordon Rock n road 43 520 knob
Schwalbe Marathon Cross 44 performance 620 semi knob
Schwalbe HS 159 44 active 645
Schwalbe Marathon HS 420 44 performance 820
Kenda Kwick Journey 44 954
Kenda Kwick Seven.5 45 834
WTB Horizon 47 515
Panaracer/ Compass switchback hill 48 Extra Leger 413
Panaracer/ Compass switchback hill 48 standard 478
Continental Race King Performance 50 580 knob
Schwalbe Marathon supreme HS 469 50 evo 600
Schwalbe Hurricane HS 352 50 performance 670 side knobs
Schwalbe Big Ben 50 performance 745
Schwalbe Marathon Almotion HS453 50 evo 750
Schwalbe Marathon Mondial HS 428 50 evo 780 semi knob
Continental Double Fighter III 50 845 knob
Schwalbe land cruiser 50 active 900 semi knob
Kenda Kwick Seven.5 50 950

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Bike Frame Design – the influence of tubing diameter and wall thickness

The usual assumption is that significant part of the ride characteristics of a frame is determined by its rigidity. An excessively flexible frame feels inefficient for power transmission and can be more difficult to control on rough surfaces or with a load. On the other hand, an excessively rigid frame rides harshly, transmits shock and vibration to the rider, and feels less lively.

Custom builders talk about the importance of using the appropriate combinations of wall thickness and diameters for a particular rider and the use to which the bike will be put (but they usually keep their design procedures proprietary). Jan Heine advocates for the improved ride quality and desirable flex in a frame constructed of extra-thin-walled traditional-diameter tubing (Jan’s blog). On the other hand, some influential figures in the business of building steel bikes downplay the importance of tube diameter and/or wall thickness to the ride of a bike (Sachs in forum discussion, Gordon link).

It is difficult to make an objective judgment about the influence of bike tubing. There are design fashions and fads, the power of suggestion, and the placebo effect. There are also confounding effects of frame angles, chain stay length, fork design, individual fit, and tire characteristics. And it is difficult to get a statistically significant sample size, both in the number of human subjects and the availability of bikes that are identical except for the factor being compared.

Without objective experiment, all we have is experience. For decades, the standard high-quality bike frame was made of Reynolds 531 tubing. The usual combination was a 1” diameter top tube with 0.8 mm wall thickness on the ends and 0.5 mm wall thickness in the thinner (butted) section in the middle of the tube (.8/.5/.8) along with a 1 1/8” diameter down tube with .9/.6/.9 wall thickness and a single-butted 1 1/8” .9/.6 seat tube. There might have been millions (I am completely making up a number here) of frames built with these specifications by Peugeot, Raleigh, Schwinn, and a host of competitors. A bike built using this tubing was responsive and reliable, and could win races, tour the world, or get the rider to work in the morning. The comparable Columbus tubing was a little heavier. The Columbus SL sticker designated tubing with .9/.6/.9 top and down tubes, and Columbus SP was 1/.7/1.

By the late 70’s, Reynolds advertised variations on the basic set of tubes with options for touring and larger frames. Heat-treated Reynolds 753 appeared in 1978, which was offered in wall thicknesses down to a .7/.5/.7 top tube and .8/.5/.8 down tube on road bikes; other tube manufacturers soon followed with their own heat-treated offerings. The mountain bike revolution created a need for steel tubes durable enough for the abuse of off-road riding, and larger-diameter tubing became available to fill the demand. Fat-tubed aluminum frames began to compete with steel. As tubing for mountain bikes and aluminum frames grew in diameter and  this look began dominating the market, the traditional tube diameters started to look oddly skinny by comparison, and road-bike builders began to use oversize steel tubing partly to fit that new aesthetic.

We are currently in a golden age for the hobby frame builder. We can buy quality steel frame tubing for road use in three different diameter standards. Traditional construction, as noted above, uses a 1” diameter top tube and 1 1/8” diameter down tube and seat tube. Oversize (OS) employs 1 1/8” top tube and 1 ¼ down tube; double oversize (2OS) uses a 1 ¼” top tube and a 1 3/8” down tube. The default for all three standards is a 1 1/8” .9/.6 single-butted seat tube, but there are a number of variations available. Wall thicknesses of .7/.4/.7, .8/.5/.8, and.9/.6/.9 are available in all diameters and 1/.7/1 is available in some sizes, along with variations on seat tube, seat stay and chain stay diameter and wall thickness. Fork blades are available from light to stout.

In spite of all this variety, I have not found any analytical methods for helping choose tube diameter and wall thickness, or even any specific guidance on ride characteristics. Clearly, a heavier-wall tube is more rigid than a thinner-wall tube of the same diameter and same material, but how do tubes of different diameters and wall thicknesses compare?

In order to answer this question, I did some rough calculating. Deflection of a tube in bending is inversely proportional to the moment of inertia (MI) of a tube. The variable part of the MI is (D^4 – d^4) (where D is the outside diameter of the tube and d is the inside diameter). Using that as a basis, I created a stiffness ratio table (Table 1) for a range of readily available main tubes. It is common practice (and this shows in most Reynolds tube sets) to use a thinner-walled top tube than down tube, so there are nuances not shown in this table. However, this seemed like the clearest way to present the information.

Tube diameter is the most important determinant of rigidity, and there was only a little overlap in the rigidity of different diameters (less than I expected). Traditional 1/.7/1 is essentially the same rigidity as OS .7/.4/.7, and OS 1/.7/.1 is very similar to 2OS .7/.4/.7 (the butted section of the smaller diameter tubes are relatively more rigid, so calling these as ties is a judgment call). I might note that traditional 1/.7/1 is not readily available, probably because of that redundancy.

The heaviest 2OS tubing (1/.7/1) is about three times stiffer than traditional .7/.4/.7. In general, progressing from one rank to the next increases rigidity by 13% to 19%. The Bruce Gordon link above suggests that most of us would not notice a change in one rank level (although Jan Heine would disagree). I would speculate that it would be easier to detect a change in 3 ranks or more—traditional .7/.4/.7 should feel noticeably different than OS .7/.4/.7 with no other design changes.

Table 1. Rigidity index relative to traditional .9/.6/.9 tubes

Table 1. Rigidity index relative to traditional .9/.6/.9 tubes

A critical piece of information in choosing frame tubes is the frame size. Deflection is proportional to the cube of tube length (based on the equation for deflection where loading is applied to the free end of a cantilevered beam). This should be a conservative approach, since frame tubes are part of a truss and not really cantilevered and torsional deflection is proportional to tube length. However, we still are not including the effect of increased weight of the rider.

The following chart (Figure 1) shows the rank (from Table 1) of top and down tubes that would be used to match the ride of a frame made of traditional tubing of specified wall thickness (everything else being equal) using the cube of the ratio of tube lengths.

If the ride quality of a 58 cm traditional-diameter 7/.4/.7 frame is desired, it is easy to duplicate in larger frames, but smaller riders are out of luck. A lighter seat tube and perhaps  a 1”  down tube might get close. On the other hand, the ride of a traditional 1/.7/1 frame can be duplicated in the full range of frame sizes. Using oversize tubing means that there is no need for lateral tubes or double top tubes on large frames even for applications that require extra rigidity.

Figure 1. Relative rigidity rank (from Table 1) that would approximate ride characteristics of a 58 cm frame constructed of traditional diameter top and down tubes of specified wall thickness.

Figure 1. Relative rigidity rank (from Table 1) that would approximate ride characteristics of a 58 cm frame constructed of traditional diameter top and down tubes of specified wall thickness.

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A new bike for me

My touring frame is a bit long in the tooth, having been built in 1978.  It was the first frame I built. It has never gotten a proper paint job and is suffering some from rust, although not as much as one might expect. Damage and changes over the years make it less reliable for a long trip. It also has a few design quirks that I would like to correct.

So I built this new frame. It has a standard touring design of 72° head angle and 73° seat tube angle. It has a fairly low bottom bracket for stability. Tubing is on the stout side for durability and rigidity with a load on rough surfaces. There is plentiful clearance for 42 mm tires.

DSC_0573

 

The Rohloff 14-speed hub with Paragon sliding rear dropouts is one feature that is a little out of the ordinary, but the biggest deviation from standard design is the long chain stays.

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Most production touring bikes have chain stays no longer than 18” (460 mm) or so (for comparison, a competition bike’s chain stays tend to be about 16” (406 mm) or a little shorter).

The longer stays on a touring bike allow the panniers to be mounted far enough behind the rider to provide clearance between the rider’s heels and a loaded pannier while pedaling without forcing the weight of the load too far behind the rear axle. It is possible to mount a rack and panniers on a bike with short stays in a manner that allows the rider to pedal without kicking the luggage on every stroke, but a load cantilevered out in space behind the bike tends to pick up the front wheel. This messes with the weight distribution, which results in vague, unstable, and/or wobbly steering. At best, this change in handling is something the rider has to get used to; at worst, it is downright dangerous. I would theorize that the tendency for Americans to try to tour on bikes with short chain stays had a lot to do with the shift in fashion from rear loads to front loads.

DSC_0574

The 21” (535 mm) chain stays on this frame place the center of a loaded pannier about an inch in front of the rear axle. There is almost no change in handling when a 50 lb. load is placed in the bags.  The stout Tubus rack also contributes to this loaded stability.

I went for a classic British three-speed aesthetic.  Most made it to the US in black, though several other colors were available. A steel VO stem and a steel Campagnolo Sport crank (manufactured briefly in the early 1970’s) fit in with the “all steel bike” theme, as do the SPD pedals styled to look like rubber pedals. The road handlebars spoil the look somewhat.

The bike hit the road in August, 2014, and it has been in regular use as a commuter/utility bike since. I have ridden it regularly on weekend excursions and two camping trips.  I am very happy with the design and, for the most part, the components.

More details than anybody cares about will follow.

IMG_0921

DSC_0756

Campi Sport Crank with Fyxo ring

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An Experiment in Frame Geometry

My utility bike is a 1974 Schwinn Sports Tourer. It has a straight gauge chromo frame with fillet-brazed joints. The frame design could have come out of the Rivendell catalog: low bottom bracket, moderately long chain stays, and 73 degree head and seat tubes.

I bought it off Ebay in 2003 or so. I converted it to a 7-speed internal hub and added fenders and lights and other utilitarian stuff. It has averaged more than 1500 miles per year since then, commuting to work and running errands.

The bike has a few faults. One is that a 35mm tire is a tight fit laterally in the fork – the sides of the tires tend to rub on the fender and/or fork blades if everything is not set just so.

The second fault is that it rides harshly over bumps, even with 35 mm tires. I would ascribe this to the relatively stout straight-gauge tubing used in the frame. My sportier bike,  with steeper angles, shorter chain stays, and narrower tires but built with standard-gauge butted tubing, is much more forgiving. It is unclear from the information out there whether the Schwinn fork is chromo, but the rear triangle is reportedly plain carbon steel.

And the third problem is that the bike cannot be ridden no-hands at any speed because of a serious shimmy.  This shimmy damps out even with light hand contact on the bars, but it is a significant annoyance.

My theory (at least I have not found anybody who states it exactly this way) is that shimmy in bikes, at least in many cases, is a harmonic phenomenon something like a torsion pendulum, with the trail of the fork, which tends to make the bike go in a straight line, acting as the spring. In a torsion pendulum, the frequency of oscillation is determined by the stiffness of the torsion spring and the moment of inertia of the system.

Bikes are a little more complex than the simple torsion pendulum example, because there are two mass/moment of inertia systems influencing the oscillation. The first is the obvious one: front wheel, tire, any luggage on the front — everything that pivots around the steering axis. The second mass and moment of inertia system is not so obvious. Because the head tube moves side to side as the as the fork is turned, all of the mass of the bike that does not pivot around the steering axis pivots instead around the contact point of the rear tire. This means that the frame, rider, rear luggage, back wheel, and any other paraphernalia influence any oscillation, with mass closer to the front of the bike or extending behind the back wheel (and thus farther from the pivot point) having greater moment than weight directly over the back wheel.

In this conceptual model, shimmy occurs when the front (pivoting around the steering axis) moment of inertia/trail system has a similar natural frequency of oscillation as the back (pivoting around the rear tire contact point) moment of inertia/trail system. Since these two systems are so different, it may also be that oscillation will occur when harmonics are similar.

I don’t know a definitive way to test this theory, but if it is a good model, changing weight distribution should affect a shimmy, as should changing fork trail without changing weight distribution. I have had experiences when changing weight distribution seemed to cause or eliminate shimmy, though other times the shimmy seemed to be insensitive to changes. The Schwinn does not have racks or baskets on the front, so I can’t change loads there, but the shimmy does not respond much to a wide range of loads on the back. I have tried added damping by adjusting the headset too tight with no change. The shimmy persists with tires from 28mm to 35mm and different front hubs.

I decided what I needed was a new fork. The fork crown would be wide enough that there would be no problem with the 35mm tires. The blades would be mid-weight chromo to see if the over-bumps-ride ride would improve over the unknown material of the original fork. And I would try a low-trail design, as championed by Jan Heine of Bicycle Quarterly (here, for example).

Here are the results.

Problem 1: Solved. There is now plenty of clearance.

Problem 2: With the new fork, the bike rides only marginally better over bumps (based on subjective observation), even with the greater offset. Maybe a fork built with lighter fork blades would have enough more give to make a difference, but I think that would be inappropriate for a bike that gets this much abuse. Then again, maybe I will try it someday just to see how much difference it does make. Anyway, the bike got a new sprung Brooks saddle to handle some of the jarring, but that does not help my hands.

Problem 3: The finished fork results in about 25mm of trail, which is at the low end of accepted practice. Somewhat to my surprise, the handling did not change all that much. It feels quick and maneuverable at low speeds and it feels a little twitchy at downhill speeds, but it still in the range of what I would call normal.

The bike now has much less tendency to shimmy – reducing the trail seems to have worked in that regard. If the above theory is correct, increasing the trail should have also worked.

And for a bonus, I discovered that brazed-on centerpulls do indeed have a nice solid feel. But this mounting did not make enough difference in braking to make up for the trouble of making the mounting studs.

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