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  1. #1
    jur
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    Why small wheels are not harsher than large ones

    My own experience clashes somewhat with the majority view here - my view is that smaller wheels eg 16" does not give a harsher ride than 700c wheels. In fact my experience is to the contrary - big-wheeled bikes I have ridden were harsher over those sharp bumps such as the edge of pavements.

    Due to the frequently expressed majority view here, I have begun to waver in my opinion, that perhaps my riding style has imperceptibly changed such that despite smaller wheels' alleged harshness, my style renders the effect of smaller wheels irrelevant. Such a riding style might be to slightly lift myself off the saddle a very small amount on encountering a rising bump, as well as giving the handlebars a small jerk to lift the front wheel over such a bump.

    Anyway, I did a mathematical analysis and some simulations of the axle displacement when riding over a square bump, the worst case. The bump I investigated is a square edge 25mm rise and later a fall (opposite sequence of a flat pothole) at a speed of about 20km/h. The findings were very interesting.

    I compared a 16" 305x50 wheel with a 700c 622x28 wheel. The 16" 305 wheel is less than half the diameter of the 700c wheel so any effect would be like a worst case. Adding a tyre changes the ratio quite a bit; the 305 wheel got a 50mm tyre (eg Big Apple) and the 700c wheel got a 28mm tyre. So the effective radius of the 16" wheel was 202.5mm and that of the 700c wheel was 339mm.

    I looked at the horizontal distance between each axle and the front edge of the bump as the wheel first touches the edge. The ratio between the 2 cases is 1.31 (700c wheel being the largest).

    First of all, the result with extremely hard pumped tyres. The vertical acceleration ratio between the 2 cases was also 1.31. Makes sense of course. The 16" wheel suffered the highest vertical acceleration as expected.

    Then I dropped the pressure on the 16" wheel by that same ratio, 1.31. So if the 700c wheel was 100psi, the 16" wheel was 76psi. For this, the 2 cases' vertical acceleration amplitude was essentially equal albeit with different shapes. Furthermore, if the pressure of the Big Apple is dropped to say 40psi, the 16" wheel acceleration drops to 60% of the 700c wheel's. The 16" wheel does experience a much bigger rebounce, though, but still very small in the grand scale of things - only about 2mm. Such rebounce would be completely obscured by something like seatpost flex.

    So as far as I can see, here is proof: Smaller wheels with everything else being the same, are harsher, but with tyres pumped to realistic pressures, the situation is much the same or even reversed.

    Last edited by jur; 05-20-08 at 10:36 PM.
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  2. #2
    jur
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    Here are the 3 cases, 300psi, 75psi and 40psi in that order. For the 1st, both cases had that pressure to establish the base line; for the other 2, the 700c wheel was at 100psi and the 16" wheel as above.

    Vertical scale in mm (25V=25mm). Axle1 and accel1 is that of the 16" wheel.
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  3. #3
    Senior Member snafu21's Avatar
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    Crikey!

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    As a new small wheel (20") rider, I have not found them harsher, but I have found them less stable. Even riding with one hand makes me nervous - forgedabout no hands! That being said, I like 'em!

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    Surprising results.

    Your point about rising out of the saddle as you spot an upcoming bump is a good one. A long time ago I picked up that tip in Richard's Bicycle Book which I read about 1980. It makes all the difference in the world when hitting a rough patch of tarmac on hard pumped tyres. Richard Ballantyne made the point that by taking your weight on the feet and holding the bars lightly, the bike pivots around the bottom bracket, transmitting much less road shock to your body since your legs act like springs anyway, bending slightly at the knees. I think this riding style puts less strain on the bike itself too rather than sitting out the bumps in the saddle like a sack of potatoes.

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    "...Such a riding style might be to slightly lift myself off the saddle a very small amount on encountering a rising bump, as well as giving the handlebars a small jerk to lift the front wheel over such a bump."


    That sounds like just an inherent part of riding any bike to me. I thought everybody did that, regardless of wheel size.

  7. #7
    Bicycling Gnome
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    Quote Originally Posted by werewolf View Post
    "...Such a riding style might be to slightly lift myself off the saddle a very small amount on encountering a rising bump, as well as giving the handlebars a small jerk to lift the front wheel over such a bump."


    That sounds like just an inherent part of riding any bike to me. I thought everybody did that, regardless of wheel size.
    ince reading that advice in the Ballantyne book, I have done it on all unsuspended bikes.

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    Harsh and noodlely are two sides of the same coin. Notice that for higher pressures although the peak of the acceleration is higher for the smaller wheel, the decay is also much more abrupt.

    In other words a larger wheel has more memory. So if you want to control and contain the impact of a series of jolts (for example, by "getting light in the saddle" or "lifting the wheel") the larger wheel will make it more difficult to do so because instead of trying to predict/compensate for each jolt individually you'd need to predict/compensate for them two or three at a time. Also, for an isolated jolt the jolt is "active" for a longer period of time, which again is more difficult to contain/control because it takes more time to contain/control.

    More than anything I think this simulation reinforces the fact that larger wheels are a form of suspension like any other.
    Last edited by makeinu; 05-21-08 at 07:21 AM.

  9. #9
    Part-time epistemologist invisiblehand's Avatar
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    Excellent work Jur.

  10. #10
    Eschew Obfuscation SesameCrunch's Avatar
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    Jur, you are indeed the Doctor!
    Last edited by SesameCrunch; 05-21-08 at 01:29 PM.

  11. #11
    cyclopath vik's Avatar
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    I rode my Tikit to the next town [25kms] for a meeting recently. I've done this ride many times on a Cannondale road bike with 23mm 700c tires. The 40-349 Greenspeed Scorchers [@ ~50/55psi] on the Tikit completely eliminated the road buzz from rough sections of pavement and even crossing over the rumble strips was considerably less dramatic with the Tikit. I had to look down several times since I wasn't getting the usual feedback from my bike that I had come to expect from these road features.
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  12. #12
    Senior Member Foldable Two's Avatar
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    Nice work, Jur, but it sounds like you've had too much time on your hands since fracturing the folding handle bar on your Downtube Mini.

    (Once the replacement part arrives, he won't have time for such "nerdy" analysis. )

  13. #13
    Life in Mono
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    Nice 1 Jur !!
    I think you may be 'preaching to the converted' here tho'
    Have you tried cross posting over on Road cycling .... they would love to hear this .. I am sure they will give you a nice friendly response

  14. #14
    Prefers Cicero cooker's Avatar
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    If I understand you correctly, you are saying the harsher ride of a small wheel bike can be offset but putting fat squishy tires on it.

    The trade off, of course, is rolling resistance. You can soften your ride but it slows you down.

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    Wheelsuck Fat Boy's Avatar
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    I would say that there is the a couple other issues to look at.

    The first would be wheels. Since the smaller wheels generally have more spokes and are all around more stout, there is going to be less wheel defection with a 16" bike as opposed to the 700c. Some 700c wheels are quite flexible. Likewise, the fork and frame of the small bike are both smaller and more sturdy which would tend to transmit more road input to the rider. Generally, we are dealing with a longer seatpost and handlebar post, so that will work in the other direction. I think it would be very difficult to estimate the total road transmissibility without throwing a couple accelerometers on the thing and going for a ride.

    My BF with 451 wheels rides nice. The Ti seat boom has a lot to do with it. It rides better than my uber stiff Cannondale TT bike, but not as nicely as my C'dale Six13 road bike. In particular, the front end of the BF bounces around a bit. Since I'm running similar tire pressures on both (Stelvio's on the BF, Mich Pro2Race on the 'Dale) I guess this is to be expected.

    Interesting analysis none the less. Great work, Jur.
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  16. #16
    SWS: Small Wheel Syndrome kb5ql's Avatar
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    I find this analysis to be utter rubbish. The best ride ever is experienced on an ordinary bicycle.

    60" wheels should be the norm for all bikes.

    PS -- did I mention that Sturmey-Archer hubs SUCK???

    Last edited by kb5ql; 05-21-08 at 12:52 PM.

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    kb, maybe ordinary back in the dark ages?

  18. #18
    Wheelsuck Fat Boy's Avatar
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    'Boneshaker' just exudes thoughts of a smooth, supple ride, huh?
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  19. #19
    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by cooker View Post
    If I understand you correctly, you are saying the harsher ride of a small wheel bike can be offset but putting fat squishy tires on it.

    The trade off, of course, is rolling resistance. You can soften your ride but it slows you down.
    I don't have the citations handy -- my apologies -- but I seem to recall that on imperfect surfaces that the increase in rolling resistance due to lower tire pressure is minimal.

    Fatter tires, however, are definitely heavier and at the very edge of the wheel. And I would guess that they require fatter rims and fatter tube. Does anyone suppose that there is a meaningful aerodynamic difference?

  20. #20
    Explorer CaptainSpalding's Avatar
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    Quote Originally Posted by invisiblehand View Post
    Fatter tires, however, are definitely heavier and at the very edge of the wheel. And I would guess that they require fatter rims and fatter tube. Does anyone suppose that there is a meaningful aerodynamic difference?
    Not a meaningful aerodynamic difference, no. But possibly the ride is affected in other ways.

    Jur, I assume your analysis and your post were at least partially motivated by our discussion in another thread, here. Your analysis is great, and your technical acumen and simulation-fu are definitely superior, but I respectfully submit that the scope may have been too narrow, and misses the big picture.

    It seems to me that the dynamics among rider, bike, and road are really, really complex. In your analysis, you model the behavior of wheels and tires on a bike moving on level ground and hitting a square-shaped obstacle head on. You included as variables wheel diameter, tire size, tire pressure, wheelbase, and speed. Your model excluded other variables that may be quite relevant. I suspect that rider/bike weight is a factor. Various profiles of obstacles. The hardness of the obstacle. The incident angle of the tire to the obstacle. Rake/trail/overall geometry. Gyroscopic effect. Weight distribution as affected by an incline. All of these contribute to the perceived comfort of the ride. I would also suggest, for the sake of the integrity of the comparison, that if you are going to choose Big Apples for the 16" wheel (arguably the cushiest tirer available in that size,) you should also do a comparison between the same 16" wheel and a 26" mountain bike rim with a 2.125" dia. tir. Otherwise the conditions of the test tend to skew the data to favor the 16" rim.

    It's because of all these variables that, in the thread I referenced, where I generalized, I said that the compromise is between folded size and comfort, rather than wheel size and comfort. I would also add that there are more things that contribute to the perception of overall comfort than just the beating that the wheels, frame, and other components transmit from the road to one's wrists and butt.

    Which brings us back to your comment, invisiblehand. It could be that the added weight of the larger tires, owing to gyroscopic effect, mitigates somewhat the twitchiness of the steering, and makes the bike easier to handle over bumps.
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  21. #21
    jur
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    I generally agree with all the comments re the effect of other parts of a bike on the overall ride quality of a given bike. But I was interested in isolating the effect of wheel size and tyre pressure from everything else, to see how much merit there is in the oft-repeated statement that "smaller wheels give a harsher ride". There are other statements which are also dubious, such as certain laced wheels giving harsher rides than others (hooey, the amount of vertical compliance is vanishingly small) and certain frame materials giving harsher rides (also hooey, the geometry and design dominates).
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  22. #22
    jur
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    Quote Originally Posted by kb5ql View Post
    I find this analysis to be utter rubbish. The best ride ever is experienced on an ordinary bicycle.

    60" wheels should be the norm for all bikes.

    PS -- did I mention that Sturmey-Archer hubs SUCK???

    The way a wheel rolls over a bump (let's say a blade-like obstacle, up and directly down again) is that the axle locus describes an arc with radius equal to the wheel radius. I didn't realise this before doing the analysis. So the bigger the wheel, the longer drawn out that bump's effect will be. Tyres in the days of the ordinary bicycle were solid, hence the development of the biggest possible wheel that you could handle (that and the gear-inch issue).
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    Quote Originally Posted by EvilV View Post
    Surprising results.

    Richard Ballantyne made the point that by taking your weight on the feet and holding the bars lightly, the bike pivots around the bottom bracket, transmitting much less road shock to your body since your legs act like springs anyway, bending slightly at the knees. I think this riding style puts less strain on the bike itself too rather than sitting out the bumps in the saddle like a sack of potatoes.
    Agreed.

    This is the reason why the majority on the forum thinks the smaller wheel is harsher. Ballantyne was talking about what one does on a road bike where the geometry of the rider is not straight up but in a crouch. When the rider is bent over, lifting your weight is easier than if you were sitting straight up like one does on a Brompton. Ritchie to his credit put a rear suspension to make the bike comfortable and it works. Moulton added suspension because he knew the ride was going to be harsh running high pressure tires even though the geometry was similar to a road bike.

    Most 16' inch wheel folders have the geometry of either a beach cruiser or comfort hybrid and become quite uncomfortable after an hour of riding. Here's the reason why. After two or three hours, you can't lift your weight straight up for the ruts, bumps and potholes. As a result, you end up hitting all the road abrasions sitting straight up!

    Of course, you can get a Brooks Champion Flyer, Thudbuster or suspension seat post to smoothen the ride.

  24. #24
    jur
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    Quote Originally Posted by CaptainSpalding View Post
    I would also suggest, for the sake of the integrity of the comparison, that if you are going to choose Big Apples for the 16" wheel (arguably the cushiest tirer available in that size,) you should also do a comparison between the same 16" wheel and a 26" mountain bike rim with a 2.125" dia. tir. Otherwise the conditions of the test tend to skew the data to favor the 16" rim.
    I did this already for a look-see; there was no big difference between the results. Interestingly, when both cases' Big Apples are pumped quite soft, the difference becomes virtually indistinguishable. So a blindfolded chap would not be able to tell which bike was which from a purely wheel feedback POV. (That is to say if he could ride blindfolded in the first place.)
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  25. #25
    Bicycling Gnome
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    Quote Originally Posted by kb5ql View Post
    I find this analysis to be utter rubbish. The best ride ever is experienced on an ordinary bicycle.

    60" wheels should be the norm for all bikes.

    PS -- did I mention that Sturmey-Archer hubs SUCK???

    I prefer my velocipede any day over the ordinary. Here is a sketch 'snapped' by an artist I passed whilst overtaking an overweight rider last week.

    Last edited by EvilV; 05-22-08 at 01:20 AM.

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