Folding Bikes - Science/performance of small wheels?
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07-01-07, 06:35 PM
Anyone have any handy links to FAQs/info on small wheel performance? Or, performance as wheels get smaller. I suppose that the details of 20" wheels are most relevant to folders, but 12" and smaller wheels are players in the folder world, too.
For instance, in a 20" wheel how would a 35mm/75# tire compare to a 23mm/115# ? To me it seems like a hard, narrow 20" tire would be tolerable or even fast only on very smooth roads. Suspension helps some---but how much?
We had s discussion on this a short while ago: http://www.bikeforums.net/showthread.php?t=309371
07-02-07, 08:01 PM
I checked out that thread. Some neat things. (Heck, I also thought I already posted here in reply but now I can't see it...) But I'm looking for more! More small wheel science... Like, that first post said that for rough roads a big cushy wheel is better than a small hard one. Well, can you design to untie those factors? What happens with a small, wide, softer wheel? Offhand, it seems like casing suppleness might be more important for small wheels than big. Then there's the energy loss from the bulge in front of and in behind the contact patch. I think there might even be a concavity in front of and behind the patch as well. Basically, the casing wrinkles/humps in a way as it flattens. Then there's the difference in flex loss between radial and bias-ply. I gather that bike tires are 'somewhat' bias-ply, not radial like cars. I've seen these factors mentioned in German material on rolling resistance. But in a cyclingforums debate last fall between Jobst and Jan the bulges weren't mentioned, though the research was quoted and translations offered... The ply issues came up obliquely. With small wheels it seems like energy losses from this stuff would get worse. But what's really going on? Help?
07-02-07, 10:05 PM
Have you asked Sheldon Brown? He might have pointers to places if he doesn't know himself.
07-03-07, 05:13 AM
My guess is there is very little well funded scientific research being done on bicycles. You might try the Society of Automotive Engineers and try to extrapolate to bicycles. The small vs big wheel thing is probably an area of significant effort in the auto world.
Here is a fun related thread we've been discussing: http://www.bikeforums.net/showthread.php?t=314693. So far, this unscientific test suggests that there is no difference between big bikes and small ones, with a huge 2 data points.
One link that might be of interest: http://www.legslarry.beerdrinkers.co.uk/tech/GS.htm
This test used a small roller to test resistance, which gets further and further from real world conditions, but might suggest something about the bulge. (Wouldn't we expect it to be more of a problem under such conditions?) It suggests that smaller wheels are more efficient.
There is a more scientific article out there in the engineering literature, but I can't remember where I saw it.
07-03-07, 08:03 AM
IIRC there is a section about rolling resistance and tires in the book Bicycling Science (http://www.amazon.com/Bicycling-Science-David-Gordon-Wilson/dp/0262731541/ref=pd_bbs_1/104-4134014-0987903?ie=UTF8&s=books&qid=1183471872&sr=8-1) by David Gordon Wilson. I would look it up, but I can't find my copy at the moment. The references in the book are also very detailed, so you would be able to look up the paper/research the data came from.
07-03-07, 10:29 AM
It's not all about science. When I was a kid, a buddy of mine had an old Alpha Romeo Spyder. It was the most fun car I've ever had the pleasure to drive. Then one day his dad got a new Corvette. It was a new body style (this was 1977) and the performance numbers were better than any Corvette preceding it. We were each allowed a test drive. I was so disappointed. My friend and I agreed that while the Corvette had better acceleration, top speed, and cornering, it wasn't as fun to drive as his beater Alpha Romeo. The engineers who designed the Vette around wind tunnel and skidpad numbers didn't know as much about the sheer fun of driving as the guys who built the Alpha.
The question of small wheel performance seems a little bit based on insecurity, harbored by a lover of folding bikes who has the comments "big wheel" buddies to contend with.
I have a nice 700c road bike. Ti frame. High end "gruppo." By the numbers, it is superior to my folding bikes in every way. It's faster, lighter, better geared, less fatiguing over long distances, blah blah blah. But it's not as fun. I've used it maybe twice since I got my first folder. Like the Alpha Romeo, my Jetstream, like many folders, has a certain je ne sais quois. And if the sensibilities of the 700c riding buddies are to coarse to discern it, well, that's their loss, isn't it?
07-03-07, 12:15 PM
My friend and I agreed that while the Corvette had better acceleration, top speed, and cornering, it wasn't as fun to drive as his beater Alpha Romeo.
It would seem that what you "like to ride" better versus what "performs" better (given the same variables - terrain, rider, gearing, distance, etc.) are two different issues. I think the real question is "Does what I'd rather be riding (a folding bike, in this case) perform as well or better than if I were on a 700c bike?" Short answer: perform as well, maybe; better, no; yet would I rather ride a folding bike? yes.
07-03-07, 12:26 PM
The "Bicycling Science" material doesn't deal with the bulges (and concavity?) around the contact patch---which I saw mentioned in German research (I'm also querying about this at RBT now). Maybe like Jobst says RR is mostly minor stuff.
OTOH, I noticed when riding a new Moulton the other day that the wheels seemed kind of loose and "bangy" and seemed to scrub/lose a lot of speed. It wasn't much of a "fun" ride from my short test. Now, I also tried an old Moulton---and that baby had solid style, a real glide feel.
Optimizing tires probably involves trade-off's. In optimizing 20" and smaller wheels one could make sure to keep the fun factor way up there.
My 20" front wheel on my full-sus lowracer seemed to roll fine and have plenty of fun factor---altho when I put a fat tire on it for more highspeed cornering grip it got a lot heavier and lost some fun. It was funnest with a narrower, lighter tire. Both were high pressure.
If one had suspended 12" wheels and had high quality tires optimized (thru testing for width, etc.) and smooth pavement could one ride as fast as with 700C? Or are there diminishing returns, limits, where you get losses that can't be adjusted for. ?
BTW, the after pic of that custom Twenty of EvilV's is SWEET!
07-03-07, 03:24 PM
I have mixed feelings about this stuff. I agree that some scientific aspects are incomplete and thus don't capture all the relevant aspects. For example, David Millar (who is a pro TT specialist) did some wind tunnel studies, found a more aero TT position, but in competition it didn't work out so he went back to his old method.
At the same time, if you are looking to go beyond the subjective impressions, you need hard numbers.
I guess the question is, do you need to go beyond subjective impressions? After all, a 20" road bike (e.g. Pocket Rocket Pro), set up the same way as a 700c road bike, will be so close in terms of performance that it's barely noticeable unless you're in competition (in which case 406/451 may not even be an option). So the question really is "which do you prefer" and "do you know enough to set up your bike to suit your preferences."
I might add that unlike some folks, I went from Hybrid to Folder to 700c Road, and in many ways prefer my 700c roadie. So Captain, if you get tired of your ti roadie and want to dump it on the cheap, let me know. :D
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