I have heard this from bike store employees: Is it true you cannot acheive the same top speed with two identical bicycles if one has much smaller tires? I want to buy one of those fold-up bikes, but they have shorter tires and I have heard that kills the high end speed.
If this is true, why, if I put super hard 100psi tires on there, can’t I acheive and maintain same top speed? I suppose I do not grasp the physics involved enough to even speculate why this could be the case. I mean, I understand the bike will not be as stable/steady due to much less gyro forces…
It’s the circumference of the tires. A smaller tire will roll along a shorter distance per revolution.
If the circumference of a smaller tire is half that of a larger tire, you’ll need to pedal twice as fast (double the revolutions) to make up for it, at the same cog/chainwheel gear ratio.
Folding bikes are meant for riding to the train station, not high speed, anyway.
This is not a concern in real life, because high-performance folding bikes are equipped with larger gear ratios. Shimano even has a part family specifically for small-wheel bikes (Capreo).
The main factors determining top speed are rider power output, air resistance and rolling resistance. Given identical tire construction and pressure, smaller diameter tires have greater rolling resistance. But rolling resistance is less important than air resistance, especially at high speed, so tire size has very little effect on top speed. More important is the fact that larger wheels roll over rough surfaces and obstacles more smoothly, with less speed loss. This can be noticeable on chip-seal roads and unpaved roads, but usually not noticeable on smooth pavement.
I used to own a recumbent with 16-inch wheels. It had 100-psi tires and decent gear ratio. It worked very well for a 1-week 500+ mile supported tour.
This is true for some folding bikes, like my Brompton, but definitely not all. Bike Friday bikes are built for speed, and have an excellent reputation. I’ve seen many Bike Friday riders smoke riders on conventional road bikes. Dahon’s high-end models are pretty nice too, at very reasonable prices. R&M and Airnimal also make nice high-performance folding bikes.
Key phrase here is “two identical bicycles.” As *gotpasswords describes, using a smaller wheel is tantamount to using a lower gear, all other things being equal (would limit top speed but facilitate hill climbing). All other things, however, are rarely equal.
For the fold-up job you’re thinking about, just do a test ride, and try to max out in the highest gear. If you’re spinning like crazy but not working too hard then this bike will limit your speed. If you’re really pushing then you would not be able to go any faster with bigger wheels anyway.
OTOH, Performance Bicycle sez, “Smaller wheels don’t have to mean slower rides, the 4-speed Shimano® Inter-4 shifting with an 80-tooth front chainwheel provides plenty of get up and go when you need it.”
There’s lot of folding bikes out there ranging from the older style of commuter to more high-performance models. So it’s hard to generalize.
A friend has this much modified Raleigh 20 among his collection, and though the frame is heavy it’s very stiff, and most people on conventional bikes can’t keep up with him, and on a fully-rigid mountain bike I really struggle to keep up while he’s just ticking over.
But I leave him way behind on very steep descents, but only because I’m much heavier while presenting a similar frontal area, and thus my terminal velocity is higher.
Instability is the main drawback of small wheeled bikes, and not just because of the lower gyroscopic forces that help keep the bike upright. Small wheels have a tendency to disappear down potholes in the road, and the shorter spokes means the wheel is much more rigid, and doesn’t absorb the shocks as well as a larger wheel. A good compromise is to have a limited degree of suspension movement for both wheels. The German Birdyfolding bike is such a beast, though the same friend who owns the Raleigh 20 had one of these while he was living in Tokyo, and one day it just flipped on him and threw him face first into the gutter, while the bike itself bounced off the rear of some Yakuza’s Mercedes.
He’s just bought a new folding bike, and he’s dead impressed with it, and more importantly it hasn’t tried to kill him yet. It folds up small enough to be taken on a train, boat or plane, which is what you really want from a folding bike. I damned if I remember the name of this fabulous new beast, but I shall ask.
I can’t see the point of having small wheeled bikes that don’t fold up, but they do exist.
Right. As scr4 has noted, there’s no reason to expect a small-wheeled bike to employ the same gear ratios as one with large wheels. The comparison should probably focus on the difference attributable to the wheels & tires, not on the notion that the bike is geared in a silly way.
And I think scr4 has covered that rather well - the difference would not be much until the road surface becomes rough.
Ummmm… why are all these bikes over $1K?
I’ve wanted one for a while to avoid car carriers and potential auto paint damage, but at these prices it’s cheaper to get my trunk lid repainted every year for 4 years and use my current bike…
Not all. The Bike Friday Pocket Tourist starts out at $800, which is a very reasonable price for a custom-built bike. I believe their frames are all hand-made in the US.
The Dahon Speed Pro is $900 (list price), and the Speed P8 can as little as $440.
The sidewalls in a smaller-diameter tire will flex more, and have more rolling resistance. The difference is small, but it is there. For racing it’s bad, but for casual use, it’s not a problem really. Smaller front wheels are a bit more jittery on loose surfaces.
Because they are hand-made and use good components. You can find cheapo Chinese imported ones for $300-$400 but they are dreadfully heavy and yet they don’t hold up well even with just casual riding. Warranty service is usually poor and often “financially unattractive” what with handling fees and shipping 50-lb bikes around the world and all.
~
There are a few advantages besides folding ability:
[ul]
[li]The overall size (length) of the bike is reduced, so it takes less storage space.[/li][li]There’s a lot of room above the wheels which can be used for luggage space. The Moulton is a good example.[/li][li]Large-wheel bikes may not fit children and shorter adults. A small-wheel bike with adjustable seat and handlebar height can be a one-size-fits-all bike.[/li][li]It looks cute.[/li][/ul]
For alternative bicycle designs (e.g. recumbent bicycles), small wheels provide more flexibility in bike design. And in some cases, smaller wheels allows you to reduce the height of the bike, which makes the bike more aerodynamic. Even though the rolling resistance is slightly incrased, the reduced air resistance makes up for it and then some. The current world human-power speed record holder (Varna Diablo) uses 24-inch wheels. And a faired (i.e. equippped with an aerodynamic fairing) Moulton holds a world speed record for upright (= non-recumbent) bicycles.
On a smooth surface, The friction in the bearings is the true limiting factor, since the hub is twisting faster than a normal sized wheel. Same bearings? More friction, which shows up at 20+ mph…
Bearing friction isn’t a limiting factor on anything - it’s insignificant compared to air resistance and energy loss in the tire. The only quantitative info I can find is the first graph on this page, but it suggests the power loss at 20 mph is around 1 watt per wheel. So the difference in bearing friction between a 20" and 700c bikes is about 1 watt total. This might be measurable if you’re doing a time trial, but otherwise a negligible factor.
I thought someone did an experiment and found the wheel-gyroscope effect was neglibible compared to something else on a normal bike, but I can’t find a description now…
No, no, nobody listen to the NASA guy that knows what he’s talking about, I’ve got some $3,000 ceramic-bearing Zipp hubs that I would love to sell you for $2,500. They, uh, fell off the back of a truck.
Save you an entire tenth of a watt per wheel at 30 mph, they will!
So it’s a good thing when I see Masters racers that are 10 pounds overweight on these things. :rolleyes:
It’s proven to be unnecessary for maintaining balance, but I don’t think it’s shown to be negligible.
In my expereince, small-wheel bikes are a little more responsive than big-wheel bikes, and also difficult to ride hands-off. But without a more controlled test, I can’t say for sure that’s caused by the wheel size (rather than, say, amount of trail).
Small bikes are inherently unstable, and tip easily, because of the smaller gyroscope effect.
This makes them quite tiring to ride, as you spend much of your energy maintaining the bike upright.
Being over the age of 5, I’ve never had any problem with fatigue caused by the effort to not keel over on a folding bike at a dead stop. I rode only a Bike Friday for a year while I lived in Belgium, using it on some 75 mile rides, and not a problem. Stopped feeling twitchy after about the first mile of the first day. Coming home after a year and jumping on a regular rode bike, though, the road bike felt like driving a unresponsive, 100-pound couch on coasters. Just depends on what you’re used to. I mean, is a BMX bike or a trials bike ‘inherently unstable’? Tell that to Ot Pi. On a pure-physics level, perhaps, but any rider with a torso and one or more limbs should be fine.
I think one of the owners of Bike Friday actually races one in the Sea Otter Classic and doens’t do badly. All in the gearing.
Some feedback from my more knowledgeable friend, who has many years of experience in all sorts of bicycles:
"The bike I have is a Brompton, they’re not cheap but if you want a folder
to use every day in all weathers that’s your man. Nothing folds as small or
as quickly, rides well enough to tour all day on and still folds properly
with mudguards and luggage system fitted. Also you don’t have to pick it
off the ground to fold it, you don’t have to grab hold of any dirty parts
and when folded the chain is tucked away in the centre, these are the kind
of details you notice when using it with your best clothes on during a wet
day.
There are cheap Chinese copies of the Brompton that have appeared in the
last year or so since their patents expired, these are half the price and
the reviews seem to reckon that they’re about half as good. Apart from
being a bit cheeky, they haven’t copied many of the quality features such
as the robust hinges and powerful dual pivot brakes and despite being made
of aluminium it’s heavier than the steel original. Might be worth
considering if funds are very limited, but to be honest I’d still go for a
secondhand Brompton from eBay for much the same price.
Although the Birdys are fun to ride I’d have reservations about
recommending one. They’re a tad lighter and a bit better to ride than a
Brompton on a long journey, but they are much more expensive, they’re much
more awkward to fold, you’ll usually get your hands dirty doing it, and
it’s difficult to fit mudguards and luggage without them interfering with
the folding. And they go OTT very easily if you do an emergency stop,
although a more sensible rider probably wouldn’t have this problem."