ICU Doc

I've now accumulated enough experience with my Bike Friday that I wanted to ask if my subjective impression that there is a more rapid loss of forward momentum when coasting with the smaller wheels than with 700c wheels is real. It seems like I slow down more quickly when I stop pedalling than I otherwise would with my other bikes. Does this really happen, or am I imagining things?

JonathanG

It should happen. In general, 20 inch wheels have a lower rotatinal mass than 700C wheels, so their inertia, the tendency to remain at rest or in motion, is less. On the other hand, with a lower rotational mass, 20 inch wheels are much easier to accelerate.
Jonathan

geo8rge

I do not know the answer but ...
You may want to study angular momentum:
https://en.wikipedia.org/wiki/Angular_momentum

The wheels have angular momentum:

Angular Momentum = Moment of Inertia x Angular Velocity.

The Moment of inertia is smaller on smaller diameter wheels (the rim is closer to the hub).

The angular velocity for smaller diameter is higher (they spin more times than a large diameter wheel to cover the same distance).

All else being equal the angular velocity of a small diameter wheel should decelerate faster, from a higher starting amount.

To compare you would also have to have the same breaking force. Not that if you are using rim breaks the same force on a large diameter wheel should produce more torque (leverage) leading to more 'breaking force'. The angular velocity of the rim may affect breaking.

How much break force is applied to each wheel may also be important.

To make things more complicated you are really talking about the foward momentum of the bike as a system, not just the wheels. The pocket rocket may weight less overall and therefore be easier to stop.

I think the breaks on the new pocket rocket are better than on the other bike, but cannot come up with a mathematical argument either way.

ICU Doc

Thanks for the input.

Regarding geor8ge's comments, I am talking about loss of forward momentum, simply when I stop pedalling, not when I actually apply brakes. Also, my Cervelo is far lighter than the Bike Friday, my Basso is the same weight, and both are equipped with Dura-Ace whereas my PRPro is equipped with 105, so any argument about the PRPro being lighter or having better brakes is moot.

I'm glad that both geor8ge and Jonathan do support my observations.

Best to all.

pxboii

you are correct about loss of forward momemteum. The 20" wheel will lose momemtum more quickly then the 26/700/27 wheels. I find I have to pedel more to keep up with other bigger wheel bikes.

dubes

Have you accounted for differences in tire widths? All else being equal, fatter tires will have higher rolling resistance and higher wind resistance (which really comes into play at higher speeds), and 20" tires tend to be fatter than 26"/700c road tires. Another factor could be riding position: many folders tend to put the rider in an upright sitting position on the bike (you don't mention whether your BF has flat or drop bars), which again creates wind resistance that will work against you.

Dahon.Steve

I think it depends on the bike itself. Folders from Dahon and Brompton leave the rider sitting straight up making every wind a head wind! There is no way these bikes are going to be fast with an inefficient geometry.

Fear&Trembling

The new S-type and P-type Bromptons provide a more aerodynamic position due to the lower height of the bars. The P-type gives a choice of heights - either (approx) 92cms or 103cms from the ground. This makes a lot of difference. The M type retains the original bars and these are certainly not suited to an aerodynamic tuck!

tharold

All else being equal, fatter tyres have a lower rolling resistance. But fatter tyres typically can't take high pressure or be made of thin material, which is what makes the narrow tyres win out. It isn't the width per se.

jur

^^^ +1

I would also vote for wind resistance which is the majority of what you're fighting against. Smaller wheels have a small amount of extra rolling resistance but once the road surface irregularities become small enough like in average roads the difference becomes negligible. In fact a very high pressure tyre may actually increase in rolling resistance slightly. There needs to be some give to roll over small bumps better.

ken cummings

I read the OP and the comments directly related to Moment of Inertia and something sounded a little bit off. Sort of an itch in the back of my mind. So I found my old college text "Introduction to Physics for Scientists and Engineers" 1969 McGraw-Hill, and looked up moment of inertia for a hoop, page 197 figure 11.9.

2
I=M(a)

where I is the moment of inertia, M is the mass of the hoop (bike tire and rim), and a is the radius of gyration (AKA in this special case the radius of the wheel). Since we just want to compare the 700 mm wheel and the 20" wheel and not calculate absolute values lets use same rim and tire type for both. That gives us 2 x pi x r = M. Put into the equation we get I = 6.28 r cubed . Since we just want proportionalitites dump the 6.28 . (350mm/[10"x25.4 mm/"]) cubed gives 2.624 .

So the standard 700 wheel has over two and a half times as much inertia as the 20 inch wheel.

Harder to spin up and better at evening out irregular pedalling forces. As far as little wheels being a handicap in keeping up with standard wheels that is a steady state case and moment of inertia drops out of the equation. I support the poster who talks about air drag and body position.

ICU Doc

actually, this is an interesting discussion about the inertia of wheels and aerodynamics, especially as I have the Bike Friday set up for the exact same body position as my 700c bikes. this would indicate to me that the more rapid decrease in speed is due to the decreased inertia of the wheels if everyone is right. turns out this may have been an interesting experiment under conditions in which my body position is the same, that brakes aren't used, and the big variable is the size/mass of the wheels.

ken cummings

Try calculating the inertia of the 20" with the (M)ass increased by filling the tubes with water. With fat enough tires you might get matching (I)nertias with the 700 wheels. Also calculate the horizontal momentum of rider, bike, and wheels and see how it compares with the strictly rotary or angular momentum of the wheels. I'm strictly guessing that linear momentum is (much) bigger than the angular momentum. Problems like this were meat and potatoes for the deceased (and much lamented) Bike Tech Magazine.

Wavshrdr

An easy way to test some of the effects is to do coast down testing. This will give you an idea of the total drag of the entire vehicle. Do it on smooth pavement so as to minimize the effect of the road surface. Do it from a higher speed and try different body positions. Easiest way is to find a hill and start from the same place each time. Aerodynamics will in general start to be a factor at 15 mph and become significant over 20. Below 15 it is almost all drag due to rolling resistance.

The last data I saw for efficiency of tall wheels vs. the 20" was about 1% more efficient than the 20" in steady state usage. As already mentioned the smaller wheels accelrate much more quickly. This also translates into better hill climbing ability. Biggest disadvantage I see and experience is the compromises in gearing. I can't easily have very tall gearing without going through a lot of hassle such as really big sprockets up front but for the most part this isn't an issue in 90% of my riding. If I were racing it then it would be.

ICU Doc - you may think your position on the bike is the same but even small changes here have a HUGE effect at faster speeds. You need to get an idea of your effective frontal area before you can really judge this. A quick and dirty way to see actual frontal area is paint a grid a wall with whatever size squares you want for the resolution you need. You could go 6" for good compromise. Sit on your bike in front of the wall. Have someone take a digital photo of you. Do this for both bikes. Compare the square footage and you have a good comparison. Much more difficult to determine will be your drag coefficient.

If your posture is VERY similar we will just assume the drag coefficient will be the same. If you even have a few % difference in frontal area you will have a signficant difference in drag. I would also suggest taking a photograph from the side as well against your grid. You can then compare the height to width and infer a few things regarding your drag coefficient. Taller = bad. Longer - good. Think teardrop shape as a good aerodynamic shape to emulate while ridiing. Aero drag goes up by the square of speed and power required goes up by the cube. To double your speed will incur 4 times the drag and require 8 times the power as a rough rule of thumbe regarding aerodynamic drag. Drag due to rolling resistance tends to be more linear.

I used to race cars and motorcycles at very high speeds and just taping the seams where fenders and doors met could be worth 2 mph or so. On one motorcycle I had during top speed runs if I just had my toes sticking down below the footpeg It would drop my topspeed from 201-202 to 199. This was consistent and repeatable. Think about that, my toes could cost me 2-3 mph on top speed. This was an issue of not just increasing my frontal area which it did but it also disturbed the boundary flow of air over my fairing in that area which then impacted the airflow around the rear wheel.

Definitely don't overlook the effects of tires too. Different compounds have different drag coefficients. Another thing to consider at that the top of the tire is traveling through the air at twice you groundspeed (assuming no wind). So at 20 mph the top part of your tire is reaching speeds up to 40mph relative to the wind. Remember what I said about drag and energy earlier? Tire width can become very significant when you look at it from this perspective.

Hope this helps understand it all a bit better.

EvilV

I'd have thought that the other forces in the equation when coasting, such as wind pressure, and wind resistance would be so much larger, that you'd be hard pressed to notice any real effect. To be honest though, you'd need some proper engineering tests to really know how much of an effect this is. Anecdotal feelings are no good because there are so many changing factors in riding around real roads in real weather.

Other factors might be:

road surface condition
tyre pressure
road inclination
wind
rider's clothing

dalmore

When I ride my Mu SL with stelvio 25s with friends on 700 23s I lose speed when coasting faster they they do regardless of the incline. I must pedal to keep up. When I ride my Speed TR with big apple 50s with the same people on their same bikes on the same routes, I rarely have to pedal to keep up while we coast. Again incline doesn't seem to matter that much but we rarely coast up inclines so that may be a factor. My seating position and aerodynamics are pretty much the same on the two bikes. The Speed TR weighs abut 10 pounds more and has much heavier tires. I vote that rotation inerti makes a huge difference based on my experiences.

That said the MU SL is a much faster bike that the Speed TR. Much faster. Maybe it's because I have to keep pedaling?

Gyrostapo

No, no no. Smaller wheels do accelerate better, but that has zero effect on climbing. The only really important factor in climbing is total bike (and rider) weight. The notion that rotationally lighter wheels are better for climbing is a flat earth type of belief.

Gyrostapo

The OP is correct that he decelerates more quickly than his big-wheeled brethren when he stops pedalling, but he can take heart in the fact that it doesn't take much effort to catch up again. The small wheels quickly lose momentum when you stop pedalling, and quickly regain it when you resume your efforts.

DaFriMon

I really don't know if small wheels are better for climbing. I'm sure that those with better physics/engineering backgrounds will be able to construct arguments that I don't know how to refute, on both sides.

I do know that I have never noticed that I climb better with 20 inch wheels than with 700c ones. Going up hills is where I'm most likely to catch other, faster, riders, but that's just because climbing is my comparative strength.