Does a 700c wheel help you go faster?

Question

Someone suggested going with a 700c wheel to go faster, and after thinking about it I am wondering if a larger wheel actually does go faster. I understand that your gears will cause a wheel to rotate once and if the wheel has a larger circumference it will travel farther, but this comes at a cost to leg power, and why not just get a larger sprocket? So cut out the gears and such and talk about the physics.

I was lead to believe that rotating weight is what slows you down, so a larger wheel will have more mass, thus making it harder to push, hence slowing you down. A bike with 12" tires going the same speed as one with a 700c will have less rotating weight so will it be more efficient? If you are on a flat track with thin, hard tires and your bikes are geared to be equally efficient to pedal at low and high speeds, which wheel will make you faster?

Answer 1

The increased weight of the larger wheel (if indeed it does increase) will of course add weight to the bike, but additional weight adds very little rolling resistance (though it does of course affect hill climbing).

It is a myth that "rotating mass slows you down". In terms of top speed there's no difference between weight on the wheel rim and weight anywhere else on the bike. Added weight on the wheel rim does reduce acceleration, but 10 grams on the wheel rim only has the acceleration-limiting effect of 20 grams elsewhere -- it's not really enough to notice. (And once you've accelerated to speed this effect goes away.)

A larger wheel can ride over slight irregularities in the road with less energy expenditure, and so can be more efficient (and less tiring) in some circumstances. And a larger wheel generally is more stable (though at the expense of maneuverability).

Answer 2

Wheel size is a trade off. In some ways, you want the wheel as small as possible, in other ways you want the wheel as large as possible. There is no "right" size really, except that it's handy to have the same size as everyone else so standards have risen up.

The benefit of a smaller wheel is they are cheaper (less materials to make them), they have less aerodynamic drag (because you're closer to the ground), the rider themselves is closer to the ground (and therefore more easy to control the bike) and they are lighter. The last one is a biggy, because the wheel is spinning which creates complex forces that I won't explain... but suffice to say the wheels are the most important place on the whole bike to reduce weight, and the weight of the wheel dramatically effects your ability to change the bike's direction while the wheel is spinning.

The benefit of a larger wheel is that the angle of the tyre when it hits a bump is better, and therefore the bumps will not slow you down as much. Also, many of the benefits of a small wheel (like keeping the rider closer to the ground) only start to become a problem above a certain size. Anyone who's ever ridden a skate board or rollerblades will point out a tiny bump can basically stop you dead in your tracks with small wheels.

To sum up, you want your wheel as small as possible... but not so small that bumps will slow you down. A BMX is much easier to throw around when doing stunts, but you won't ever get it to go fast without a perfectly smooth surface (I have ridden a BMX with high gearing once... it required a lot of effort to keep up with someone on a typical adult bike)

Cars have good suspension, so they can get away with smaller wheels. But notice that off road motorcycles/cars tend to have larger wheels, sacrificing on road performance so they handle bumps better (you can really feel the extra weight of the wheels on an off road motorcycle, especially above 50kph/30mph. Even switching to tyres with deeper tread completely changes the handling characteristics of mine).

At the end of the day though, you should stick to the most common size wheel, because it's just more practical. You'll have more choice and better prices when buying tyres/tubes/etc, and the standard sizes are perfectly fine.

Answer 3

Rotating weight does not slow you down. It has a flywheel effect, that is, for a given weight, the bigger the wheel, the stronger the tendency to keep rotating at its current speed. If the current speed is 0, that means you need to work harder to overcome it (ie, acceleration is harder). It doesn't mean the wheel is always going to be slower.

As pointed out by others, smaller wheels ride relatively poorly over bumps (which saps a lot of energy). One of the very few makers of small-wheel road bikes, Alex Moulton, uses suspension to compensate. Suspension weighs more than bigger wheels.

Smaller wheels might have other added losses as well. I can speculate that the way the rubber in the tire deflects when it rolls onto the ground has higher hysteresis.

Answer 4

Does a 700c wheel go faster? Hmmmm...

Kinda hard to give a definative answer to this one as you have lots of variables to contend with.

Let's assume that both wheels being tested have the same mass.

Well a 700c wheel typically refers to a race wheel which is going to have a very high inflation rating and a thin profile. Both of these items allow for a very small contact point on the road (foot print). The smaller the foot print you have the less road resistance you will have and the easier it is to move the wheel along the road.

If it's easier to move the wheel along the road you can get a higher effiency power transfer to the wheel. If you have a higher efficiency then you waste less energy moving forward and so have more energy to go faster longer. So based on the sole assumption that the previous wheel has a larger foot print than the 700c wheel then I'd say that the 700c wheel does in fact have a greater likelyhood of traveling faster.

Now let's look at both wheels from the stand piont of 'moment of inertia' (MoI). That's the amount of force (torque) it takes to increase or decrease the rotational velocity.

If both wheels have a mass of 1kg and the 700c wheel has a radius of 311mm then the moment of inertia is calculated to be about 9.6kg m^2. If we compare that to a BMX wheel having the same mass and foot print (ya right, a skinny BMX tire?! Let's just go with it for the example) and a radius of 203mm we get 4.1kg m^2 needed to change it's velocity. Moment of Interia Calculator

Theoretically the BMX wheel should be able to go way faster because it needs much less energy to increase it's angular velocity. The problem is what enables it to be able to accelerate so quickly is a double edged sword because it also allows it to be deccelerated just as easily.

Because we don't ride in vacuums, we need to contend with wind resistance as well as road resistance. So while it takes less than half the energy to get that BMX wheel up to speed, the counter effects of the wind and road resistance are acting at more than twice that of the 700c wheel at slowing you down. Also to remember is that the force from air drag is calculated from the square of your velocity so the faster you go the drag you need to overcome grows exponential. If you were thinking that you'll just go fatser to make up for the difference, think again.

It's a trade off. Do you want to be able to sprint up to speed really quick but have that speed bled off just as quick? Or do you want to be able to maintain your speed over longer distances, taking more effort to get up to speed, but be able to maintain it with less effort over the long haul? Like the tortoise and the hare... if the race is short then the hare wins everytime. Make it longer and the hare burns himself out.

If you want to look at the effects of how the distribution of mass affects the torque on a wheel then check out the claculator for inertia of a thick walled hoop. You'll see how moving the mass closer to the hub affects the MoI.

Hope this helps.

Answer 5

If you are on a flat track with thin, hard tires and your bikes are geared to be equally efficient to pedal at low and high speeds, which wheel will make you faster?

The recumbents that do really well on human powered vehicle speed tests tend to have smaller wheels so that they can get as low to the ground as possible and keep a small wind-profile.

It's not because the wheel is smaller though. One thing to keep in mind is that smaller diameter wheels tend to be made wider so that you get a decent size contact patch to corner and brake. This may offset any aerodynamic gain. Aerodynamic gain comes from making the whole bike shorter (as in a recumbent). If you have to make the wheel thicker than the frame, then you're actually adding to the wind-profile.

The main arguments for large wheels off the track are

• Smaller wheels do worse on uneven road surfaces -- imagine a wheel that hits a pothole that is deeper than the radius of the wheel -- you're going to endo.

• Smaller wheels have a lower volume of air, so don't behave as well as shock absorbers.

Answer 6

Doesn't this become a chicken & egg thing? As 20 inch wheels accelerate faster than a large wheel but larger wheels maintain their momentum longer, over a given road distance with climbs, turns & descents the advantages should cancel each other out. Of course if top speed is your goal then in my opinion the larger wheel diameter will be faster but at the cost of poorer handling through the turns.

Answer 7

Assuming that the tire widths are the same, a larger tire will make you go faster. One revolution of an axle that has a tire with a 4-foot circumference will travel 4-feet, whereas one revolution of an axle that has a tire with a 6-foot circumference will travel 6-feet. Also - in order for the 4-foot circumference tire to travel the same 6-feet distance as the 6-feet circumference tire, the bike operator of the 4-feet circumference tire would have to pedal 1&1/2 times the number of pedal revolutions as the bicycle operator of the 6-feet circumference tire. The larger tire (we're talking only CIRCUMFERENCE here) travels a greater distance with each revolution of the axle. Think of rotating a skateboard wheel one single revolution (the "axle" rotating one time) versus rotating the wheel/axle of a monster truck just one time. The wheel of the monster truck travels a MUCH greater distance with each revolution of the axle, which translates into greater "speed".

Answer 8

Smaller is faster? yes with limited parameter or ideal calculation. There is complex calculation in the real 700c vs smaller faster things. For example the energy stored in a rotating weight is not gone, once it is rotating at certain constant speed all you need is dealing with frictions that mainly coming from wheel-road contact, bearings and the wind. You can go faster with fine tuned 700c wheels. A 700x25c at 100-120psi has the lowest friction to normal paved road. A 600 wheel at the same 25 size may have similar friction at 150psi and this only at flatter (better) road, as in normal road there will be another friction caused by small bumps.