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Bike tire rolling resistance and speed

Old 07-24-21, 09:09 AM
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nrsmd
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Bike tire rolling resistance and speed

Assume tires with a rolling resistance of 20 watts. Now replace with tires with a rolling resistance of 10 watts. What speed increase, if any, can I expect all things being equal?
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Old 07-24-21, 09:26 AM
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Depends on speed, frontal area, Cd.
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Old 07-24-21, 09:28 AM
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All things being equal. Is there a speed increase that is measurable?
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Old 07-24-21, 09:35 AM
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What will be more measurable is how less tired your legs will be. Why don't you just try them and see what you get. Average the best 6 out of 12 of the same rides with one tire set against 6 best of 12 on the other tire set. Then you'll know.

I'd bet similar to my experience most of the positive will be how much better your legs feel and how much more energy you feel like you have left. Maybe you'll get an 1/2 to 1 mph average on a 30 mile ride.
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Old 07-24-21, 09:36 AM
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Originally Posted by nrsmd View Post
All things being equal.
The amount of speed increase will still depend on those things even if they're held equal across the two cases.

If you're talking about rolling resistance per-tire, and we spitball a bike+rider with a CdA of .32 previously doing 20mph on level ground, you'd see somewhere in the neighborhood of a .8mph increase in speed at the same power.

Is there a speed increase that is measurable?
Yes, if your measurement is precise enough.
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Old 07-24-21, 10:16 AM
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So, that is helpful and a Go.

Now, what if I purchase aero rims.? Will I get a measurable boost in speed adding carbon fiber rims (assume cost about $400-$500 for the pair of rims)?
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Old 07-24-21, 10:28 AM
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Originally Posted by nrsmd View Post
Assume tires with a rolling resistance of 20 watts. Now replace with tires with a rolling resistance of 10 watts. What speed increase, if any, can I expect all things being equal?
Not a lot. Less than 1/2 mph, depending on initial speed and power input.

Bike Calculator
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Old 07-24-21, 10:37 AM
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One can only wonder... don't be disappointed if you make changes and still finish mid-pack. Changes that suggest minimal performance improvements 'in the lab', do not always translate to measurable improvements 'on the road'. So many, many variables. Overcoming air resistance is the primary solution to greater speed.

Eddy is attributed with saying - 'Don't buy upgrades, ride up grades.'
That said, the best place spend $$$ to improve ride quality is tires and wheels.

What you may need most after the wheels is a more aero position on the bike, skinsuit, shoe covers. Assuming a high level of fitness and low body fat measurement.

When splitting hairs, be sure to shave those legs, too.

Last edited by Wildwood; 07-24-21 at 10:42 AM.
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Old 07-24-21, 10:55 AM
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Is that 10W per tyre or for both tyres? Also what speed is that 10W saving quoted at? If you know the CRR coefficients for both tyres I could give you a fairly accurate estimate of their relative performance. I recently did this exercise myself. The gains were pretty marginal, not something I really noticed out on the road.
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Old 07-24-21, 11:01 AM
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Originally Posted by nrsmd View Post
So, that is helpful and a Go.

Now, what if I purchase aero rims.? Will I get a measurable boost in speed adding carbon fiber rims (assume cost about $400-$500 for the pair of rims)?
You will notice a difference.
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Old 07-24-21, 11:36 AM
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The break even point is around 15 MPH. That's the speed at which aero drag passes all other factors combined that are typically holding you back. As you accelerate beyond 15 MPH, aero drag becomes progressively more and more significant.

If you typically ride 10 MPH or less, you might be able to sense a speed difference in low drag tires. If you are able to ride in the middle to upper 20's on the flat, the rolling resistance factor, while still present, will become such a small fraction of the drag factors holding you back as to become insignificant.
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Old 07-24-21, 12:16 PM
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Going from 20 watts to 10 watts implies a 50% reduction in Crr. This would be a massively noticeable change.

You'd go from 11 mph to around 13 mph if everything else is "normal"

On my recumbent at 30 mph, I need 300 watts of which about 55 watts are rolling resistance. Cutting the rolling resistance in half would increase my speed by just around 1 mph.

Last edited by GhostRider62; 07-24-21 at 12:19 PM.
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Old 07-24-21, 12:20 PM
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Originally Posted by nrsmd View Post
All things being equal. Is there a speed increase that is measurable?
Yes
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Old 07-24-21, 12:36 PM
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Originally Posted by GhostRider62 View Post
Going from 20 watts to 10 watts implies a 50% reduction in Crr. This would be a massively noticeable change.
This is true. If this change is per tyre then you are talking about the difference between a dedicated road race tyre and the slowest of armoured commuter tyres. Would be quite an obvious difference.
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Old 07-24-21, 03:57 PM
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I just changed from 50mm Schwalbe Super Marathons to smaller width, smoother, softer compound tires. I doubt I'm going to do the two dozen rides prescribed by Iride, but I'll be happy to report back here. Probably just muddy the waters.
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Old 07-24-21, 05:47 PM
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Originally Posted by nrsmd View Post
Assume tires with a rolling resistance of 20 watts. Now replace with tires with a rolling resistance of 10 watts. What speed increase, if any, can I expect all things being equal?
Originally Posted by HTupolev View Post
a bike+rider with a CdA of .32 previously doing 20mph on level ground, you'd see somewhere in the neighborhood of a .8mph increase in speed at the same power.
Originally Posted by PeteHski View Post
If you know the CRR coefficients for both tyres I could give you a fairly accurate estimate of their relative performance.
Originally Posted by GhostRider62 View Post
Going from 20 watts to 10 watts implies a 50% reduction in Crr.

You all have no idea how comforting it is to know that these are matters which I will never spend a single second of my life considering.

Seriously -- and this isn't snark -- I thank you for reminding me of the sheer, simple pleasure of just going for a bike ride!
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Old 07-24-21, 06:14 PM
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Originally Posted by Rolla View Post
You all have no idea how comforting it is to know that these are matters which I will never spend a single second of my life considering.

Seriously -- and this isn't snark -- I thank you for reminding me of the sheer, simple pleasure of just going for a bike ride!
Notification: you must further assert that you have never worn and will never wear Lycra/spandex for official authentication of your post.
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Old 07-24-21, 06:17 PM
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Originally Posted by Rolla View Post
You all have no idea how comforting it is to know that these are matters which I will never spend a single second of my life considering.
!
Except you just did!
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Old 07-24-21, 06:52 PM
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Originally Posted by GhostRider62 View Post
Going from 20 watts to 10 watts implies a 50% reduction in Crr. This would be a massively noticeable change.

You'd go from 11 mph to around 13 mph if everything else is "normal"

On my recumbent at 30 mph, I need 300 watts of which about 55 watts are rolling resistance. Cutting the rolling resistance in half would increase my speed by just around 1 mph.
Using bike calculator a 20 watt (10 per tire) difference with a 170 pound rider putting out 200 watts on the flat looks more like a .7 MPH difference.
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Old 07-24-21, 07:44 PM
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Originally Posted by MattTheHat View Post
Using bike calculator a 20 watt (10 per tire) difference with a 170 pound rider putting out 200 watts on the flat looks more like a .7 MPH difference.
Except I listed 11 mph to 13 mph and not 200 watts in the first example. In the second, my CdA is under 0.150 m^2.

I have no idea what your assumptions were.

The bottom line....a 50% reduction in Crr would be huge. At 200 watts, the reduction would be more like 40 watts down to 20 watts. OP formulated the question very poorly
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Old 07-24-21, 07:48 PM
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Originally Posted by Rolla View Post
You all have no idea how comforting it is to know that these are matters which I will never spend a single second of my life considering.

Seriously -- and this isn't snark -- I thank you for reminding me of the sheer, simple pleasure of just going for a bike ride!
Seriously-and this isn't snark-I thank you for reminding me of the sheer, simple pleasure of just not going for a ride with people like you.
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Old 07-24-21, 08:27 PM
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Originally Posted by nrsmd View Post
All things being equal. Is there a speed increase that is measurable?
If you are not riding in the drops the majority of the time, you are losing speed.

Everything you do will help, but aero rims will not overcome a non-aero you.

John
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Old 07-24-21, 11:34 PM
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Originally Posted by 70sSanO View Post
If you are not riding in the drops the majority of the time, you are losing speed.
Well, there’s also the “hands on the hoods, forearms horizontal” position.

Some say that’s even more aero than the “hands in drops” position.
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Old 07-24-21, 11:40 PM
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You'll notice the difference, although as other folks noted above around 15 mph wind drag becomes a more important factor.

It'll be a nicer ride too. Tires with lower rolling resistance are usually more supple and comfortable, especially at slightly lower pressures without sacrificing much rolling resistance. The rolling resistance tire test site has plenty of data. Tires differ in how much the resistance changes with pressure.

Thinner, lighter butyl tubes will help a bit too. So will latex tubes. Ditto newer tubes like the Tubolito and Aerothan. Or going tubeless.

Aero rims help a bit in some conditions. In cross winds high profile rims can make handling tricky.

Regarding position, riding the hoods rather than drops, with forearms parallel with the ground, is more aero than using the drops, per wind tunnel tests. But it's physically taxing and harder to hold for extended periods. Plan on doing some pushups, planks and core strengthening to hold the hoods/parallel forearms position for long stretches. Riding the drops can be fast if we keep our heads down, but as soon as we look up to check the road ahead it's like unfurling a sail. Everything is a tradeoff between efficiency, comfort and safety.
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Old 07-25-21, 01:11 AM
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Originally Posted by nrsmd View Post
Assume tires with a rolling resistance of 20 watts. Now replace with tires with a rolling resistance of 10 watts. What speed increase, if any, can I expect all things being equal?
You can just barely measure a speed difference between a pair of GP5000s and Vittoria Corsa Speed tires which is supposed to be about 10W difference or so. Given the difference in durability and puncture resistance and (especially wet) grip which all heavily fall in favour of the GP5000s, it's not a clear cut decision. I have a wheelset with one and one with the other.

Differences of a few watts between top shelf road tires would, by extension, be even harder to measure without applying more rigorous methods.

Aero wheelset is also a similar difference - detectable on segment times but barely - but it's a more clear cut decision to use one because there's literally no downside to using something like 45mm deep aero wheels except price.
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