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I understand that around 20% of the energy while cycling is lost by the deformation of the wheels. I also understand that in smooth and dry roads people use plain tires (no tread pattern) as these tires will get more contact area with the ground, therefore needing less deformation in order to equilibrate the forces (with P= F/A).

Now based on that, I was wondering if there is any estimate on much gravel bike tires (with some light tread pattern) might contribute to the loss of energy compared with road bike tires (plain) for the same conditions (dry, plain and smooth road). Is the difference going to be substantial and noticeable, or just a small percentage barely noticeable in efficiency? or more generally, is there any study quantifying the loss of efficiency as the tread pattern gets more and more predominant?

In principle I would expect that if the tread is close to "plain" but with some light pattern it shouldn't affect strongly the performance, but I am not sure if this is correct.

I am an amateur cyclist, and I will use my future bike mainly on the road, with a 5 to 10% gravel, so I need to make a decision based on this.

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    Tire weight, quality and designed use along with compound probably account for a significantly larger difference. i.e. a high end Gravel tire designed for performance will have better rolling properties than a low quality road tire designed for puncture protection.
    – mattnz
    Aug 31, 2020 at 22:56
  • I find that I’m able to hold around 30km/h on my gravel tires vs around 33-35km/h on my road slicks.
    – MaplePanda
    Aug 31, 2020 at 23:29
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    It's difficult to do a scientific comparison because there are a number of variables that go into a tire's rolling resistance. But there is a website that compares a bunch of road tires and a fair number of gravel tires. According to their tests, a top performing gravel tire uses ~17 watts at "high" inflation; a top performing road tire uses ~7 watts at 120 psi (which is very high). bicyclerollingresistance.com
    – Adam Rice
    Sep 1, 2020 at 1:30
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    If it's not too wet and you're not racing over the gravel, 28-32mm road tyres will do fine on a lot of gravel. I'd change mine for a mainly-gravel ride, but often build gravel tracks or rougher into a road ride
    – Chris H
    Sep 1, 2020 at 9:58
  • One of your statements is wrong: F=PA, or P=F/A as it's usually defined. Sep 1, 2020 at 17:24

2 Answers 2

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Now based on that, I was wondering if there is any estimate on much gravel bike tires (with some light tread pattern) might contribute to the loss of energy compared with road bike tires (plain) for the same conditions (dry, plain and smooth road).

You don't need to estimate. You can measure.

For example, here's one gravel tire: https://www.bicyclerollingresistance.com/cx-gravel-reviews/panaracer-gravel-king-sk

Its rolling resistance at high pressure is 21.7 watts per tire for a load of 42.5 kg and speed of 29 km/h. If the total bicycle load (bike + rider + cargo) is 85 kg, you get twice this or 43.4 watts.

In comparison, the most reasonable road tire today is probably the Continental Grand Prix 5000 in 32mm width: https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison

At 100 psi, it loses 9.7 watts when used with butyl tubes. If you think 100 psi is too much for 32mm, you can pick the 28mm width (10.3 watts) or reduce pressure of 32mm tire to 80 psi (11.0 watts).

So a road tire uses at most 11 watts per tire or 22 watts total.

Therefore, you lose at least 21.4 watts when using gravel tires.

Is this much? A quick simulator (taking into account riding uphill, on level ground and downhill in approximately correct proportions and simulating the uphill resistance, rolling resistance and air resistance) I wrote in Matlab shows that with the 21.4 watts extra rolling resistance, your average speed reduces from 22.4 km/h to 21.1 km/h. The simulator assumes the rider produces 90 watts on level ground, 180 watts on uphills and 0 watts on downhills.

I'd say this is much. For example, in 10000 km distance, you lose 27.5 hours when using gravel tires.

If you assume a pair of road tires costs 80 EUR and lasts 10000 km, you have to pay only 2.9 EUR per saved hour.

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    And if you use solid steel wheel on railroad tracks you will loose almost zero. Those tests are made assuming aperfectly smooth surface, perhaps a pine velodrome. Real roads are not like that. Sep 1, 2020 at 13:14
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    @VladimirF Bicyclerollingresistance is quite aware of that. However, we still have the pure rolling resistance as underlying losses. Juhist already tried to compare wide tyres. Taking suspension loses, in the way Jan Heyne defines them, into account the much less supple gravel tyre will have even more losses.
    – gschenk
    Sep 1, 2020 at 13:35
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    @gschenk I do not beleieve that your final implication is so simple. Jan Heine quotes this: "As tire width increases, tire pressure decreases. So a wider tire performs better in terms of rolling performance." and says "It is understood that to offer good performance, the wider tires must be supple, otherwise, you lose too much energy to flexing the tire casing at it deforms with each wheel revolution." Of course, a gravel tyre will be slower. GCN calculated that a gravel bike will be slower even at Paris-Roubaix. But I do not believe it is so simple, more supple is not more losses. Sep 1, 2020 at 14:37
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    @VladimirF indeed. If the calculation were simple we wouldn't fiercely debated tyre width for a decade. It's good to have measurements for rolling resistance on a flat smooth surface (the drum has some embossed parts afaik). This part of friction always plays a roll on compact surfaces. The measurements also allow to compare compounds. They are not the whole picture, which is too complex and variegated to answer, but still it makes a substantial part: Have pressure and width equal, the tyre with faster compound and supple casing will be faster.
    – gschenk
    Sep 1, 2020 at 14:48
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    @VladimirF technically the surface used for the tests is a rough diamond pattern plate that is intended to simulate asphalt surface. The comment about suppleness refers to the fact that wide tires used to come with thick treads and low density casings, two things that did not make them supple. This is not any more the case for all tires.
    – ojs
    Sep 2, 2020 at 8:11
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I understand that around 20% of the energy while cycling is lost by the deformation of the [tires]

No not really. Rolling resistance will increase linearly with velocity while drag increases with the cube of velocity, so rolling resistance is not a fixed percentage of total power lost. At higher velocity drag will completely dominate.

use plain tires (no tread pattern) as these tires will get more contact area with the ground, therefore needing less deformation in order to equilibrate the forces (with F=P/A)

That's not completely true. The tire carcase is is somewhat stiff and each tread block contacting the ground supports an area greater than its own cross section area above it. Because the forces are concentrated down trough the blocks the area actually in contact with the road is lower but each block exerts a higher pressure against the road surface.

There is additional power loss through greater flexing of the tire and tread blocks though so there is additional power loss compared to a slick tire.

A factor I don't think you have taken into account that gravel tires are typically wider than road tires and run at a lower pressure, which leads to higher rolling resistance.

I can't point to formal research but the Global Cycling Network YouTube channel has some some tests.

They ran with a power meter, on rollers:

  1. 28mm GP 5000, 90 PSI, 45 KPH 299W
  2. 28mm GP 5000, 70 PSI, 45 KPH 327W
  3. 40mm Terra Speed 70 PSI 45 KPH 449W
  4. 40mm Terra Speed 40 PSI 45 KPH 516W

The deltas were significant.

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    Could you add a short summary of the results of the test from the linked video ?
    – Puck
    Sep 1, 2020 at 7:04
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    Two corrections: Rolling resistance increases linearly with speed. Secondly: Good road tires have ~10W losses at 30km/h and 40kg load. To ride at 30km/h you’d usually need something like 150W total power, which would mean 13% losses from the tires. Not too far away from OP’s 20%. Cheaper road tires can easily have twice the resistance.
    – Michael
    Sep 1, 2020 at 7:40
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    To be precise: Rolling resistance force is constant, rolling resistance power is linear. Likewise, drag force is quadratic, and drag power is cubic. So, either you say that rolling resistance is constant and drag is quadratic, or you say that rolling resistance is linear and drag is cubic. Just don't mix these statements. Sep 1, 2020 at 8:31
  • Another correction: the answer assumes that you're riding alone. In a real road race you'll spend at least some time inside peloton or in paceline where air resistance is significantly less, which increases the proportion of rolling resistance.
    – ojs
    Sep 2, 2020 at 9:38
  • Weight distribution across the two tyres is not equal. More weight is taken by the back tyre and therefore it accounts for most of the rolling resistance. Sep 9, 2020 at 20:54

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