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One of the runs I do on my roadbike is a very straight, flat, very smooth strip of road. Everything I read recommends wider tyres because of less rolling resistance.

Where does tyre weight come in? How much do the aerodynamics make a difference? Because the road is so smooth does that make a difference?

I've recently been trying some (admittedly not great) 28's in place of my 23s and they have seemed quite a bit slower on this road.

I'm getting up to speeds of between 30 and 50kph (if the tail wind is right..) to give you an idea of wind resistance..

Perhaps it's all in my mind?

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    The tread may be as important as the width. Smoother / slicker tyres will roll better than ones with more raised / grippy pattern. You can also decrease the rolling resistance by increasing the pressure, so the tyre deforms less. Higher pressure smooth tread 23mm may be better than lower pressure grippier 28mm. – KeithWM Nov 2 '16 at 11:39
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    I heard something about that thicker tires are more aerodynamic so pros started moving to them. In gcn (you tube channel ) they mention that several times also there are some tests. I guess that it affect acceleration but heavier tyre should also have more momentum so it will keep speed longer and make it actually easier maintain speed – kifli Nov 2 '16 at 11:41
  • Yeah, there are a LOT of factors, which is why I ask. I'm not very happy about this blanket wider = faster thing as I don't think it applies directly to the real world in all situations (namely the one I'm asking about). The aerodynamics are affected by the rims too.. I'm currently considering getting some 25's as a compromise as I'm not always on this stretch of road (though when I am, I need to go as fast as I can!) – John Hunt Nov 2 '16 at 11:44
  • Basically, the more tread you have, the more energy is lost in deforming the rubber as you roll, plus there is more frictional loss in the tire-road junction as the rubber "squirms" slightly. Other than that, tire pressure is the main factor controlling rolling resistance, because the lower the pressure the more the above factors become significant. In terms of width, moderation is probably advised. Except for very heavy tires, tire weight is a negligible issue. (You're probably running a lower pressure in the wider tires, which upsets the comparison.) – Daniel R Hicks Nov 2 '16 at 11:51
  • Here's one good dive into the subject: velonews.competitor.com/2014/12/bikes-and-tech/… and another: competitivecyclist.com/learn/25-vs-23 – Carel Nov 2 '16 at 12:59
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The key problem in this kind of analysis is holding all of the possible variables equal while you change only one factor. Weight, construction materials, tread pattern and inflation pressure are all at least as important if not more important that tire width when it comes to rolling resistance. That said, VeloNews has an interesting article where they tested various tires at different pressures.

From Resistance is futile: How tire pressure and width affect rolling resistance

Tire rolling resistance on the road is caused by (1) internal friction and hysteresis (deformation over time) within the tire’s materials, and (2) on rough roads, small bumps lifting the bike and rider slightly (uphill) on each little impact.

AND

If a wider tire is made of the same materials in the same thickness as a narrower one, it will roll faster, because (1) the internal friction and hysteresis within the tire’s materials will be lower, and (2) because the surface imperfections in the road will be absorbed into the tire more easily (since it has more deflection available), thus lifting the bike and rider slightly less with each little impact.

The key in these tests was that they compared the tires at different widths but at the same pressure. What you are experiencing is likely that you are running the 28mm tires at a lower pressure than the 23mm ones. On a smooth road, minimizing the area of the contact patch becomes more important to rolling resistance, so the narrower tire at the high end of pressure will be fastest. That's why you will see velodrome riders using specialized tires that have a max psi of 185 or more.

On rough surfaces, however, a tire at lower pressure is able to absorb more of the bumps than a tire at higher pressure, with less deflection of the bike and its rider. This is the same “sprung vs. un-sprung weight” argument that demonstrates why suspension makes a bicycle faster on rough terrain — it takes less energy to keep the bike rolling if only a small amount of weight is lifted (like a small section of the tire) than if the entire bike and rider is lifted by the bump.

In addition, it's unlikely that your 28mm tires weigh the same as your 23mm tires, so the additional weight is going to increase your resistance. In all, it looks like if you are trying to minimize your rolling resistance on this smooth section of road, experiment with the narrower tires at different pressures. Just be aware that wind resistance is far more important at any significant speed. Wearing a slightly flappy shirt may make a bigger difference than the 5-10psi change, or the 5mm of extra rubber.

  • Excellent. This is sort of what I was thinking. This is just one of the optimisations I'm planning on making, all spurred on by having a quick go on a TT bike on Saturday and being blown away and upset by the fact I can't afford to buy one! It sounds like the optimal thing to do is have different tyres for different scenarios (if I can be bothered to change them often..) or just go 25mm as a catch all. – John Hunt Nov 2 '16 at 13:36
  • If you are trying to approach the performance you had on the TT bike you may want to look at some after market aero bars to make your position more similar to that of a dedicated TT bike. That will make a much bigger difference than tires for likely less money – Gary.Ray Nov 2 '16 at 13:42
  • With all these comparisons, it's worth highlighting every if. Quite often these conditions aren't actually true, but the consumer rarely has the information to know for certain – Chris H Nov 2 '16 at 14:50
  • I have got clamp on aerobars. What I will do though is lower my stem, I realised that my position on the bike, while comfortable is certainly not conducive to an aerodynamic position. I think armed with this information and a few other changes I can make a noticable improvement in performance without breaking the bank. Thank you all who commented and to Gary for the excellent answer. – John Hunt Nov 7 '16 at 14:37
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I would say that for a full analysis, you need to consider:

  1. Weight. Thicker tires weigh more.
  2. Air resistance. Thicker tires have more of it.
  3. Rolling resistance. Thicker tires have less rolling resistance, but then again this may be offset by the increased air resistance and weight.
  4. Ability to get the tire to the operating pressure with a pump you can reasonably carry with you.

For me, (4) is the dealbreaker for 23mm and 25mm tires. By choosing 28mm tires, I can run them at lower pressures, and therefore, have an easier time inflating the tires with the pump I carry.

  • You know, you could just get a carry a decent pump. – ojs Jul 29 '17 at 20:44
  • Yeah, I use a co2 canister for > 100psi in 25c's when out and about. Honestly, I've never looked back in 5 years. You still need a light manual pump as a backup but never needed it. – John Hunt Jul 31 '17 at 12:11

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