I’m sure we’ve all read that 23c tyres are less efficient than 25c, but why is this? What is the science behind it?

My son asked me yesterday how this could be, and frankly, it makes no sense to me. I mean, instinct suggests that the less rubber in contact with the road, the less the drag.

Well have a read: http://www.competitivecyclist.com/learn/25-vs-23

Tyre deformation is more important than friction “drag”. Wider tyres deform less.
Wider tyres are also more aero (in relation to the whole tyre/wheel/bike/rider).

Hysteresis innit.

It’s not that funny.

Bear in mind they mean equivalent tyres. So no use comparing a half kilo of 28mm Marathon using a 200 gram tube with a ~200 gram 23mm Veloflex running on latex tubes…. (which i have seen done)

25c tyres are more efficient because they wont fit in your frame – meaning you will need to buy a new one

i really struggle with understanding rolling resistance, which given that I’m a physicist is upsetting. On road it makes a lot more sense but thinking about it offroad is a bit of a head scratcher.

e.g I was thinking the other day whether on muddy courses would mud tyres actually have lower rolling resistance than all-round tyres in the soft dirt/mud as there are fewer knobs to sink in/break free? In other words when the ground is soft enough to allow the tyre knobs to sink in, the total contact patch is less for a beaver/mud x than a ron or similar

I also wonder if its slightly misleading to say you’ll go faster, I imagine more realistically you’ll go the same speed (same gears and roughly same cadence) but will expend more effort doing so and thus fatigue quicker? or is the percentage difference in resistance so much that you would end up in a different gear?

Also on an mtb, I guess it only makes a difference on certain parts of the course (fire-roads and climbs that aren’t super technical)

have a read of some of Jan Heine’s articles on the subject, there are also various (mostly German?) tests that have been published over the years. The only caveat I’ll give you is that ALL of the published material is lacking in some ways. Jans stuff is very well thoguht out and tries to focus on real-world testing, but leaves his results open to interpretation and difficulty in repeatability and some questions over variable control. The Germans have mostly resorted to drum testing which although very controlled and repeatable, is lacking in the real world aspect and doesn’t take the human on top into account at all.

But in a nutshell, the friction/drag of the contact patch is only one aspect of it, carcass (and tread) deformation, bump absorption and vibration all play a big part. This is why the casing is just as if not more important in some cases as the rubber compound and tyre size.

The circumstances and specifics of the tyres dictates the balance between them but ‘rolling resistance’ is a result of the entire systems and it’s interactions, not just how much rubber is in contact.

This is also why the qualifier ‘when compared like-for-like to a tyre of similar construction and pressure adjusted to suit’ should always be included when saying wide tyres are faster.

ie: a 35mm Marathon Plus is NOT faster than a 25mm GP4s, the two tyres are vastly different in construction and design. But a 25mm GP4s is likely faster than a 23mm GP4s.

Also, remember rolling resistance =/= speed, they are related but you also have to take into account the human on top, and the impact on shaking and bouncing that human around over long distances which mean that ‘speed’ is also not simply about how fast the tyres are, especially once distance and time on the bike come into play, same argument with speed vs comfort, ie: a TT bike and aggressive aero position is ‘faster’ but if you can only maintain it for a few hours at a time before you’re crippled and need to stop/rest then it may not be ‘faster’ over the long haul.

Real world experience…….. Just taken off 32 Marathon Plus tyres and put on 28 Durano.

The D’s are less than half the weight of the MP’s
This resulted in an instant benefit when cranking the pedals, it picks up quicker, it climbs better and handles better, it’s lighter easier to point and shoot, squirt through the rougher road sections.

The MP’s are heavier, slower BUT descend quicker. On a steep descent from a rolling start on a hill I use I roll down and get further up the climb from the dip with the bigger tyres the duranos roll in but loose energy roughly 5 to 10 metres up the steep incline coming out.

Probably why those compass tyres are selling so well. Massive air volume, very high quality carcass and tread, very light (considering the only alternatives were generally tyres that wouldn’t look out of place on a 99 quid hybrid!) so you can get veloflex type performance in a 32 mm tyre……

Not sure who makes them (have heard two or three different names mentioned) but they are all premium manufacturers.

All other things being equal a wider tyre will have a shorter contact patch with the road.

This means the tyre is “rounder” so rolls better.

That’s the simple explanation

Real world variables like weight, pressure and tyre contruction mean its not always as clear cut though

Probably why those compass tyres are selling so well……Not sure who makes them (have heard two or three different names mentioned) but they are all premium manufacturers.

Pretty sure they’re all made by Panaracer based on previous writings from Mr H, not using standard casings, but built to spec for them.

Ok, i’d heard veloflex and vittoria/challenge. But you know what internet gossip is like.

Well I might be wrong, (I’ve been wrong before!) but all the chatter in the articles seems to refer to Japan and Panaracer

Doesn’t really matter, they are v.good tyres. Who ever makes them!

As well as the lower rolling resistance, you also benefit from 20% larger volume (25 vs 23) which means you can run lower pressure (less harshness), reduced chance of pinch punctures and the more rounded tyre profile provides better, more predictable handling IME. Better still if you run tubeless as there’s significant friction / hysteresis losses between the inner tube and tyre carcass.

For offroad tyres, you also need to factor in the energy losses from skinny tyres physically displacing the soft surface and the extra drag it creates – comparing the tracks between a regular MTB tyre and a fatbike tyre over a soft surface it’s pretty obvious which one takes more effort. Obviously, taking account of relative tyre weight evens it out.