Fat Tyres versus Thin Tyres of the Same Type

[Bolt]

I've been experimenting with both 35mm and 50mm Schwalbe Kojaks, running the 35s at around 70psi and the 50s at around 50psi on rides up to 200km.  The most obvious difference between them is the additional comfort with the wider tyres and although I haven't got enough comparable data to know for sure, the fatties do seem to be slightly faster despite their extra weight?  Although I know the theory and that the perceived difference in performance could be possible, my brain is still telling me that narrower and lighter must mean faster   Has anyone else gone down this path before?

[mattc]

Have you done a roll-down test? If not, I'm very sceptical of your "slightly faster" statement (but open-minded!).

There are many variables...

[Mr Larrington]

Testing on a roller showed that the 2.1" Conti Avenue had lower rolling resistance at the same pressure than the 1.3" version, but that takes no account of either revolving mass or aerodynamic drag.

[LittleWheelsandBig]

At the same pressure or different pressures and how rough is the road surface? Both can tip the balance.

[Bolt]

Have you done a roll-down test?

Now that is a good idea and embarrassingly not one I'd thought of   It is simply freewheeling down a hill without braking isn't it?

This will be the test course, hopefully long enough to get some meaningful results over the weekend.
https://www.strava.com/segments/6927887

[LittleWheelsandBig]

Make sure it isn't too steep (otherwise wind resistance dominates), maintain a constant position on the bike and ride the exact same part of the road each time, otherwise there may be differences in surface roughness. Do multiple runs of each configuration with no wind and no traffic. Try to do it all within a short period on the same day to avoid temperature and barometric pressure differences.

[Rampancy]

I've seen a couple of comparisons on the web.

http://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28

First a lab test of Conti GP4000S 23, 25 and 28. The conclusion was that wider had less resistance.

https://janheine.wordpress.com/2012/06/13/bicycle-quarterly-performance-of-tires/

Second was a rolldown test on rumble strips, also found wider was faster.

[Dave_C]

Dave McGraw in Edinburgh has written a few interesting articles on increasing performance:

http://mccraw.co.uk/optimising-for-long-distance-speed/

http://mccraw.co.uk/bike-weight-performance/

http://www.cyclingweekly.com/news/product-news/are-wider-tyres-really-faster-160403

[rogerzilla]

I've often read that 25mm is faster than 23mm (except maybe at TT speeds, when wind resistance is far more significant) but TBH the tyre construction makes a lot more difference.  I get a much, MUCH faster and more pliant ride from the 23mm Vredestein Fortezzas on the clubman than I do with 35mm Marathon Pluses on the Inbred.  M+ are very stiff and tough, although I'd still recommend them heartily for anything involving psyclepaths.

[Samuel D]

Fatter tyres have lower rolling resistance at a given pressure, but matters are complicated by the need to use lower pressures to achieve equal comfort (because the casing tension is higher at the same pressure) or better.

There is also the problem that tyres of the same model but different widths do not always have identical casing and tread construction.

Rolling resistance is not the only thing slowing you down and it’s likely that the effect of aerodynamic drag is underplayed in the current industry-wide fixation on wider tyres.

As a general rule:

• the lighter you are,
• the faster you ride,
• the smoother your roads are,
• the less aerodynamic your rims are,
• and the faster your tyres are,

… the narrower will be your ideal tyre.

So for someone like me, who’s 65 kg and rides mostly on smooth tarmac, often in groups, with practically box-section rims and high-quality, supple tyres, I conclude that tyres over 25 mm are probably counterproductive.

Audax riders know more about long-distance riding than I do but I still think their tyre choices are often irrational. At slow speeds rolling resistance absorbs proportionally more of your power and on long rides the time gains from fast tyres are much larger than time lost to punctures. Besides, fast tyres are usually also more comfortable and that is its own gain.

The main thing is to use fast and comfortable tyres rather than tyres of some specific width, because the net effects of width are marginal among likely candidate widths.

[Brucey]

I agree with much of the above post.

It varies with the bike, the rider and the setup, but aero losses usually dominate  at any speed over about 15mph.  By the time you are doing 20mph they are vastly more important than rolling resistance.

So one way of looking at it is that at some (probably quite low) speed the extra aero drag of wider tyres is liable to outweigh any benefit you might get from a small improvement in rolling resistance on good surfaces. On bad surfaces wider tyres make a lot more sense.

BTW rolldown tests may or may not give a clear indication of rolling resistance; aero drag is different but since wider (but otherwise similar) tyres are also heavier, the wheels have a larger flywheel effect too.

cheers

[hubner]

Testing on a roller showed that the 2.1" Conti Avenue had lower rolling resistance at the same pressure than the 1.3" version, but that takes no account of either revolving mass or aerodynamic drag.

In the real world you don't use the same pressure, if you did the wider tyre would be rock hard and narrow tyre would be too soft, which would account for the wider tyre having less rolling resistance!

At slow speeds rolling resistance absorbs proportionally more of your power and on long rides the time gains from fast tyres are much larger than time lost to punctures. Besides, fast tyres are usually also more comfortable and that is its own gain.

The main thing is to use fast and comfortable tyres rather than tyres of some specific width, because the net effects of width are marginal among likely candidate widths.

Well said. It's ironic that slower riders get the most benefit from fast supple lightweight tyres! The downside is lack of durability and high cost.

[vorsprung]

Audax riders know more about long-distance riding than I do but I still think their tyre choices are often irrational

Seems a bit unfair to single out Audax riders as irrational!  Surely all categories of bike riders are irrational, not just about tyres but all forms of choice

[DrMekon]

I think tyre construction makes a huge difference. I've tried a bunch of paper thin tyres , and the width thing is great but i cant tell the difference between 35mm bon jons and 44mm Snoqualmie Pass. Both are astonishingly thin cased. They feel amazing at 40-50psi, and not in the least bit draggy. In contrast, the very soft rubber of the 32mm Bontrager aw3hcls gave similar grip levels, but felt stodgy at lower pressures.

I'm relatively light at 70kg, but the North Yorkshire roads I ride on are shocking. I think the locals must have around at night on Landrovers blasting shotguns in to the road. Moving up from 26mm evo paves, 28mm gp4000iis to super supple wide rubber has been revelatory.

I'm tempted to try some other Compass tyres in 32s, to see where the sweet spot is for me.

As for aero gains, getting low / using tri bars seems like an easier option for me than going back to 90psi skinny tyres.

[Bolt]

In the real world you don't use the same pressure, if you did the wider tyre would be rock hard and narrow tyre would be too soft, which would account for the wider tyre having less rolling resistance!

Not so true in my planned experiment with 35mm and 50mm width tyres, I've found that the 35s run best for me at 70psi, any higher and the ride becomes too harsh.  With the 50s I started at around 45psi but found that taking them up to 70psi gives a faster but still plush ride. With an average speed of just over 20-25kph I'm not a fast rider, I'm also no lightweight at 85kgs and have a relatively relaxed position in the saddle, so perhaps I'm well placed to take advantage of the benefit of wider tyres.

[Brucey]

re the tyre pressure/comfort thing; I think it depends what shape bumps you are going over.   If you are (say) riding over small protrusions/stones that lie on top of an otherwise smooth firm surface, the result is that each object pushes into the tyre locally; the way this feels varies mostly with tyre pressure and less with tyre width.

Other types of bump cause the tyre to deflect differently and then the way the tyre feels will vary with both pressure and tyre width.

BTW part of the comfort story is to do with the natural frequencies of the frameset (especially the fork), handlebar, and tyre. If the tyre allows vibration frequencies to pass  through that the other parts of the bike cannot absorb, the result is discomfort.

Not all frames and forks work the same way and not all riders sit on the bike the same way thus 'the tail wags the dog differently'.

 However if you feel your fillings being shaken loose on a regular basis, this is very fatiguing for the rider and it will also be hard work (high effective Crr).

cheers

[Jakob W]

(Slightly OT:) Is there much (anything?) good in the public domain about the aerodynamics of bicycle wheels? A lot of the forum warrior assertions about how 'aero' particular wheels and bikes are rely on assumptions about simple steady-state flow. At best the airflow round a bicycle wheel will be no less complex than round a helicopter rotor; add in the effects of the ground plane, boundary layer, frame etc, and I'd not trust any simple explanation without a great deal of empirical validation (and that ideally in a tunnel rather than just CFD).

[hubner]

I'm sure a new separate thread on the aerodynamics of bicycle wheels would be of interest!

[vorsprung]

I'm sure a new separate thread on the aerodynamics of bicycle wheels would be of interest!

or not

I doubt that there's a dozen people on yacf who would really benefit from aero marginal gains, except on the ubiquitous 10 mile time trial

[mzjo]

I think that mudguards, particularly over wide tyres, could have a considerable effect on the aerodynamics. As we all know many use mudguards in real life (apart from when racing of course).

[Morat]

Very close fitting mudguards can be an aero bonus, until you end up on your face.

[rogerzilla]

A £40 disc cover on the rear makes a noticeable difference to drag - I had one - but was not legal for time trialling because it was non-structural and therefore a "fairing".

[zigzag]

^^ rear wheel fairing saves about half a minute per hour at tt speeds, barely noticeable while riding, but if the tt is long it's a worthwhile upgrade. tight fitting clothes and no bags on a bike would be a better place to start.

[fd3]

I think tyre construction makes a huge difference. I've tried a bunch of paper thin tyres , and the width thing is great but i cant tell the difference between 35mm bon jons and 44mm Snoqualmie Pass. Both are astonishingly thin cased. They feel amazing at 40-50psi, and not in the least bit draggy. In contrast, the very soft rubber of the 32mm Bontrager aw3hcls gave similar grip levels, but felt stodgy at lower pressures.
...

This.

I tend to find that my 35mm kojaks are as comfortable as 23mm ultremos on same wheels/bike.  Being same mass as drM I wonder whether some effects are felt stronger by different sized riders.  E.g. Different size tyres and psi makes not that much difference to me, but drop the handlebars 2cm...