Tyre size and rolling resistance

geoff_ss

Not sure if this is the right place for this but there doesn't seem to be a general on-topic section (as opposed to the Cake Stop, which seems to be an off-topic section)

My wife and I have just bought a Cannondale touring tandem which sports 26" wheels with massive 2" Schwalbe semi-slick tyres which must weigh a ton. We're experienced tandem riders though a little out of practice and our old tandem is a handbuilt 531 with 700c wheels with 32mm tyres usually (though we did race it with 18mm tyres).

We've been out a few times with other tandems and on every occasion we descended (freewheeling) faster than any of them despite the fact they had what were ostensibly 'faster' ie narrower tyres. It goes without saying we out-paced any single bicycles (we paid for it on the climbs!).

I wonder if the heavier tyres actually help when descending with more inertia (?) once the bike gets rolling. Any thoughts? Crew weights were broadly similar btw. Perhaps it could be that because our bike is new with only a few hundred miles on the clock the bearings are smoother?

Geoff

andrewgturnbull

Hi there.

Given two identically manufactured tyres, inflated to the same pressure, the wider tyre will be faster.

I think this is due to the contact patch which is deformed as the tyre rolls being wider rather than longer with a wider tyre. The energy taken to deform the tyre across the way is apparently less than the energy taken to deform a longer section of tyre.

Or something like that. Narrow tyres are only going to be faster at higher speeds where ayrodynamics come into play.

Cheers, Andy

Buckled_Rims

But fatter tyres have both rolling resistance and aerodynamics against them. The differences mentioned could be a whole factor more...gearing, weight, experience, stiffness of frame, forks...etc.

keef66

"Given two identically manufactured tyres, inflated to the same pressure, the wider tyre will be faster."

But in practice you wouldn't inflate the wider tyre to the same pressure. The main benefit of wider tyres is being able to use lower pressures for reasons of comfort.

My guess would be the weight (of the tyres / wheels) and / or aerodynamics (of you and Mrs Geoff)

geoff_ss

Buckled_Rims wrote:

But fatter tyres have both rolling resistance and aerodynamics against them. The differences mentioned could be a whole factor more...gearing, weight, experience, stiffness of frame, forks...etc.

The tandem we were out with yesterday was also a Cannondale and the crew is, if anything, more experienced than us. They still hold the mixed tandem 25 club record. And I'm talking about freewheeling so gearing is irrelevant. We have our tyres at 80 lbs/squinch which 10 over the number on the side wall

I just find it curious that our down hill speed with these enormous tyres is counter intuitive to the normal convention that light, narrow tyres are faster.

Geoff

andrewgturnbull

Geoff_SS wrote:

I just find it curious that our down hill speed with these enormous tyres is counter intuitive to the normal convention that light, narrow tyres are faster.

Geoff

Hi Geoff - what I'm trying to tell you is that this is not the convention!

Cheers, Andy

geoff_ss

Yes, I see that Andrew but I'm talking about quite high speeds - 30 to 40 mph is quite common on a decent tandem descent. We've clocked well over 50 at times before faint heartedness and old age (from the back seat sometimes) means I start applying the brakes.

I intend leaving these tyres on for the Winter but I've got some 1.5" slicks in stock I was intending to fit next year. So you're saying the bike will be faster on the fat tyres and I'll be wasting my time?

Geoff

Nuggs

Geoff_SS wrote:

30 to 40 mph is quite common on a decent tandem descent. We've clocked well over 50 at times

:shock: Bloody hell, I need a new chamois at around 40mph on my own - chapeau for 50 on a tandem!

andrewgturnbull

Geoff_SS wrote:

Yes, I see that Andrew but I'm talking about quite high speeds - 30 to 40 mph is quite common on a decent tandem descent. We've clocked well over 50 at times before faint heartedness and old age (from the back seat sometimes) means I start applying the brakes.

I intend leaving these tyres on for the Winter but I've got some 1.5" slicks in stock I was intending to fit next year. So you're saying the bike will be faster on the fat tyres and I'll be wasting my time?

Geoff

Hi geoff.

It depends...

When this argument is usually trotted out, it is more usually the distinction between 19mm and 23mm road tyres we're talking about.

I guess on a tandem you've got twice the weight to put through the tyre so you need a bigger volume of air in there - hence the wider tyres. Twice the weight does however imply twice the force down through the tyre and hence greater deformation. So maybe rolling resistance is _more_ important on a tandem?

The problem with really wide tyres is that they're not usually designed to be 'fast' i.e. the casings aren't going to be as supple as a fast tyre. If your current wide tyres are the same design as the 1.5" ones then stick with them.

Slicks will always be faster on the road - any tread squirming around underneath you as the tyre roll will cost you energy.

Cheers, Andy

geoff_ss

Nuggs wrote:

Geoff_SS wrote:

30 to 40 mph is quite common on a decent tandem descent. We've clocked well over 50 at times

:shock: Bloody hell, I need a new chamois at around 40mph on my own - chapeau for 50 on a tandem!

The Cannondale is very stable at speed on a descent - much more so than my Kinesis single (or any of my 531/653 steel bikes for that matter). Also the hydraulic front rim brake coupled with a mechanical Avid disc at the back offers excellent braking. I think the wide tyres tend to give some confidence as well. Having said that, the 50 mph was reached on a 531 handbuilt tandem when we were young and foolish

Now I'm on threshold of my 7th decade I think caution may well trump high speed thrills in future

Geoff

desweller

As you know, in general there are three distinct scenarios: ascents, descents and flat riding.

How fast you can go (discounting acceleration) can be crudely summarised as follows:

On ascents the most important ratio is power/weight.

On descents the most important ratio is weight/drag.

On the flat the most important ratio is power/drag.

(where the 'drag' term is a combination of aerodynamic loads, bearing friction and rolling resistance in the tyres).

On fast descents, the aero load far outweighs tyre rolling resistance (as the power required to overcome aero loads increases as a cube function of speed, whereas the tyre rolling resistance is more or less constant). So you're either somehow vastly more aero efficient than other tandem riders, or you're just heavier. Tyre rolling resistance is probably not a primary factor.