Tyre Size

NWLondoner

I currently use 700 x 38 schwalbe marathon plus.

I have NO desire to go right down to 25 as they scare the s**t out of me, would i notice a difference by going down to say 30 or at a push 28??

gundersen

And what is scary about a tyre?

A bike won't fall over even if it is a 700X18.

Garybee

Friction is independant of contact area. What this means for bike tyres is that you can get the same grip from a narrow tyre as you can a wide one, it's just not as comfortable. For practical proof of this you just need to look at the downhill road championships, won on a road bike with skinny tyres. They're not confidence inspiring but they do work.

To answer the original question, I believe you would notice a difference, yes.

topdude

If you want your bike to feel faster / lighter / more responsive then you need to make a fairly big jump down in tyre size. I expect 700 x 28 will be the smallest you could safely fit on your rims anyway if you currently have 700 x 38. Try panaracer pascela, they are a good compromise between hybrid tyres and race tyres and tough as old boots.

Steve I

"Friction is independant of contact area"

I don't quite know what you mean by that statement Gary, but I think it's misleading. The greater the tyre contact area the greater the friction. Wider tyres grip the road better, that's a fact. Wider tyres can be run at lower pressures giving more comfort, they can also run at higher pressures and potentially have less rolling resistance than narrower tyres run at the same pressure. Disadvantages of wider tyres are; heavier, therefore slower to accelerate, they can't be run at the very high pressures narrower tyres can and hence have higher rolling resistance.

Steve I

Gary, I apologise for questioning you, I was a bit hasty. After reading around a bit, it seems the whole area of tyre grip is quite a complcated one and I might have been talking boll0x. It seems thinner tyres potentially have greater grip due to the higher contact pressure over a smaller area versus a wider tyre. The whole issue is further complicated by different types of friction eg static and rolling, it's all way above my head .

Garybee

Steve, I understand your initial scepticism, it runs against what would appear to be logical thinking. What you find is when you reduce the contact area the extra pressure generated between two surfaces means that the friction returns to exactly what it was. As you say, it is complicated with rolling friction though. Another thing to consider is that if you run wider tyres you can use a softer compound to achieve the same wear rate, so in that respect wider tyres would help. With bicycle tyres though it is better (in my opinion) to have a softer compound on the narrow tyre and just accept the poor wear rate.

whyamihere

Gary's right. The equation for friction is F=uR, where u is the coefficient of friction (a constant value between two materials) and R is the reaction force (essentially the vertical component of the weight). There is nothing about surface area in the equation, because the surface area doesn't matter. Not that easy to get your head around, but it's true.

aracer

Oh dear. You're all wrong (though Steve was right first time round). That equation is only valid for solid hard surfaces. Rubber behaves in a totally different way, and more surface area does equal more friction. Why do you think racing cars use such wide tyres?

Garybee

aracer wrote:

Oh dear. You're all wrong (though Steve was right first time round). That equation is only valid for solid hard surfaces. Rubber behaves in a totally different way, and more surface area does equal more friction. Why do you think racing cars use such wide tyres?

Get back in your box. Racing cars run wider tyres to allow them to use a softer compound. If you're going to display that level of arrogance, make sure you're right.

aracer

Maybe you should try following your own advice, Gary. Have a look at the at the top of page 169 in http://books.google.com/books?id=aMAt8w ... PPA1168,M1
"In practice rubbers do not normally obey these rules and the coefficient is a 'variable constant', it's value depending on the real contact area..."

Hudster

Contact area depends on tyre pressure, not tyre size. It's all very complicated and therefore a proper explanation is never going to happen on a forum.

"Race" cars (of which there are many types - all running different sized tyres) run all sorts of sizes of tyres depending on conditions and rules of the competition and are very different vehicles to bikes. Rally cars on the snow run very narrow tyres, for example. And remember those formula 1 cars with two rows of narrow tyres at the front?

Anyway, back to the original question - there is no reason to be scared of 23 tyres.

aracer

Hudster wrote:

Contact area depends on tyre pressure, not tyre size.

That is indeed true, but in general wider tyres are run at a lower pressure, keping a constant length of footprint.

Rally cars on the snow run very narrow tyres, for example.

Indeed they do - for similar reasons for traction in mud, mountain bikes often use narrow spiky tyres, which have very little contact area on a hard surface, but we were talking about road cars and bikes where you are putting rubber on tarmac.

Anyway, back to the original question - there is no reason to be scared of 23 tyres.

Indeed not - no particular reason to be scared of 20s or 18s - I've cornered pretty fast on those. But I doubt there's much advantage to the OP either in going that narrow - I have 28s on my training bike at the moment for extra comfort, and they're not exactly slow.

NWLondoner

Thanks for all you replies.

Totally confused the hell out of me.

When my current tyres need replacing i'll try out some 28c's.

Cheers