This whole thread has been interesting to read, but when will we discuss tyres ??

Mike, weren't we there already, or was that a different thread. In any case it's a tyred olde joke

You're asking me ? I'm fortunate to be able to find my way home, most days......and it's not from rolling resistance.

Try lowering your tie pressure and raising your blood pressure a bit. Raise it until you have less than 15% tired droop.

Since this discussion was pretty recent, I thought I'd throw this in here.

Discuss.

http://velonews.competitor.com/2012/...5c-tire_218011

"...27mm-wide Bontrager Aeolus 5 D3 rims...":eek:

There's a lot of repetition in this thread, but even after two and a half pages, nobody's made this point:

You don't have to run the same size front and rear. Like, hello? The rear typically carries 60% or so of the weight, guys.

Also not getting enough play: The weight of the tyre is a factor. IMO this is a bigger deal than rolling resistance or aerodynamics by a long shot, particularly concerning front braking.

And never mind swapping out your frame for a comfier ride, how about a carbon seatpost? I have a fat ally bike that would buzz every little bump into my bum, except my seatpost makes it feel like my rear tyre's half flat. The more slope on your top tube, the stronger the effect.

Multiple gears, chainrings under 42T, lighter frames, armrests on the bars, and now squishier tires... racers just keep getting wussier, huh? ;)

That's going to really piss off Carbonfiberboy.

I've seriously considered it, but for one thing: It would negate the ability to always put the new tire on the front when replacing a worn out tire. I don't replace a tire until I see cords, and I'm not going to run a front tire down to that point.

Mixed tires make sense and I often ran them in the past. The only reason not to is a practical consideration. I run tubulars on the road bike, so need to carry a spare. My SOP has always been that once a tire is mounted, it's there for keeps (bad karma to remove a good tire).

These days, my commuter is 26' clinchers, and I could run mixed tires, and sometimes have, but here again, that would mean being ready with 2 replacements at any given time. So now both tires are the width I'd want for the rear. IMO it's better to have a front that's wider than necessary, than a rear that's too narrow.

Au contraire. If you're running aero carbon 24mm tubular rims, 25c tubulars make a lot of sense. Zipp's been recommending special 24c tires for their clincher rims for quite a while. I run 25c Ultremos right now - on my tandem. I don't see any data showing an advantage for 25c over 23c in rolling resistance on a 19mm clincher rim, again and again, unless they are foolishly inflated to the same pressure, which no one does.

I believe the advantage being sought with these thick rims is a better airfoil section. Zipp's thick rims actually provide thrust rather than drag under certain cross wind conditions. Isn't that a trip?

Apparently you do, if you inflate to the maximum sidewall pressure. I have plenty of tires that have the same maximum pressure written on the side for 23, 25, or even 28mm. So which is it?

Can you expand on this? Braking, in particular, is about bring the total weight (bike + rider) to a stop. How does weight of a tire effect this?

Cheers,
Charles

What about tires like the RiBMo? That have a smaller contact area per width?

The mass of a tire wheel system introduces rotational forces to your bikes movement. Without getting into a physics sort of lecture, your bike is effectively riding on two gyroscopes. If you have held a spinning gyroscope you have experienced the effects of a rotational force. The larger the mass under rotation, the larger the rotational force. This is ok if you do not want a object to change velocities or directions quickly (the heavier the gyroscope, the greater forces needed to change its direction). IF you want a bike to be responsive to your input, you need to try and limit the rotational forces acting upon the bike, or you need to apply a greater force to overcome the rotational forces ( increased pedal,brake,handle bar forces).

All this is true, but the steering affects of precession are very small compared to the rest of the forces acting on a bike, (though they are significant for motorcycles). However precession doesn't affect braking, because the axis of rotation isn't changed.

I currently ride on RiBMo 700x32 tires, they do indeed have a sort of pointed shape, that being a narrowing of the profile that approaches the contact area. I have only had the tires for a few hundred miles. Before the RiBMo, I used Vittoria Randonneur tires which I would overinflate on the rear. I believe that the pointed shape of the RiBMo will introduce more tire distortion as opposed to a traditional shaped tire.

This sounds correct. The tire must distort until the contact patch is the right area, regardless of the shape of the tire. A 'pointy' tire, then will possibly deform until it has a longer and narrower contact patch than other tires, but since this will still likely be more distortion, there will be more rolling resistance. I doubt it would be a significant difference though.

I understand the physics of rotation. I don't understand why you think it affects braking. The mass of the tire is so small compared to the bicycle that this is really just a non-effect (as chisler) points out.

As an interesting aside, I've ridden a bicycle that had no angular momentum - both tires had another tire on top spinning the other way. The bicycle was harder to balance, but not because of the lack of angular momentum (you could lift both upper wheels so they stopped turning).

A quick crude work up of the work required to spin up a 300 gram tire vs a 700 gram tire to a speed of 20km/hr provides results of about 0.25J and 0.5J respectively.

And compared with moving a 50 kg (110 lbs) man + bike, the same speed, 0.5 m v^2 = 0.5 * 50kg * *5.5 m/s)**2 = 771 joules. I didn't check your math (edit: or for that matter my own before posting :D), but as I've said before, it is the getting the whole bike to speed that matters. Aside from angular momentum arguments, I just don't see how tire masses matter in the big picture (aside from how they add to the total mass).

p.s. Note to self:

20 kph = 20,000 m / hour = 20,000 m / 3,600 s = 5.5555 m/s

your math is correct, I calc. my wishfull mass out (90kg) and I'm at about 1350J. The math does confirm that the tire mass is rather moot, especially considering a rider as myself.

I think this may be the first witnessed admission that someone might be wrong on the internet. I feel like I should contact Al Gore or something.

I'm pretty sure higher pressure in a smaller tire gives lower rolling resistance on a perfect surface. Let me know if you ever find such a surface on any road rides. To me it's about comfort and not unduly penalizing myself on rolling resistance.

After one terrible 3 hour stretch on 23's at 110psi rolling over about ten thousand expansion joints, I swore off tiny, high pressure tires forever. Never again. Ever.