ergott wrote:youngs_modulus wrote:So yeah, mountain bikes don't have more stopping power because their tires are bigger. Once your brake is strong enough to lift the rear wheel--on road or off--max stopping power is dictated by where your center of mass is. Low and far behind the front wheel is what you want here...this is why tandems can stop faster than single bikes.
I'm not saying that you meant this, but it's worth pointing out.
ergott wrote:The difference between wider tires on a car and wider tires on a bike is typically bike tires have increased volume as well. Wider tires on performance cars tend to have lower profiles so the volume is similar or even less (not the case with Formula 1 tires). Resulting tire pressure is about the same, 35-40psi for most road cars.
I point that out because as a result, wider bicycle tires are run at a lower pressure indeed resulting in a larger contact with the ground.
That's right; also, car tires exhibit Hertzian contact between a cylinder (the tire) and a plane (the road). Bike tires have curvature about two axes, so their Hertzian contact is closer to that of a sphere on a plane. The effects of changing tire width are therefore more highly nonlinear on a bike than a car.
ergott wrote:I run my disc road bike at 35psi (650X42) so I have more than double the rubber the typical 700cX23-25mm tire setup has. That results in more grip for braking, cornering or whatever combination of the two. That's why I prefer that setup this time of year when grip is generally lower due to lower temps, wet, and increased debris. It's not so much slower that I can't keep up on the group rides.
Well, it's not that simple. In fact, tire dynamics is a deeply complex subject that swallows engineering careers whole. The short answer is that your 650x42B tires give you more grip than your skinny tires, but that's due to their lower pressure, not their increased contact patch. On roads with debris or big bumps, fat, low-pressure tires conform to those things in a way narrow, high-pressure tires can't. On a smooth, clean road, there's not much difference in absolute grip between the same tire in two different widths.
It's also worth remembering that rubber tread compounds are profoundly sensitive to temperature...below about 40 degrees F (5 degrees C) most of us would do better on a tire designed for cold temperatures than on our usual training or racing tires.
The myth that tire grip is simply proportional to contact patch area is as persistent as a B-movie zombie, and about as clever. I'm not making fun of people who believe it--there are a lot of good reasons to think it's true. But there are a lot of good reasons to think that the sun orbits the earth, and yet it isn't so.
Tires are largely made of elastomers, so the relationship between contact patch area and grip is complicated and subtle. Contact patch area does play a role, but it's extremely complicated...if you know the RMS roughness of the road surface and the size of the asperities relevant to your particular tire, you're part of the way to a good start. Tire grip can also be deeply counterintuitive: Michelin put a tread pattern on its Pro4Grip not to pump water away but to decrease contact area and therefore increase contact pressure. All things considered, it's less wrong to say that tire grip is independent of surface area than it is to say it's proportional to surface area.
Brake pad materials are much stiffer (and profoundly more linear) than tires, and so there's less hand-waving required: pad surface area and braking power don't have much to do with each other.
Here's a paper with an overview of some of the factors that make one tire "stickier" than another: https://deepblue.lib.umich.edu/bitstrea ... sequence=2
And here's another, more technical one: https://hal.inria.fr/file/index/docid/5 ... /vsd05.pdf
youngs_modulus wrote:Well, it's not that simple. In fact, tire dynamics is a deeply complex subject that swallows engineering careers whole. The short answer is that your 650x42B tires give you more grip than your skinny tires, but that's due to their lower pressure, not their increased contact patch. On roads with debris or big bumps, fat, low-pressure tires conform to those things in a way narrow, high-pressure tires can't. On a smooth, clean road, there's not much difference in absolute grip between the same tire in two different widths.
I know what you mean about the tire conforming more (also subject to rubber compound). I also believe lower pressure setups are less prone to cuts and punctures in general due to this so they end up being more reliable. Compound that with sealant more likely to work at lower pressure and you have a much better setup for any riding short of racing. More comfortable too.
Oh and by the way, my rims are 335g and my tires are 355g (another 20g of sealant). That combination makes them competitive with most alloy 700c setups (tubeless or clincher) that people ride everyday so I'm still keeping the WW dream alive;-)
jeanjacques wrote:Is it the same with the braking area of the disc ?
I will leave the answer to the physicists, but did want to comment that from my CX experience, when it is wet out, highly vented rotors have a tendency to trap bits of debris and silt which in turn acts as an abrasive. I observed a brand new set of organic pads worn nearly all the way down to the pad holders in the span 60-minutes during of a wet cyclocross race, and a set of semi-metallic pads equally worn after only 4-days of bike touring in wet weather using Ashima super light/heavily vented rotors.
I think what you’re rieally trying to ask is “will both rotors generate the same braking force for a given lever squeeze?”
The answer is yes. There will be some variation at first as the pad wears to match the rotor. Also, different stainless steels will have slightly different coefficients of friction with a given brake pad. (I’m pretty sure Ashima uses 416 stainless, but I don’t know what other companies use).
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