I was riding my motorcycle through the Angeles Forest Highway, in both directions, and there are a lot of curves with steep elevation changes. Psychologically I was not able to go as fast around the same curve going down hill then I did when I was going uphill. It felt, to me, that I had less traction going down hill. Is there anything to this or is my slight acrophobia coming back to haunt me?
going downhill your rear doesn’t have as much contact as uphill.
There is also a psychological element. If something goes wrong when you are going downhill, it can get out of hand really quickly. If you are going uphill and something goes wrong, you slow down quickly rather than speed up.
The center of gravity always shifts downhill, so to speak, so the lower wheel(s) always have more load on them, thus more traction than normal. Going uphill, the traction in the rear increases, and vice versa.
For a motorcycle, I guess it’s correct that it has less overall traction going downhill because most of the load is on the skinny tire in the front, but in a vehicle, overall traction doesn’t change much, it just shifts (well, generally speaking, I don’t know if the math is 100% equal).
This is important when offroading in a vehicle. When descending a steep incline, most of the weight is on the front while the rear is pretty light. The front has great traction so if you hit the brakes, it can often slow faster than the rear and they will try to change positions. Going uphill, the weight shifts to the rear enough that it’s common for the front wheels to raise off the ground.
“All other things being equal,” I think overall traction stays about the same, but downhill on a motorcycle isn’t equal. It means it’s relying on the skinny tire more than the big fat one for traction and it’s not going to have as much as the rear or both tires together.
The width of the tire shouldn’t matter. Yeah, you’ll have less contact area, but you’ll also have more pressure, which will (in typical circumstances) cancel out.
It’s not just the total friction that matters, though, it’s also the distribution. In a cycle, you often can’t use all of the front wheel’s traction to brake, or the rear wheel will lift off the ground and the whole thing will flip over the front wheel (I don’t know about motorcycles, but this is often the limiting factor for braking a bicycle). And going downhill, more of the traction is already shifted to the front wheel, giving less available braking before the big flip, and vice versa for uphill.
Thanks for all your responses. You got me thinking more about my question. Although now I see how braking and acceleration will be effected but how would lateral acceleration be effected? Should I be able to have as much lateral traction going uphill as I do going downhill on the same curve? Can I lean as far? In my experience the answer is no but I think its more psychological than reality.
You are correct, you have less traction going downhill than when going uphill. That’s not because of weight distribution (although that might play a part, too), but because of a change in the total force between your tires and the ground. How much change depends on your speed.
When you go uphill, your motorcycle is being lifted up against the force of gravity; that requires additional force. When you go downhill, your motorcycle is falling, reducing the contact force with the ground.
To prove this to yourself, here’s a thought experiment: Suppose you’re cresting a hill, and the slope on the other side drops away steeply. In fact, it just steep enough that you almost, but not quite, actually lose contact with the ground. How much traction do you have? Then, at the bottom of the hill, turn around and climb the hill at the same speed. How much traction now?
zut, that’s nonsense. Normally one is not accelerating up or downhill, and Newton’s Second Law tells you a = 0 implies F = 0.
zut, that’s nonsense. By not accelerating a body downwards at 9.81 m/s² you are applying a force on that body. You forgot to take gravity into account
Gah. You’re right, of course. In my defense, I was thinking of the effect of the curvature of the hill, but that’s neither in the original question nor in what I wrote above. So ignore what I said.
First, we are all talking about the friction effect with the road, but based on its root the word “traction” could be taken to mean pulling force - the thing a “tractor” has lots of, the thing that keeps an orthopedic patient “in traction”. There would of course be more of that when going downhill.
But I understand that is not what the OP was getting at.
I think the best view is that a motorcycle would be in trouble if either wheel started slipping, so the question becomes whether the tangental forces required of each wheel (which depend on the mass distribution and angular and radial accelerations) are in the same ratio as the normal forces on each wheel (which depend on mass distribution and the angle with respect to the pitch axis).
As a further refinement, the coefficient of friction approach is likely a good starting point, but it isn’t all that accurate. I understand that with tires the tangental force increases less than proportionally with the normal force. That’s why dragsters have such wide flat rear tires.
On a bicycle, you have significantly more actual traction going uphill. Maybe you notice it more than on a motorcycle because you can feel the pedals and hear what’s going on. Note that (most) bicycles are rear-wheel drive.
The key issue, which I think you’re all disregarding, is that real roads have patches of loose material on top of them.
If you’re going uphill, and you hit a patch where there is a little sand on the road, your rear wheel might slip a little bit, but it will eventually catch on something and off you go.
If you’re going downhill, especially on a curve, if you’re wheels slip on anything, you’re in trouble–largely because you’re likely leaning a LOT more. If your wheels start to go sideways and you can’t correct immediately, you go down.
So all the theoretical and mathematical stuff is very interesting, but automagic is quite correct in thinking he had less traction going downhill. I don’t know if the Angeles Forest Highway actually goes through forest, but forested roads tend to have a lot of loose patches on them.