Ai_1 wrote:
carlosflanders wrote:
If the ply direction made a difference it would probably be best for the tires to be mounted in opposite directions. Front to optimize braking and rear to optimize for accelerations.
Why would this be the case?
Yes, braking and pedalling apply shear loads on the tyre and at the contact patch, and differ between the front and rear tyres, but I don't see how you make the leap to saying this would necessarily be the overriding criteria for optimisation or that it would make any sense to mount the front and rear in opposite directions. For one thing the front tyre is generally not under much shear load. The vast majority of the time you are not braking, and it's simply supporting the load and providing steering (so some shear loads there but not in the same direction as braking). I would expect carcass directionality, if present, to be directed primarily at reducing rolling resistance (with or without consideration of propulsion) with braking only taken into account in terms of durability and no significant loss of grip. Simply rolling possesses directionality, before accelerations are ever considered.
carlosflanders wrote:
There's a report out there from November bikes that wid tunnel testing shows one direction to be favorable over the other. I find this a little strange because the wind tunnel should see the pattern symmetrically. The top should be opposite to the bottom etc.
Not so. The ground has a boundary layer, or wind gradient. Except in still air, there will be a difference in windspeed, and probably direction, depending on height off the ground. If the wind tunnel testing took this into account, which it certainly should for yaw tests, there would be a significant difference between the flow over the upper and lower portions of the wheel.
carlosflanders wrote:
Wind tunnel doesn't capture rotational drag and this is where there might be a directional difference. Here's an experiment: mount a front wheel with a gp4000 tire in a trueing stand. Spin it clockwise 10 times and measure time to decrease from 30 mph to 20 mph equivalent, spin it anti clockwise 10 times and do the same measurement. See if there's a repeatable difference. One can measure the speed by attaching a magnet and speedometer or better by using an optically activated relay that's triggered by a passing spoke or reflector.
Should be possible to estimate the moment of inertia and the drag and accuracy, and then put an upper and lower bound on any difference.
Interesting idea, but how do you take any asymmetries in the bearings, spokes, rim or valve out of the equation? You'd also need to perform this in a wind tunnel to get a realistic and repeatable flow. I don't think it's as simple a test as you suggest given the likely small relative magnitude of the forces involved for aerodynamic drag differences.
Good to see someone thinking about this.
My comments about directionality were from the point of view of durability and handling. Front tire does most of the braking so optimise the front for that. I don't have strong opinions about it, just thought it was worth a mention.
Wind Tunnel: This is interesting. The question is: is the gradient in the wind tunnel the same or similar to IRL? I don't have the answer to that but I would expect there to be a tunnel-to-tunnel variation. Plus the rear tire is going to see quite different air from the front - most of the WT tire tests have been done on stand alone wheels that only simulate the front tire. More than happy to be educated on this but my expectation is that IRL flows and gradients will be quite different - plus tunnels don't measure rotational drag.
Rotational drag: The test I proposed is purely to measure rotational drag and could be a high school science project. Some back of the envelope calculations show rotational drag to be about 1/3 of translational but you don't need a tunnel to measure it. Should capture very purely the effect of direction without worrying about gradients or yaws. If you see a difference then you've established that mounting direction has merit and you can get an estimate of the magnitude. No, you don't need to do this in a wind tunnel.
Taking into account any difference in directionality of spokes, rims, bearings etc is easy. Do you measurements in clockwise and anti-clockwise, then mount the tire in the opposite direction and repeat.
I think there's a huge merit to doing a test that requires negligible $ to invest, only time, versus a test with limited time and huge capital cost.