Tim.Wright wrote: ↑
Sun Dec 01, 2019 10:56 am
Tyre energy cannot be directly measured and is not a simple thing to calculate. There are a few different sources of it and for each one you need to know some of the tyre characteristics. For example, energy due to the cornering or braking forces are the forces multiplied by the contact patch slip speed.
But getting the tyre forces and slip from the realtime data is pretty tricky. They have sensors on the car for the lateral slip so this then must be transformed to the contact patch slip. Then with wheelspeed sensors you can (try to) get the longitudinal slip. Force might be measured via strain guages otherwise it needs to be estimated using a handful of vehicle sensors and a load of vehicle information. Once you have the total contact patch force and slip speed you multiply the two together to get the energy in Joules.
That doesn't tell you the full picture though, there are some other sources of energy loss such as rolling resistence and hysteretic losses from carcass deflection but I'd guess the major percentage comes from the tyres forces and slip.
Thank you very much Tim.
So…energy due to the cornering or braking forces are the forces at the contact patch multiplied by the contact patch slip speed…
I’m trying to work backwards, let's see if I'm getting it right…
The equation for the lateral and longitudinal slip velocity are:
Vlat= V*sin alpha
Vlong= V* cos alpha –Vb
Where Vb is the circunferencial velocity of the tire under traction or braking.
I can potentially measure slip angle and steering angle and with that calculate tyre slip angle (considering you could measure/calculate V and Vb).
I think you could use:
1- 2 correvit type sensors (vehicle’s co-ordinate frame). The system measures the forward and lateral velocities directly and compute slip angle using the formula.
2-You could use dual-antenna GPS and/or inertial navigation systems (both earth-based co-ordinate frame).
Am I right!? Am I missing any other way!? Accuracy will change with speed, right!? these instruments measure velocity with a relatively fixed accuracy, so…the smaller the speed the more significant the error.
Is this approach correct so far!?
*Would it be possible to calculate slip angle base on accelerometers!?
So…you won’t be getting a real value but just an approximation, right!?
With regards to the forces…as you said, strain gauges/load cells are the way to go. My question is how do you calculate forces in the contact patch considering your strain gauge is in the pushrod!? Am I missing something or is it just a trigonometry problem!? The resolution will depends upon link geometry, which generally changes with wheel position…So, the push rod load by itself can be resolved into vertical load, but…the result will be "affected" by loads reacted by other suspension arms and wheel position…
In any case, again, you are just getting an approximation of what’s happening at the contact patch...right!?
*Just as side note, you could also use linear pots to measure suspension travel (though you'll be neglecting shock loads, which may or may not be a big deal) to calculate force, which could be good enough for some scenarios…
You multiply both results and that’s just one part of the total energy…
You could calculate/measure rolling resistance and I have no idea on how to calculate/measure hysteretic losses…
P.S: I think I'm missing something though...power is equal to force times velocity, and energy is power multiplied by time