F1/GP2 tyre construction changes.

[AWF]

I was aware that Pirelli have been changing the construction of these tyres most seasons and i had a question maybe DaveW could answer .
If from one season to the next lets say the rear vertical and lateral stiffness increased, what would the theoretical requirement of change to take this into consideration as far as damping and spring rates are concerned.
I know from many years of unfortunately now restricted Rig testing that when I increase an axles stiffness with spring rate I generally need to increase the damping at the other end due to the shift in ability to disperse the energy input to the car. Now considering that the Tyre K / Spring K has changed, then am i right to assume that less energy is dispersed via tyre squash and its limited damping ability and therefore more input is put through the suspension, so should I then need to increase damping on that axle as now it has a greater need to disperse the energy. I know that this will probably affect the front as well. Any theories greatly appreciated!

[Jersey Tom]

Not to say it's the best option, but you could of course change tire pressure, get your spring rate back, and then not have to screw around with the rest of the car.

[SiLo]

Probably not the best option though. I imagine the teams get very detailed information about the tyres stiffness and pliability / flexibility quite early on so that the can make changes accordingly.

[Jersey Tom]

For sure not the best option. I also don't think the best approach is to be doing all this purely on "energy dispersion" analysis either.

There's value in simple approaches though and not getting lost in minutiae.

[DaveW]

For sure not the best option. I also don't think the best approach is to be doing all this purely on "energy dispersion" analysis either.
There's value in simple approaches though and not getting lost in minutiae.

I think there is a world of difference between GP2 (& F1) vehicles & the sort of vehicle you work with....What is the best approach in your case?

Tyre stiffness varies with weight, down-force, spring stiffness, damping coefficient, camber, geometry, temperature, inflation pressure, time, & of course, construction. It also varies (massively) with test procedure and processing technique. Looking at previous results (with a consistent technique & starting in 2008), I concluded that vertical stiffness of both front & rear tyres were similar in 2009, the rear tyre stiffness increased in 2010, tyres in 2010 & 2011 were similar, and the stiffness of both tyres was reduced in 2014. Crucially, the ratio of tyre stiffness was reasonably stable (varying between 1.13 and 1.22, rear/front). I have no results for 2012 or 2015.

Briefly, the suspension of an "Aero" vehicle must control the ride height & rake at high speeds (aero set-up), and the contract patch load variation (mechanical set-up) at low speeds, probably starting at 50 mph. The mechanical set-up is important, because it defines corner exit speeds, which have a disproportionate effect on lap time. Also important is the transition between mechanical & aero set-ups, which ideally should not affect lateral balance. Some of the "tools" to affect set-up are springs, bars, "spring aids" (e.g. bump rubbers), damping coefficient, and damping "structure". Tyre and "installation" stiffnesses place limits on the suspension parameter values (increasing both gives the engineer more scope). Ballasting the vehicle can also help if the tyre stiffness ratio is not correct. An inflation pressure "split" can also be used (within limits), but contact patch area and running tyre temperature will probably be affected.

A mult-post rig test is one way of helping to "optimize" the mechanical and transition set-up, but a theoretical model can also be used (usually validated by a rig test).

AWF: Your question is not easy to answer. Tyre damping is small (is the grand scheme of things). Increasing tyre stiffness will decrease tyre deflection & increase spring deflection, hence the damper will be more effective (arguing for a reduction in damping settings), but the tyre stiffness ratio will also change. Ultimately, depending on the set-up, stiffness ratio might dominate - calling for changes to the other axle, or position of the c.g....

A model might help you.

p.s. For what it's worth, my own sensitivity analysis suggested:

Code: Select all

     Tyre stiffness       Damper settings
      Front   Rear         Front    Rear
      0%       0%           0%       0%
      0%      +5%          +4%      -2%
     +5%       0%          -2%      -4%
     +5%      +5%           0%       0%

Hope this helps (apologies for the layout)...

[AWF]

Many thanks DaveW. I have used a certain rig many many times in the past , however current regulations do not allow rig testing in a 3 year period and so it is getting increasingly hard to determine the true correct path to take when tyre construction keeps changing most years.
I understand that tyre damping is very small, ok maybe on high profile tyres there may be a little more than WEC style low profiles. The damping requirements are critical in this championship due to tyre heat generated tyre degradation and so I shall try to see if we can test a few things to reduce this. In the past a more linear and reduced damping has worked for traction at the detriment of lateral control but I am sure we can sort that. As you say the biggest issue could come from the tyre/sus stiffness ratio, we will try and address this from previous rig tests where we altered this and see if there is a direction there as well.
One last question, if the suspension travel is now increased due to this as springs in series and so on, if i was to reduce bump damping and increase rebound, am I correct to assume my contact patch load variation and mean load will be reduced assuming we are not well outside the correct working envelope or not?

Best Regards.

[DaveW]

One last question, if the suspension travel is now increased due to this as springs in series and so on, if i was to reduce bump damping and increase rebound, am I correct to assume my contact patch load variation and mean load will be reduced assuming we are not well outside the correct working envelope or not?

Moving damping from rebound to compression (particularly at the rear axle) consistently improves the "grip" parameter during a rig test. This appears to translate to track performance. There is an issue, however. Too much compression damping will launch the chassis over large inputs - which is not a good idea for aero reasons. An adjustable "blow off" control can help to contain that. As it happens, the dampers used by GP2 have a high speed bump control that can implement blow off quite effectively.

I hope this helps.

I did try to estimate the effect of damping style on tyre heat rate. It appeared to be small, but my model uses viscous damping, rather than the (mostly) hysteretic damping of the real tyre, so the results are questionable.

[Rustem 1988]

Why is more compression damping bad for aerodynamics?