XRAY - Model racing cars

Guys,

Hoping for a little clarification here. The setup book says:

"The amount that a chassis rolls in a corner depends on the position of the roll axis relative to the car’s center-of-gravity (CG). The closer the roll axis is to the center of gravity, the less the chassis will roll in a corner. A lower roll center will generally produce more grip due to the chassis rolling, and the outer wheel “digging in” more"

"More chassis roll equals more grip"

but then things like this:

"Anti-roll bars are used to adjust the car’s side (lateral) grip. They can also be used in conjunction with a softer spring rate to handle bumpy tracks more efficiently without excessive chassis roll at mid-corner. Anti-roll bars resist chassis roll and by doing so transfer wheel load from the inside wheel to the outside wheel. The stiffer the anti-roll bar, the more load is transferred. However, as the outside wheel is not able to convert the extra wheel load into extra grip, the sum of the grip of both wheels is actually reduced. This changes the balance of the car to the axle at the other end of the car; increasing the stiffness of an anti-roll bar on one particular axle (front or rear) decreases the side grip of that axle and increases the side grip of the axle at the other end of the car."

These statements contradict each other, one says more tire load = more grip, the other says more load = less grip.

The second statement is something i'm familiar with from full size car stuff and makes perfect sense to me given the tire lateral grip efficiency reduces as load increases.

The first statement is the opposite of what i would expect and contradicts the second.

Chris..

Lowering the rollcenter at one axle reduces the resistance against chassis-roll. Same does a softer anti-roll bar or a softer spring.
So the effect is similar. Its not the same (all elastic resistance against chassis-roll can only come up when there is at least a little bit of roll while geometric effects of rollcenter-height work at the very first moment when lateral forces begin to act. Not to talk about bumps...)
In steady-state cornering (midcorner) on a flat surface its the same.

The effect described in the Setup Book "as the outside wheel is not able to convert the extra wheel load into extra grip, the sum of the grip of both wheels is actually reduced." explains the effect of both changes. This is true in most racing-conditions. I wouldn't give too much to the explanation with the outer wheel "digging" in more...

However there are tracks where it is the other way (the axle with more resistance against chassis-roll gives more grip). We had this at the EC in Luxemburg for example. I don't know what the physical explanation of this "wrong" behaviour is, but i felt it on the track and doublechecked it more than once...

All this is about car balance. Overall grip is a completely different story...

Thanks for the clarification. I really struggle with the idea that loading the outside tire more creates more cornering efficiency as described in the "digging in more" example. This would mean that a heavier car with corners faster than a lighter car!! At our track in the spec classes we went from 8.5t to 10.5t and 13.5t to 17.5t motors but lowered the weight from 1525g to 1425g. Even with the slower motors the cars are still breaking track records which says lighter is faster (our track is very technical so most time is spent cornering). I have also found siuations where weird things seem to work beter, like running heavier springs in the rear than front but the car still being planted in corners.

I can think that maybe when using a control tire that is too hard for conditions you might use these things to hit the tire harder when it can take it to generate more overall grip (for that unsuited tire) but that's about it.

Can you discuss more about overall grip?

In racing you won't ever find much difference in overall grip. Even the hardest and softest springs don't differ much about this.
The biggest difference in overall grip is tyres and additives, effective camber in roll, track conditions.
There is also some influence by aerodynamics and if the track is bumpy by spring/damper setup (you won't have any grip if a tyre is in the air) and center of gravity-height.

OK you are getting into an exceedingly complex area and you need to recognise the difference between the overall grip of the car and the relative grip between the front and rear axles at any given time which controls the balance.

Roll or sway bars increases the overall grip of the car, period.

At the simplest level if you go around a corner and an inside wheel lifts, you have less overall grip with 3 tyres in contact with surface than with 4.
At a more complex level it revolves around the properties of rubber and there are complex algorithms with which you can calculate this stuff. The general principle is that when you load up the outside wheel you will increase the grip of that wheel. BUT the increase in grip can never equal the loss of grip on the inside wheel so your overall grip reduces. You don't see F1 or racing sports cars rolling much do you!
A sway bar reduces the relative loading and unloading, ie between the outside and the inside tyre, increasing the grip for a given situation. To be absolutely accurate, it is reducing the loss of overall grip.
I have never fully got my head around stiffer rear sway bars increasing front grip. Have never studied it but I have a niggling suspicion it might be something to do with reducing the roll to a point where you are not using the full potential of the tyres. So what is actually happening is that you are inducing oversteer which maybe gives the impression the front has more grip. But I am not convinced about that, I am sure someone can offer a better explanation!!

I understand it's complex, i'm an engineer so i have trouble believing without knowing

The key to this is the vertical load vs lateral force tire characteristic itself.

For real car tires at peak lateral force it is possible to have more lateral force generated than you have vertical force acting on the tire. However, once you get to a certain vertical load the peak lateral force becomes lower than the vertical force. So, after that, the more you load the tire these less lateral force can be generated. This is a proven fact for full size tires.

As an example an F1 tire can have a lateral vs vertical force ratio of well above 1 at light loads.

This is the only opening i can see for those who say more roll and tire load = more grip and i honestly don't know how a Sorex32 loaded at a few hundred grams compares to a Goodyear Eagle loaded at hundreds of kilograms. Full size racing tires operate mostly in the overloaded state and all the literature is written around managing that situation. If RC tires don't spend most of their time overloaded then great, but my tires look pretty shredded after 3-4 rounds on most tracks.

The explanation i have for full size cars on your rollbar question is this:
- if a car is understeering it means the outside front tire is overloaded and is reducing the total front grip, so that the front has less grip than the rear. A stiffer rear rollbar transfers more of the total roll force to the outside rear wheel (overloading it more). So you get the effect that the outside front is loaded less now (becuase the rear outside took more of the load) meaning the total front end has more grip. Additionally, becuase the outside rear is loaded more (and became less efficient than it was before) the total rear grip is less. The net result is more front grip and less rear, but it is a very fine line as to whether the overall grip even changed. (again back to the tire characteristic and the camber effects at each end of the car).

I have 3 very good books on full size cars.
- Chassis Engineering, Herb Adams
- Race Car Vehicle Dynamics, Milliken
- Tire and Vehicle Dynamics, Pacejka