Yup, VSA gets really freaked out by changes in geometry. On my car (lots of camber & caster, roll-center & bump-steer correction), VSA goes nuts on even moderate corner entries. I can only leave it on when driving around town or on the highway. The slip angle curves and the wheel speeds all get affected when you start moving things around.

 

Any further comment SakeBombGarage or RobRob (great site btw!)? You guys are fairly vocal usually so I would just like to know if you acknowledge what I said or if you disagree with it. Thanks.

 

Peter / SBG,

what are your thoughts on the increased scrub radius associated with the wider track via wheel offset?

on one hand you have reduced weight transfer, on the other hand you have increased the scrub radius and the undesirable effects that comes with it.

I guess there is a balance point somewhere, the question is where?

has anyone had real world back to back testing on track S2K with wheel offset?
I'm currently on 17 x 10 +38, and i'm thinking to keep it like this or to go with +50 offset wheels.

 

Close but not exactly. ARBs effect the ratio of load transfer each axle sees, this is called the Lateral Load Transfer Distribution or LLTD for short. You are correct that they don't effect the lateral load transfer because that is just a function of lateral acceleration and track width, however if you accept that fact they also can't effect front to rear load tranfer as that is simply a function of longitudinal acceleration and wheelbase!

Think of it this way:

Take a 2x6 piece of wood a foot long, bolt one end of the board to something rigid. Now take that board and twist it along the long axis, all of the torque you have applied will be absorbed by the bolted end.

Now take the other end and use a rubber band to attach it to something rigid. Apply the same torque as before, the bolts are still significantly stiffer than the rubber bands but because the rubber has a bit of stiffness, a very small amount of torque will be absorbed by the rubber bands.

As you increase the stiffness of the rubber bands, the torque absorbed by that end of the board will increase and the loads on the bolts will decrease.

Eventually the rubber bands are replaced with steel bars and the torque is absorbed by both ends equally.

In this example the bolts are equivalent to a massive front anti-roll bar, the bar is so stiff that any roll moment (torque) on the car is absorbed only the front axle. This would mean that the front axle takes all of the load transfer and the rear axle's wheel weights are unaffected by cornering.

Please let me know if this helps or if you need further explanation!

 

This question depends entirely on the use case of the car. Ignoring fender/bbk clearance, etc. type issues, heres a quick list of pros/cons of low offset wheels

Pros of low offset:

• Wider track width reduces weight transfer
• If used on front axle, load jacking is increased with steering (more on this below)

Cons

• Increased frontal area -> more drag
• Less predictable behavior over bumps

When the front axle is steered, the inside wheel is pushed down and the outside wheel is pulled up. By decreasing the offset of the wheel, this effect is magnified because the wheel is further away from the kingpin (axis between upper and lower balljoints). This reduces understeer at higher steering angles by decreasing front axle load transfer (rear load transfer increases as a result).

This is a powerful tuning tool that allows you to set up the car based on steering angle, which is a good estimation of corner radius.
Low speed -> large steering angle -> lots of load jacking -> oversteer allows the car to rotate around tight hairpins
Mid speed -> medium steering angle -> some load jacking -> balance mostly tuned by springs, bars, tires
High speed -> low steering angle -> minimal load jacking -> balance mostly tuned through aero

What "Low speed" means largely depends on the type of track you are on, an auto-x may have extremely tight turns at 15mph (therefor higher steering angles -> very low offset wheels may be too much load jacking), a road course will tend to have larger corners and therefor less load jacking for a given wheel spec. Its also worth mentioning that increasing castor has a similar effect.

 

You are correct that the lever length equation is a simplification, when we measure motion ratios for development platforms we directly measure the motion ratios, we do not calculated them. Ultimately the motion ratio is Wheel Displacement / Spring Displacement (or Spring/wheel depending on British or English conventions). The thing to note is that the offset does not effect the lever method calculation because the wheel offset is playing a roll in the camber gain based wheel motion. As the camber increases the wheel moves up, lower offset wheels increase this motion while higher offset wheels decrease the motion.

All things considered it is far to complex to explain in a few forum posts. Here is a quick summary of ways that motion ratios are effected

RCA -> Increased Camber Gain -> Decreased MR
Lowered Car -> Increased Camber Gain -> Decreased MR
Lower Offset Wheel -> Increased Lever For Camber Gain to Act On-> Decreased MR

 

Yes yes i know you can tune the handling with staggered track width, and yes i know the wider front is more understeer, wider rear is more rotation etc. etc.

No offense, I'm just wondering if anyone has done back to back testing at the track with low offset wheel and high offset wheel on the S2K, and if an ideal offset range was found. Or if there was any significant change in laptime at all.

Sometimes i feel like too much theory just muddying things up abit.

Thanks !

 

No, no one has measured what an ideal offset is on this platform that ive ever come across in this forum. There is too many variables, as I think your starting to gather, and at the end of the day most drivers are focus on other aspects that dictate lap times more, such as what tires are on those rims and driving.

I agree, sometimes theory's get analyzed to death. There is a place for understanding principals of course to have some kind of direction, but sometimes you just have to get out and do some trial and error to really know what works best for you for the cars application.

 

To me it is far more important to optimize offset to allow you to run the widest rim that will fit and get the most out of the given tire.

 

That's my general rule as well. Get as much tire under the car with the best profile to optimize it, which will dictate the rim width and offset.