Rear Spring Location Analysis

[Norm Peterson]

So then, in conceptual terms (no numbers ) is a chassis mounted watts link going to be less stable (towards oversteer) under braking while turning and more stable under (towards understeer) acceleration while turning due to the change in RC height above the ground even though the RC remains fixed relative to the CG? Or is that over simplifying what is going on a little too much?

All else the same including the static geo RC height, that's what I would expect.

In the S197, the variation of axle roll steer with rear ride height is slightly slower with the diff-mounted Watts. This effect would be additive to the above (also being looser in, tighter off). Mid-corner/steady-state understeer/oversteer balance with a diff-mounted Watts ought to be just a little less sensitive to the throttle. Of course if you're good enough to notice the difference . . .

I guess what I'm asking is, would a Fays2 be better in situations where more rotation is necessary under braking but more grip for acceleration compared to a differential mounted unit?

Possibly.

My assumption is that with a fixed RC relative to the CG (chassis mounted watts) that your roll contributed to that "arm" between the RC and CG is constant. I get that, but given both, how does the RC height (again, still fixed relative to the CG at all points in roll) above the ground translate into grip under braking and acceleration?

It affects how much total load transfer happens. All else equal, the greater the lateral load transfer (which ties up X amount of the tire's total grip), the less remains for longitudinal. If you let the rear RC drop slightly on corner exit, you're transferring less load through the rear RC leaving a tiny bit more for forward acceleration.

Or if you start with braking causing the rear RC to rise slightly, the forward load transfer limits the rear tire total grip and the higher rear RC means that the rear lateral grip that remains gets "eaten up" quicker (IOW, the tire would run out of grip at a lower lat-g).

The stiffer your rear springs are, the smaller these differences will be and the closer the behavior of the two Watts arrangements should be to each other. Less ride height variation and less axle steer variation right off. Less roll if you're also rebalancing the front to suit stiffer rear springs (which you might do if you were trying to keep the same sta-bar or bar setting).


Norm

 

[DTL]

My apologies if this question isn't exactly on-topic, BUT:

Can these discussions and math be used to determine what spring rate you would want when converting from coil over shock to coil on housing? Example: If I had a 400lb coilover spring on the shock, what rate would equal that as far as wheel rate and roll behavior if the spring were moved to the stock on-axle location?

 

[SoundGuyDave]

Short answer is: No. Not because you can't do the math and convert one rate to the other, but because the two spring locations behave radically differently in roll. As an example, the 400lb/in springs you mention when looked at in pitch (accel/braking) give a net wheel rate of approximately 394lbs/in on an inclined coilover, or 400lbs/in on the stock axle perch. In roll, however, the same springs (400lb/in) give you a 277lb wheel rate as a coilover, but only 152lbs/in in the stock location.

Yes, you could do the math and get a spring in the stock location that gives the same 277lb/in rate in roll, but the "ride rate" would be through the roof. The closest standard rate match would actually be a whopping 725lb/in...

 

[DTL]

Yikes! Thanks for the explanation. It sounds like trial and error will be the order of the day.

 

[SoundGuyDave]

So, you're changing from a coil-over spring to the stock perch? May I ask why? Any data you can add to this thread would be appreciated!

 

[Norm Peterson]

Yikes! Thanks for the explanation. It sounds like trial and error will be the order of the day.

Along with adding or upsizing the rear sta-bar. Like Dave said, the ride rate goes crazy if you try to maintain the same roll rate using only the springs. But you can choose softer springs, maybe even retaining the same ride rate as you currently have with the C/O's, if you're willing to make up the lost roll rate with a bar.

If you work with lateral load transfers through both the springs and the bars and match the total of some new springs-on-housing plus new bar to the existing C/O springs plus existing bar, you ought to be close and not have to buy too many sets of springs.


Norm

 

[Philostang]

Stupid Q Time. If you're trying to come up with a close match to some specific roll rate, can you simply add the expected wheel rates from the new springs to that of a new roll bar?

Apart from the difficulty of getting an accurate figure for the roll bar, that sounds too simple. Are we in luck?

 

[Sky Render]

Roll stiffness created by springs is different than that created by roll bars. A car with stiff springs and soft bars will handle differently than a car with soft springs and stiff bars.

 

[DTL]

So, you're changing from a coil-over spring to the stock perch? May I ask why? Any data you can add to this thread would be appreciated!

I don't know how much data I can add, LOL. Basically, The coilovers are fine, but I need the extra tire clearance in there, so I'm moving back to a big spring setup.

I've got an adjustable rear bar on the car now and I typically run it in the full-soft position. I'll try keeping the same spring rate and stiffening up the bar a touch.