Suspension Optimization and Tuning Help

[Ron Sutton]

Tyler,

We have a lot to cover & some things to figure out. I'm clear on some things & not on others. I'm going to map out what I see and let's start some discussion to help us both figure it out.

What I see so far ...
1. You have a lot of stuff right or close to right. I think you're on track with your thoughts and plans.
2. You have some things not correct ... some are close ... others are not.
3. You have several things off a little ... enough so that it pushes too easy.
4. We may ... or may not ... find you have one big thing "off." And that would be what I call dynamic toe, which is a combination of ackerman, static toe out & "bump out" (toe out from bump steer). I can't tell for sure, because we haven't mapped out the steering ... yet. But that spindle has anti-ackerman in it ... so I have concerns.

5. The two fenderwell camera videos in post #1 ... show me different dynamic tire angles ... which has me a bit perplexed. Are they from the same day? Were there any changes between those runs?

* The geometry shows plenty of camber, but one of the videos shows the car driving on the outside edge of the outside front tire. Any changes between the date of the video & the date of the geometry capture?


6. If I understand the order of things ...
a. You had stiffer springs & the 28mm sway bar, then ...
b. Went to softer front springs & the same 28mm bar, then ...
c. You switched to the 33mm bar ... but haven't run it yet.
... is this correct ?

If so, "a" was your "conventional set-up" ... "b" was a high travel & high roll set-up ... and "c" will be somewhat of a high travel/low roll set-up.

7. Even though the two fenderwell camera videos in post #1 show me different dynamic tire angles ... I still think you have the wrong geometry for that much roll angle. But it sounds like you want to go the high travel/low roll angle route ... so the geometry change will be less.

I typically shoot for about -1 degree of static camber, 0 toe in/out, and according to the FasTrax Adjustable Caster Camber Gauge I can currently register about 8.5-8.6 degrees of caster.

8. My calcs ... and the software show you have much higher caster than your specs ... and different from side to side. 11.5° on the left & 10.6° on the right. So either your ball joint measurements are off or your caster measuring process is off.

I am also battling a relatively high scrub radius due to my 17x11” wheels with just 4.25” backspacing. Using the method in Lance’s thread, I measured the scrub radius at 4 13/32”

9. That is a ton for an AutoX vehicle. Where it hurts the most is in the tightest corners ... and especially if you have to give the truck some additional steering input when you're in the corner. You can band aid this some by running an higher/wider entry line ... turning in later & harder ... and apexing later. This will help, but the solution is longer control arms & deeper back spaced front wheels to get that scrub radius down closer to zero.


10. There are several differences in your geometry settings from side to side. What this does is make the dynamic geometry different on LH turns versus RH turns ... which makes the truck handle differently on LH turns versus RH turns. This is common ... especially in production car ... but never good. My focus is on "how bad is it" and does it merit reworking & TLC'ing the geometry points to correct it.

Yours is off significantly. Your RC migrates to the left quite a bit (21") on LH turns ... and a ton (184") on right hand turns.

11. The front roll center is too low one direction (Not 25" .... .25" below ground level) ... and way too high (5"+) the other direction. If all other things were neutral, this would make the front end of the truck roll more on LH turns & less on RH turns. With the numbers I'm seeing, the effect won't be small, so you should be feeling it & seeing it.

P.S. All things are not neutral, as you have 3" of split in the panhard bar. I'll address that in the next post.

For a 2.5°-3° high roll angle/Autox set-up I feel around 1.5"± above ground would be closer to optimum. Both directions obviously. For a 1.0°-1.5° low roll angle set-up, you'll want to be around zero (ground level).

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This is hard left hand turn ...




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This is a hard right hand turn ...


As you can see ... there is some work to do ...

I typically shoot for about -1 degree of static camber, 0 toe in/out

12. We definitely want to switch to running some toe out, but we need to look at the whole picture. Static toe out helps give the inside tire "some" slip angle ... which makes the tires have more initial turn in grip ... and the steering becomes way more responsive.

13. We also need to look at what your bump steer does through your realistic travel. Have you bump steered each side individually yet?

14. The only reason to run dynamic toe (ackerman, static toe-out & "bump out") is to force the inside front tire to achieve the optimum slip angle ... for optimum grip. This is HUGE ... especially for cars running tight road courses & autoX tracks. There is no "one best setting" because it varies with both tire design and suspension strategy.

Low Front Travel/High Roll Angle suspension set-ups work the inside front tire less ... and require a larger combination of ackerman, toe-out & bump out. High Front Travel/Low Roll Angle suspension set-ups work the inside front tire more ... and require a smaller combination of ackerman, toe-out & bump out. But both need to be tuned until optimum front tire grip is found.

15. Remember earlier when I said ...
"You have a slight mismatch in front & rear roll angles. Ideally we want the front roll angle to be .3° to .4° less roll angle than the rear. This creates a balanced handling competition vehicle that is disengaging the inside rear tire more than the inside front tire. Yours is off a little. It is rolling slightly less in the rear than the front ... I'd say around .2° more in the front ... which is about .5°-.6° from ideal."

What this means is as you work out your spring rate & sway bar rate combo ... you need rear springs & sway bars to achieve about .3°-.4° more rear roll angle than front. As an example, i have a set up that runs the front end at 1.2° & the rear at 1.55° roll angles. This is keep the inside front tire working better & disengage the inside rear tire "to a specific degree" to help the truck turn. Make sense?

16. Also mentioned earlier ...
The final ... but BIG issue, is your front shock valving doesn't have enough low-speed rebound. Meaning the front shocks let the inside front suspension roll too much and come up too quick when you step off the brakes. To achieve your AutoX goals, you need front shock valving with both stiffer low speed rebound valving & some degree of rebound valving at zero, before it moves.

17. As you correct all these things to make the truck turn better ... if nothing is done in the rear, you'll lack forward bite off the corners. Two things that will need to be addressed are shock valving (stiffer low speed rebound) and the height of the IC for "lift & plant" leverage.

After you read all of this over ... digest it, answer my few questions ... and let's figure out what we need to do make your truck an even better handling machine.

Next I'll post some info on tuning what we call "Split" in the panhard bar to achieve neutral handling.