8 second OEM 4 link drag race chassis setup

[Alky V6]

Shock baselines

I'm going to start with baselines that are completely different from what has been used with the car in the past.

Clicks from full soft.

Front QA1 single adjustable (rebound), left and right.
Old setting: 0 (full soft).
New baseline: 24 (full firm).

Rear Afco double adjustables, left and right.
Old settings: compression 13, rebound 3.
New baseline: compression 16, rebound 8.

 

[Alky V6]

For the record, this is the link arrangement I went with. This is with the car resting.

 

[Alky V6]

This is the same link arrangement with some rear end and front end rise.

 

[Alky V6]

Same link arrangement with some drop in the rear and rise in the front. I don't expect the car will actually do this.

 

[Alky V6]

Baseline heights.
Rear spring height loaded;
LR 10 63/64", RR 10 61/64". 1/16" preload on RR.
Front spring height loaded;
LF, RF.

Ground to highest point on fender lip;
LF 26 15/16", RF 26 5/16". 5/8" higher on LF.
LR 30 1/16", RR 29 7/16". 5/8" lower on RR.

Tire to bottom front of fender lip (horizontal);
LR 1 9/16", RR 1 1/2".

Highest point of fender lip to tire;
LR 1 1/2", RR 1 1/8". 3/8" less on RR.

Roll bar preload;
2 flats preload on RR.

 

[Alky V6]

I went with 4 turns on the roll bar preload before running the car today.
It proved to be too much preload.
The launches were flat, a little tire chatter, some corner to corner rocking. Steering correction was needed on the top end as I got off the throttle and onto the brakes.
The launch rpm/boost levels were low to medium (125-145 kPa map) on these runs.

Suspension tuning that was done today.
4 flats taken out of the roll bar preload on the RR.
Rear tire pressure increased from 9 psi to 9.5 psi.
5 clicks looser on both front shocks. Now the adjuster is at 19 clicks from full soft.
The launch with the above changes felt real good. No video on this run. The car launched very flat and smooth, even though the rpm/boost was on the high side (5900+ rpm/200+ kPa). 1.28 60'. Didn't have to wrestle with the car on the top end when getting off the throttle and onto the brakes.

 

[cruzn57]

so?

would you say your making progress on overall susp setup?
1.28- 60ft is HOT!
did the car wiggle when you let off?
impressive!!!!!

 

[Alky V6]

I haven't put the boost to it yet. I'm only using a peak of 275 kPa map for these initial runs. When I think I have something I'll increase the boost.
I'm starting to see some of the things I've been studying up on. Too much spring or roll bar preload on the RR will cause wiggle when you let off. When you preload the RR, you are also preloading the LF tire. When you let off on the top end and apply the brakes, the weight transfer to the front end, along with the extra preload on the LF will want to steer the car, which is exactly what happened. After I took 2/3 turn of preload out of the RB the wiggle on the top end was much less. In fact, I don't recall any at all. It's amazing how sensitive that adjustment is. I'll have to wait for more runs to verify the change.
Stiffening the front shock rebound setting really helped control front end lift. When I'm done, I may not need the wheelie bars. I'll be slowly softening the front shocks until I find a level of front end lift and/or 60' time I'm happy with.
Matching the tire pressure in the slicks to the suspension plays an important role too. The 1/2 pound change I made really helped smooth out the launch.
I haven't started playing with the rear shock adjustments, yet. After I have some good video to study, I'll start doing that.

 

[charlief1]

I'm starting to see some of the things I've been studying up on. Too much spring or roll bar preload on the RR will cause wiggle when you let off. When you preload the RR, you are also preloading the LF tire. When you let off on the top end and apply the brakes, the weight transfer to the front end, along with the extra preload on the LF will want to steer the car, which is exactly what happened. After I took 2/3 turn of preload out of the RB the wiggle on the top end was much less. In fact, I don't recall any at all. It's amazing how sensitive that adjustment is. I'll have to wait for more runs to verify the change.

Stiffening the front shock rebound setting really helped control front end lift. When I'm done, I may not need the wheelie bars. I'll be slowly softening the front shocks until I find a level of front end lift and/or 60' time I'm happy with.
Matching the tire pressure in the slicks to the suspension plays an important role too. The 1/2 pound change I made really helped smooth out the launch.
I haven't started playing with the rear shock adjustments, yet. After I have some good video to study, I'll start doing that.

I highlighted the section in red for a reason Donnie. This is part of what I've been harping about. Get a set of scales and this will help you get everything set. One of the other things you can try is to preload like you did but also preload the RF wheel at the same time to compensate for the rear bar. You need to get a better idea of where the weight is sitting so you can make it handle better.

 

[Alky V6]

I'll work on that, Charlie. It will be interesting to see what I find after feeling out some ideas.

 

[Alky V6]

Tomorrow I will finally have the numbers!

 

[Alky V6]

This is what I figured out to place my new holes for the LCA axle mounting somewhat accurately.

 

[Alky V6]

A couple more.

 

[Alky V6]

I'm going to start slowly extending the rebound travel limiter on the LF until the videos show the two front tires leaving the ground at the same time on the launch.

Something else I want to experiment with. Using the shocks more in place of some of the roll bar preload. Stiffening compression on the RR, and rebound on the LF.

 

[Alky V6]

OK, Charlie. Here are the corner weights. The roll bar has been neutralized. Driver, helmet and full fuel load. They were set using spring heights in the rear only, when I first upgraded the suspension. Haven't touched the springs since, except to lower the front end a tad.
I originally set up the springs to favor a little extra weight on the RR. How much that extra weight really was, I didn't know for sure. I only had the difference in rear spring heights to go by. The rear spring rates are 125 lbs/in. So .200" should equal 25 lbs.
This is what I have today;
LF 955.5
RF 829.5
LR 741.5
RR 764.0

The difference in LR to RR is 22.5 lbs., with the more weight favoring the RR. Absolutely amazing.

Next, calculating the CG.

 

[Alky V6]

CG height = 21.7"
Other vehicle specs calculated using the new data;
Total vehicle weight: 3290.5 lbs. with suited driver and full fuel cell load (15 gallons).
Percentage of weight on the rear axle: 45.75%
Percentage of weight on the left side: 51.57%

 

[Alky V6]

Of course, the new CG has changed the IC calculations. Here is the updated IC and AS%.

 

[Alky V6]

RB neutralized with the slack taken up to favor preload for the RR.

The next project is traction dyno testing with chocks.

 

[Alky V6]

I'm thinking of trying a combination of things to control roll at the launch and equalize loading of the rear tires.

Asymmetrical 4 link adjustment. Putting the left side LCA axle mounting back in the old location, leaving the right side LCA axle mounting in the new location. This will provide some dynamic roll control.

Increasing spring preload on the RR corner to make up for the little bit that this particular asymmetrical 4 link adjustment may be short by in controlling roll. To start, the spring cushion will be put back in the right side, along with any spacer at the bottom spring mount to reach the target weight for the RR corner. A combination of weight jacking between the LF and the RR will be done to maintain target RR corner weight target and ride height.

Shock adjustments in the LF and RR corners to control rate of roll, if there is any left.

RB will be set to 2 flats of preload on the RR corner.

Left side LCA will need to be lengthened 20 flats with the old mounting location.

 

[Alky V6]

After doing some computations, all I should have to do is install the right rear spring cushion without any extra spacers, and raise the left side LCA axle mounting 1.000" higher than the original mounting hole to get a perfect mix of dynamic and static loading of the rear tires.
This will give me;
66.2 AS% left side.
114.5 AS% right side.

The spring cushion added to the RR will give me;
716.5 lbs. LR corner.
788.7 lbs. RR corner.
980.2 lbs. LF corner.
804.6 lbs. RF corner.

RB with 2 flats preload RR.
Shocks with symmetrical settings left to right.