Printable Version of Topic

Posted by: yeahmag Dec 4 2018, 12:31 AM

I’m moving up from 225lb to 300lb springs in the rear to try and combat the rear rolling over and the large amount of front, inside wheel lift I’m getting. Is there a general rule of thumb for when I should seam weld the rear shock towers?

Posted by: Chi-town Dec 4 2018, 07:50 AM

What valving are you running in the shocks?
What sway bars are you running?

Street car, Auto-X, Track car?

Posted by: yeahmag Dec 4 2018, 09:31 AM

Car is a narrow bodied autocross car.

Tires:
Hoosier A7

Front:
Raised spindles - 19mm built by Tangerine Racing for an EP 914-6
Rebel Racing (teflon) bushings
Bilstein custom valves
22 mm torsion bars
21 mm sway bar

Rear:
Rebel Racing (teflon) bushings
225lb rear springs (going to 300lb)
Koni Yellow
No sway bar

Posted by: Chi-town Dec 4 2018, 10:45 AM

If you're getting that kind of weight transfer have you thought about stiffening the rear valve stack before going heavier spring rate?

I'm worried if you go heavier spring on that valving you're going to start to run in to tire bounce or funky snap oversteer.

Posted by: yeahmag Dec 4 2018, 10:50 AM

I definitely need more spring. 300 may be too much, but I think it will be close to correct. I will likely end up with custom valved bilsteins in the rear, but shock valving won’t cure what I’m chasing.

Posted by: Chi-town Dec 4 2018, 11:28 AM

I'm confused by your thoughts on this? Can you explain?

Not trying to question, just trying to understand your train of thought.

I'm a track guy not an auto-x guy for the record.

My thoughts:
Stiffer compression valving slows the rearward weight transfer motion and reduces the front tire pick up.

Heavier spring rate will try to eliminate any rearward weight transfer and apply extra load to the tire carcass possibly overloading contact patch (aka my snap oversteer comment earlier)

Having thought about it a bit more, why no rear sway and have you thought about travel limiters for the rear shock like the GT used to use or shorter stroke shocks in the rear?

Posted by: yeahmag Dec 4 2018, 01:22 PM

Please! Ask away! Debate!

So, we are having rear traction issues due to the car rolling around in the rear. The rear is rolling over enough to cause issues including the front wheel way off the ground, all the time, and (we think) rolling over the outside rear tire enough to break traction.

Traditionally, autocross cars don't run a rear bar unless they have an LSD (which I do not). It's our opinion (my co driver is a hot shoe and worked for HRG as a tech and knows more about suspension theory than I ever will) that we need more spring to help plant the rear and stop the excessive tripoding.

I personally think that I will actually distribute the weight overall better. If I do end up defeating some absolute traction at the back, I feel the rear end will be more predictable than what it is now. It's really sloppy back there now...

We ran a ton of front bar to try and combat this (working the diagonals), but have reached the limit of what that can do.

Posted by: eeyore Dec 4 2018, 05:32 PM

Do you have oversteer or understeer with the current setup?
Adjustable sway bar?

Posted by: brilliantrot Dec 5 2018, 02:33 AM

I'm the co-driver. The simple answer is both.

To make the car drivable and faster we pretty much maxed out the adjustable 21mm front bar. The car has some push now but that was the only way to keep the rear end behind the car in sweepers and put down power as the car has roll oversteer also. Tightening up the front bar resulted in pulling front wheels close to 6" in the air as the rear is still rolling a lot as we got rid of most of the droop in the front by doing this.

I would like to play with shocks to get things dialed in more but that's not on the table as an option right now. I think 300lb/in from 225 is too big of a jump and think the Konis will have a hard time controlling the rebound but who knows, I do like a loose car as long as it is predictably so which it was with a looser front bar, it just wasn't all that fast as it wouldn't put power down.

The car is fast as I have gotten the top PAX time in it the last two events we have run, it just could be even faster.

https://www.youtube.com/watch?v=3hRf0cg7RNU

Posted by: sixnotfour Dec 5 2018, 02:52 AM

22 mm torsion bars
21 mm sway bar

wow like driving a Camaro

Posted by: ChrisFoley Dec 5 2018, 06:08 AM

If you add that much rear spring you may run into the other problem - rear wheel spin when trying to apply power coming out of a corner.
I would try 250 lb rate first.

Posted by: Andyrew Dec 5 2018, 10:13 AM

[quote name='brilliantrot' date='Dec 5 2018, 12:33 AM' post='2671195']
[quote name='eeyore' post='2671049' date='Dec 4 2018, 03:32 PM']

https://www.youtube.com/watch?v=3hRf0cg7RNU
[/quote]


Damn I wish my car turned in like that!! Quick ratio rack or anything?

Posted by: yeahmag Dec 5 2018, 12:34 PM

It does, but that really only helps on things like hairpin turns. It does noticeably speed up the steering when you are playing "catch and release" with the back end.

I think what you are seeing more than anything is just a well set up car ;-)

Posted by: PlaysWithCars Dec 7 2018, 12:00 AM

I'm running the same spring rates that you are and considering going stiffer. The jump to 300# does seem mighty big though. I was targeting something more in the 250#, 275# range.

Posted by: eeyore Dec 12 2018, 09:39 AM

This problem is similar to the issues JP and Britain had with the Sh!tbox.

As for the original question, there have been much back-and-forthing about benefits or necessity of welding & reinforcing shock towers.

Try bigger springs for an AX or two. If they do what you want and stuff starts to tear, weld the towers.