Rear end bouncing

[mattxr2]

Wondering if anyone has any ideas on why the rear of my car bounces badly over bumpy surfaces

I'm running 500lb front 650lb rear standard arbs with power flex bushes and gaz coil overs, rear suspension is on a soft setting. Rear camber is at 0.5deg neg and fairly low

I used to run 400lb front and rear with the shocks a bit stiffer and the wheels would leave the floor

Any suggestions to solve this? It's perfect on smooth tracks but a handle full if a bit rough

[Demlotcrew]

Shocks are valved too high on the rebound.

[mattxr2]

How do I adjust this ?

[Demlotcrew]

It depends on the damper, if you have adjustables you would tweak the high frequency.

Otherwise dampers have to go back to manufacture for re valving.

[mattxr2]

They are gaz gha, thanks for the input. I'm assuming they will have to go back

[400motor]

Did you try adjusting rear dampers to stiffer settings, is there any difference? It sounds like your dampers can't control the springs leading to bounce.....

[mattxr2]

Not yet, I was running them soft to get as much grip as possible. Might try some 600lb springs and adjust the dampening. Just trying to keep the car balanced. Think it's time to invest in some uprated away bars

[Altrezia]

I agree with Demlotcrew - your valving isn't right. Gaz can re-do it for about £100 per pair.

[mattxr2]

The shocks are currently 5 clicks from the lowest setting, I'll set them at the mid point and see if it still bounces on the road.

Any views on springs/sway bars I'm getting a lot of body roll I want to get rid off but not loose grip

[mattxr2]

Set them to 25 clicks and it better but still bounces on acceleration, going to try 600lb springs

[stevesingo]

Are your rear springs in the stock location or mouted concentric to the damper, i.e coil over?

What is imprtant is the rate at the wheel and not the spring rate itself.

The Motion Ratio is required to work out the wheel rate.

Fnt 0.926
Rr 0.67

The wheel rate/spring rate is MR (motion ratio) squared.
That multiplied by the spring rate is the actual wheel rate, spring force on the wheel.

If we use the E30 M3 as an example

0.926^2 = 0.86
0.86 * 100 = 86lbs/in effectively on the wheel with a 100lbs spring on the front.

0.67^2 = 0.45
0.45 * 300 = 135lbs/in effectively on the wheel with a 300lbs spring on the rear.

The ratio between front/rear 1.57:1 higher spring rate on the rear.

I would suggest you don't want to stray too far from the stock ratio fnt/rr unless you have made a significan change in weight distribution.

Using the same motion ratios applied to your spring rates in the stock loacation

So for example 0.926^2 = 0.86
0.86 * 500 = 428lbs/in effectively on the wheel

So for example 0.67^2 = 0.45
0.45 * 650 = 291lbs/in effectively on the wheel

The ratio between front/rear 0.68:1, so much softer spring rate on the rear.

If you have them mounted in the coil over position, then the rear motion ratio is 1.06:1

Example

So for example 0.926^2 = 0.86
0.86 * 500 = 428lbs/in effectively on the wheel

So for example 1.06^2 = 1.12
1.12 * 650 = 730 lbs/in effectively on the wheel

The ratio between front/rear 1.70:1 higher spring rate on the rear.

A 600lb/in rear spring will give the following wheel rates:

Stock location 269 lb/in Ratio of 0.63
Coil Over 674 lb/in Ratio of 1.57

Once you have the spring rates correct, you can then look at damping. An unscientific way of doing this is to use a Go-Pro mounted to look at your whees.

[mattxr2]

Thanks for the information, it's somthing I'll have to look into more using your advice

I spoke to gaz and basicly I was using specs from cars with an inbound spring so I was way over. Currently 650lb front and 400lb rear on a true coilover. Stock arbs

They told me to max the front gha shocks on the dampening to control the spring as there at there limit. Rears are 20 clicks up and it feels pretty good and is gripping well atm

[stevesingo]

I make the wheel rates to be...

Fr- 557lb/in
Rr- 449lb/in

A ratio of 0.81:1 to the rear, so proportionally much softer on the rear.

This seems to be where the GpN cars went.

From the Grp N catalog.

Front:

31 33 2 222 655: 685lb/in (120 N)
31 33 2 222 801: 799lb/in (140 N)
31 33 2 222 802: 913lb/in (160 N)
31 33 2 222 803: 1027lb/in (180 N)
31 33 2 222 654: 1141lb/in (200 N)
31 33 2 222 804: 1255lb/in (220 N)
31 33 2 222 805: 1369lb/in (240 N)

Shocks: 300/300

Rear:

33 53 2 222 699: 1027lb/in (180 N)
33 53 2 222 530: 1141lb/in (200 N)
33 53 2 222 531: 1255lb/in (220 N)
33 53 2 222 532: 1369lb/in (240 N)
33 53 2 222 555: 1483lb/in (260 N)
33 53 2 222 556: 1597lb/in (280 N)
33 53 2 222 557: 1711lb/in (300 N)
33 53 2 222 558: 1825lb/in (320 N)

Shocks: 200/200

These springs are mounted in the stock location.

Taking the softest springs wheel rates

Wheel rates

Fr- 586 lb/in
Rr- 460 lb/in

Ratio-0.79

Taking the stiffest springs wheel rates

Fr- 1173 lb/in
Rr- 819 lb/in

Ratio-0.70

It would seem the stiffer they went the softer the rear was in relation to the front.

I don't see why the location of the spring should effect the damper as it is the wheel rate what matters, not the spring rate. A 1000 lb/in spring in the stock loaction gives the same wheel rate as a 400 lb/in spring in the coil over location.

I'm no expert mind you.

some interesting reading here.

http://blackartdynamics.com/Chassis_Art ... prings.php