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post #1 of 14 (permalink) Old 04-13-2015, 05:20 AM Thread Starter
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Suspension Tuning

I have a 2011 Exige that I run Hoosier A7 205s on the front and 225s on the rear. The springs are half way between stock and what BWR recommends. The shocks are DA Ohlins and have been re-valved to give more compression and rebound. The ride heights are around 125mm Front and 130mm rear. The diagonal in the car is 49.95% with left side at 51% and rear at 60%. Front camber is about maxed out around 2.7 deg and the rear is about 3 deg. Rear is also toed in about 3/32. I have BWRs road race sway bar on the car.

So here is the problem; when the sway bar is on the middle set of holes, the car has a push on road race tracks and low to medium speed autoX turns. The car is planted when going through solemns. When I soften the sway bar one setting, the push goes away on road race courses and is very nimble and responsive on autoX turns. The problems is that the rear becomes unglued when going through solemns or high speed transition.

I am looking for some advice on how to solve this issue.

Thanks,
Eldon
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post #2 of 14 (permalink) Old 04-13-2015, 06:13 AM
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I would suggest beginning with taking tire temps after running hot laps on the track. Get inside/middle/outside of each tire with a probe pyrometer. This will help to determine if your toe/camber/tire pressure is set proper for those particular springs and ride height.

The more data you can gather, the easier it will be to diagnose...

Also, Do you have any additional aero on the car, or is it stock?



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post #3 of 14 (permalink) Old 04-13-2015, 06:44 AM
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re: Suspension tuning

1. Adjusting the sway bar as you did moves the handling balance from understeer to oversteer,as you have experienced.

2. If you have reached the limit of adhesion on the tire, that is what you get. Adjust to the balance that you like/go faster. You are done with this set-up.

3. Taking tire temps would indicate if you are or are not at the limit, as in (2).

4. 3 degrees of camber in the back seems a little too much to me, but I run bigger tire. The bigger the tire the less camber required... Camber in the rear is not good for traction. It is a compromise to compensate for body roll.

5. More rebound or compression dampening is not the best for traction... (especially compression). Do you have the optimum for your spring/tire/bar/track combo?

6. AutoX set-ups are never best for the (bigger) track. Always too twitchy.

7. You can try a little toe-in in the front and less bar... Tire temps would be key here.

8. My favorite pet subject where I go against the common advice on the forums, is that the cars fall-over in the back and take a set after a lot of time has elapsed. Bigger front bar is a crutch to get around this problem. I subscribe to the old soft-spring-stiff-bar theory. So a rear bar with a proper front bar will make the chassis react faster. Rear bar without changing the front will decrease rear traction. The trick is to achieve a degree of balance and adjustment, since the ratio of rear to front roll stiffness (bars) determines the overall balance and the limit. See new V6 cars. I would try this as a last resort...


You need to get the tire temps before trying anything. Without tire temps these are philosophical general discussion.


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Quote:
Originally Posted by eldonz View Post
I have a 2011 Exige that I run Hoosier A7 205s on the front and 225s on the rear. The springs are half way between stock and what BWR recommends. The shocks are DA Ohlins and have been re-valved to give more compression and rebound. The ride heights are around 125mm Front and 130mm rear. The diagonal in the car is 49.95% with left side at 51% and rear at 60%. Front camber is about maxed out around 2.7 deg and the rear is about 3 deg. Rear is also toed in about 3/32. I have BWRs road race sway bar on the car.

So here is the problem; when the sway bar is on the middle set of holes, the car has a push on road race tracks and low to medium speed autoX turns. The car is planted when going through solemns. When I soften the sway bar one setting, the push goes away on road race courses and is very nimble and responsive on autoX turns. The problems is that the rear becomes unglued when going through solemns or high speed transition.

I am looking for some advice on how to solve this issue.

Thanks,
Eldon
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post #4 of 14 (permalink) Old 04-13-2015, 03:29 PM
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If you can break up your symptoms in terms of corner entrance, mid corner and corner exit that would be more helpful in terms of diagnosis or changes to make.

With the softer front bar, you say "the push goes away". I'm going to guess that's a steady state improvement. You also say it's loose in transitions, which are more akin to the corner entrance phase. Based on this evidence, I would recommend the following:

Increase rear toe in
Increase front compression damping
Increase rear rebound damping

The suggested damping changes will reduce the rate of weight transfer from the front to the rear, slowing how quickly grip builds. Using compression will do by supporting the chassis. Using rebound will do so at the sacrifice of overall grip. Taking either adjustment too far will likely cause loss of traction and other handling issues.

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Last edited by jdawson; 04-13-2015 at 04:12 PM.
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post #5 of 14 (permalink) Old 04-14-2015, 12:24 PM
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Put it on the nimble setting with the swaybar and then double or more your rear toe-in.

Enjoy

Also, Jake understands shocks.
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post #6 of 14 (permalink) Old 04-14-2015, 06:43 PM
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Quote:
Originally Posted by jdawson View Post
If you can break up your symptoms in terms of corner entrance, mid corner and corner exit that would be more helpful in terms of diagnosis or changes to make.

With the softer front bar, you say "the push goes away". I'm going to guess that's a steady state improvement. You also say it's loose in transitions, which are more akin to the corner entrance phase. Based on this evidence, I would recommend the following:

Increase rear toe in
Increase front compression damping
Increase rear rebound damping

The suggested damping changes will reduce the rate of weight transfer from the front to the rear, slowing how quickly grip builds. Using compression will do by supporting the chassis.
...
Taking either adjustment too far will likely cause loss of traction and other handling issues.
^This^
(Sort out the medium speed "steady-state" first.)

I would suggest starting with writing down the dampers settings.
Then backing off to get to where you want to be.

It is normal for a car to understeer at lower speeds and oversteer at higher speeds. Usually trail-braking deeper at lower speeds is a driver workaround.
If you optimize it for lower speeds it is usually too sharp at higher speeds.


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Originally Posted by jdawson View Post
... Using compression will do by supporting the chassis. Using rebound will do so at the sacrifice of overall grip.
...
I am not sure about that. I thought lower rebound increases overall grip?
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post #7 of 14 (permalink) Old 04-15-2015, 05:44 AM
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Holmz, I think you and Jacob are saying the same thing about rebound.

What do you mean when you say the rear becomes unglued in high speed transitions? Is it too slow to transition from one side to the other and then snaps loose? Do you see this effect in long high speed sweepers or only in transition?


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post #8 of 14 (permalink) Old 04-15-2015, 07:39 AM
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Quote:
Originally Posted by holmz View Post
I am not sure about that. I thought lower rebound increases overall grip?
We are in agreement. I meant using _more_ rebound to control the platform will work, but it will do so at the sacrifice of grip.

2006 Lotus Elise
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post #9 of 14 (permalink) Old 04-15-2015, 07:56 AM
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Quote:
Originally Posted by jdawson View Post
We are in agreement. I meant using _more_ rebound to control the platform will work, but it will do so at the sacrifice of grip.
Yeah we are in sync. I liked your assessment
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post #10 of 14 (permalink) Old 04-15-2015, 08:25 AM
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In the last year, I made a decision to actually learn how shocks are used to tune weight transfer. I've understood how they actually work for quite a while and have rebuilt a few shocks... but knowing when to apply more or less damping is more involved than learning about the mechanics of the shock. There is also a lot of misinformation out there, so I had to unlearn a lot of forum/internet garbage and start fresh with damper tuning. In the off season, I've been working with a local FSAE team, turning damper knobs every chance I get and listening to their feedback. It's been interesting, to say the least. Softening rebound has consistently yielded a positive response from the drivers.

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post #11 of 14 (permalink) Old 04-15-2015, 12:44 PM
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Quote:
Originally Posted by jdawson View Post
In the last year, I made a decision to actually learn how shocks are used to tune weight transfer. I've understood how they actually work for quite a while and have rebuilt a few shocks... but knowing when to apply more or less damping is more involved than learning about the mechanics of the shock. There is also a lot of misinformation out there, so I had to unlearn a lot of forum/internet garbage and start fresh with damper tuning. In the off season, I've been working with a local FSAE team, turning damper knobs every chance I get and listening to their feedback. It's been interesting, to say the least. Softening rebound has consistently yielded a positive response from the drivers.


Yes!

Although I would change the wording to be "reducing rebound damping" rather than softening, because it is counter-intuitive since people tend to think that a "stiffer" suspension handles better. Too much rebound damping just prevents the tire from following the road, and prevents the mass of the car returning to center from large weight transfers. It really just fights against the compression damping, and together they would prevent the suspension from moving very fast at all.

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post #12 of 14 (permalink) Old 04-15-2015, 03:31 PM Thread Starter
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Quote:
Originally Posted by fzust View Post
Holmz, I think you and Jacob are saying the same thing about rebound.

What do you mean when you say the rear becomes unglued in high speed transitions? Is it too slow to transition from one side to the other and then snaps loose? Do you see this effect in long high speed sweepers or only in transition?
Going through a solemn, the rear will stay under control for about two turns. After that, the next turn, the rear will break traction and start to slide thus broadsiding the cone. When the sway bar is stiffer, it appears that the bar helps hold weight on the inside rear tire. Thus, you can carry very high speeds through the solemn.

When traversing a sweeper, the rear feels as if it wants to break loose with the bar in the softer setting. The same feeling is present on high speed sweepers on a road race courses and causes you to not maintain acceleration.

Before stiffening the front bar, I started to dial out rear rebound and this helped. The problem is that the shocks are missing the valving to separate compression from rebound. So, as I dial out rebound I loose the nose of the compression at low speed. This is going to get corrected, they just did not have the needles the last time I had the shocks off.

My feeling is that the car likes the softer roll stiffness of the front but lacks rear roll stiffness. Normally, I would fix this by working with a rear bar. Forward traction is extremely good to the point that it is difficult to break traction when coming out of a corner. The thought was maybe the car has too much bump steer in the rear but after looking at the geometry, I do not believe this is the case. The rear toe links have been upgraded to Sector 111's setup.

Before changing from the stock springs, the sway bar was run at the current stiffer setting. After shooting some video of the suspension travels, it was noted that the car would carry the inside front while exiting the corners without picking up a push. The car was also fairly stable through the solemns. Also, the comment about 3 degs being too much rear camber was put to rest after watching the outside rear tire on a sweeper. It flattened the tire and you could see the distortion of the side wall.

I am sure I can find a way to soften the behavior of the car but I would rather do this without hurting the other end. That is why I am asking for suggestions because I am not that familiar with setting up a mid engine car. My experience is with front engine, rear wheel drive.

Thanks,
Eldon
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post #13 of 14 (permalink) Old 04-15-2015, 04:38 PM
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I have ohlins Da on my car. At first the rear felt floaty and loose on track in high speed corners, started adding more rebound . the problem then was on bumpy corners the rear would feel like it was collapsing. Too much rebound damping and the car was jacking itself down.

So went to an adjustable swaybar at the advice of Fred at BWR. Now I can use less rebound and the stiffer bar keeps the rear planted. But now I have the same the problem understeer. Course an understeery car is easier to drive, but the front tires now run down to the cords pretty quickly.

Now that the rear feel Ok, will try less compression to get some more front gripin the front and maybe more negative camber. I have no toe in the front and half a degree rear,
Car is corner balanced and RH is 188mm in front 122 rear.
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post #14 of 14 (permalink) Old 04-15-2015, 08:49 PM
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Quote:
Originally Posted by jdawson View Post
In the last year, I made a decision to actually learn how shocks are used to tune weight transfer. I've understood how they actually work for quite a while and have rebuilt a few shocks... but knowing when to apply more or less damping is more involved than learning about the mechanics of the shock. There is also a lot of misinformation out there, so I had to unlearn a lot of forum/internet garbage and start fresh with damper tuning. In the off season, I've been working with a local FSAE team, turning damper knobs every chance I get and listening to their feedback. It's been interesting, to say the least. Softening rebound has consistently yielded a positive response from the drivers.
Yeah - softer is often a good place to start.

I have some strain gauges to install, as well as linear potentiometers. Sometimes a data driven approach helps... But I do not have the data yet, so I cannot yet recommend it with authority at this point in time.
It sounds like you are clued-in so maybe this is worthwhile to mention.
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