New Free On-line Training Videos......
 
"Every Racer's Guide to Suspension Tuning"
 Recommended for every one interested in handling, especially for racing and high performance
 
Dale Thompson, December 2015, www.suspensionsetup.info


 

Shock Absorber Tuning - Our Model for Tuning for Balance with Shocks.  (All the information below is superseded)

I think it is possible to draw a distinction between two types of shock adjustments you might make.  The change is either for:
(a) Response, control of the chassis platform, and/or "wheel control"
(b) Balance. 

And we should probably add another.....
(c) Specific insights from dynoing and working with shocks.
We should be aware that some changes we make under (a) can change balance.

There is general agreement amongst racing people about the aims of shock tuning for response, control of the chassis platform and tyre compliance.  We want quick response, but not too quick for the driver to handle, good control of the chassis platform, and good tyre compliance.  We could consider adjustments to rebound particularly, but also bump, to improve response in initial turn in, improve power down, and control the car in roll, dive and squat.  These are the most important shock adjustments.  A general procedure for baselining shock settings, as recommended by most racing shock manufacturers, is shown at the bottom of this page.

Smithees Shock Tuning for Balance Procedure:            

With shock tuning for balance, you are choosing a particular transient (corner phase), a movement of the car, where you can wedge or de-wedge the car, sometimes to increase overall grip and at other times, to make the car more progressive for the driver.  For example, you may want to improve turn in, or reduce the onset of oversteer on corner exit.  Note that improving one corner phase may influence another corner phase for the better, or worse.

1.  Draw a diagram of your corner phase showing where the weight is going - an arrow from the shock(s) in rebound to the shock(s) in bump. 

2.  Consider the affect of weight transfer for the front wheel pair first, and then the rear wheel pair seperately.  Is the weight (as indicated by the arrow on your diagram) moving towards the inside wheel, or the outside wheel?  This will tell you if you are adding to inside percentage, or decreasing inside percentage, for each of front and rear wheel pairs. 

3.  Consider whether the increasing or decreasing inside percentages at front and rear are adding to your aim of wedging or de-wedging the car, or subtracting from it.

4.   If the weight movement is helpfull, stiffen that shock (or shocks). This will speed up the weight transfer.

(Note added after Claude Rouelle Seminar, 2004)  This a complete error.  Stiffer shock will slowdown weight transfer)

If the weight movement is in the wrong direction, soften that shock (or shocks). This will delay the weight transfer.   For road racing, we change both front and/or both rear shocks so that the car behaves the same for RH & LH corners.

We know it is inside weight percentage, front vs rear, that affects balance.  So for the diagonal weight transfers, if we soften one corner, we always stiffen the opposing corner, and vice versa.   There are any number movements of the race car which you could influence using our tuning procedure.  As with all suspension tuning, the chassis must be stiff enough to allow the loads to build, and the shocks need to be in a tuneable range.

Shock Tuning Concepts           

 

1.  With shock tuning for balance, we are wedging and de-weding the car (or leaving wedge unchanged),  the shocks are only applying loads while they are moving in bump or re-bound (transients).

Too hard to say what happens.  Weight transfer is slowed, tyre load peak may be reduced but will be spread over a longer period.

2. The stiffer shock always transfers weight faster than the softer shock, in both bump and rebound.  

Wrong

If a shock is bound up ie shock stiffness goes to infinity, weight transfer is almost instantaneous, like hitting a hard bump stop. If the shocks have failed completely ie shock stiffness is zero, weight transfer is slow and spongy (and of course the chassis hunts around in an uncontrolled manner).

The effect of a shock hitting full droop is interesting. Any weight left to go must instantly be transferred to the other side.  At the rear this could cause snap oversteer, and lack of tyre compliance on the inside rear wheel for acceleration. All bad. So why would we do it on the front - the so called "zero droop" front suspension?    There are a couple of positive effects I can think of.  The car would stay jacked down at the front, helping aerodynamics (less air under the car), and negative camber in droop would be reduced, helping grip on the inside tyre. 

3.  The stiffer shocked wheel pair will always transfer more weight than the softer shocked wheel pair (just as for wheel pair stiffness from springs, bars and suspension geometry).  Each shock adds to wheel pair stiffness, whether in bump or rebound

4.   A stiffer shock transfers the same amount of  weight as a softer shock.  Think of it like this.  As weight is transferred in the car, and the chassis rolls or pitches, the tyre load builds or reduces depending on the springs and bars. 

5.   In the last phase of roll or pitch, the stiff shock in rebound is still be moving, while a softer shock would have already stopped.  The chassis is still moving.  Won't the stiff shock still be transferring weight?   

Yes, it will

6.   "Less front rebound allows for a greater amount of weight transfer to the rear under acceleration."    The same amount of weight will transfer for stiff or soft rebound in the front. 

Yes, but it will be faster weight transfer with softer rebound

So why would drag cars go for very soft rebound in the front?  I guess it would be to pitch the car high in the front, so as to overshoot the front springs in extension.  This, along with some pro-lift in the rear suspension, might lift the centre of gravity, and this will transfer more weight.  The exagerated chassis movement might also create some inertia to help rear tyre loadings.   I think they call all this "wrapping the tyres".  None of it applies, except at the drag strip.

7.  Let's very quickly revise weight transfer in pitch.  Most racing people understand weight transfer in roll well enough.  Pitch is the same as roll turned around 90 degrees.  So, instead of front and rear wheel pair stiffness, the amount of weight transfer will be in proportion to RH and LH wheel pair stiffness.  For road racing, RH and LH wheel pair stiffness will be the same.  So weight transfer in pitch is 50-50 each side.   The car is not wedged.  For asymetrical speedway set ups, if the car is RH stiff, the RHS side transfers a greater percentage of the weight transfer. The car is wedged in forward pitch, and de-wedged in rearward pitch (in relation to LH turns).   If the car is LH stiff, the car is de-wedged in forward pitch and wedged in rearward pitch.

8.  We need to work with the shocks showing the greatest movement, otherwise the effect we are after will be cancelled out.  This might be determined by driver description, track side observation, or from data acquisition.

Here's one easy tuning adjustment that always works.  Where the whole car unloads over a rise in the track, all 4 corners go into rebound.  For instance, as you come off the dogleg at Oran Park, the car goes light and you are still turning left.  (Too many Konica V8 Lite Supercars have looses here.)  Set your front low speed rebound stiffer at the front than the rear.  The stiffer front end will unload the most.  So our car will be momentarily wedged (tighter).  If shock settings were stiffer at the rear, the car would momentarily de-wedge (or be loose).

If it were possible to work out which end of the car was moving the most, either by observation or on-board data, you could change one end only. 

 

More on Shock Tuning:

The shocks must be in sync with your springs.  If you increase your spring rate you probably decrease shock bump, and increase shock rebound, and vice versa.  You need shock dyno graphs to show that your shock adjusters are working as expected, and matched between shock pairs.   You will probably want a fair bit of "nose" on your graphs, irrespective of whether you have low speed adjustment or not.  This is to give you good low speed control over the chassis platform.   As a result, you may be able to run softer springs to increase tyre compliance with the road.

Your bump curve on the dyno graph will probably be digressive ie flatten out with increasing shaft velocity, so the car is not too harsh.  Your rebound curve should show increasing force pretty much in proportion to shaft velocity, and only digress at higher shaft velocities.  You will want as much rebound as you can, short of jacking the car down excessively, lifting wheels clear of the road, or where too much rebound affects power down.

Hopefully, your car will work over a reasonable range of shock settings.  Only then could you consider using shock adjustments for balance, as per (b).

 

PLEASE READ THIS.....
There is a huge caveat on all of this as it relates to shock tuning - you must work on the shocks with sufficiently greater movement for it to work.
Also the driver must be consistent with the way he balances the car as he drives, otherwise the movements of the car may be different. 

Speedway teams are at less risk of getting it wrong.   They only need to change one side at either front and/or rear.

Consider the following table from the Carrerra Shocks website:-

Volume 2   Issue 1

Fall 1999

Page 4 of 4


Tech Tips

If you only run at one track, you’ve probably got your set-up down pat. However, if you travel to different tracks getting your car handling well, quickly, is a must if you want to win. 
Here are a few tips to help you get close.

 

from the
pros! 

ASPHALT HANDLING TIPS

• Short Tracks:  Softer all the way around.
• Flat or Tight Turn Tracks:  Softer rears.
• High Bank Tracks:  One step stiffer on RF & RR.
• Push Going In:  Softer compression on RF or 
stiffer rebound on LR.
• Loose Coming Out:  Softer RR or stiffer rebound on LF.
• Loose Going In:  Softer rebound on LR or stiffer RF.