Wheel Rates - Historic
In looking over my car during the summer break, I found that my front 450# springs are really 500#, which could explain my continual low speed understeer.
This meant I had a front wheel rate of 132, and 136 rear. Current weights per corner are 222# front, 310# rear. Car is a 1973 Hawke DL11.
Before I go off and get some new springs, I thought I'd canvas the experts.
Tyres are treaded Avon ACB10, hard compound.
Thanks in advance.
What have you tried to achieve the balance you want with the springs you have on the car now?
Steve,
I've softened the front bar, got the rear bar on full hard. Changed the bump and rebound valving to softer front, and harder rear.
Made the front upright mounts higher to improve the movement of the front roll centre. Increased the rake.
How much did you change the rake? What is the height of the front roll center?
How does the car handle in the braking zone and then in the transition to mid corner?
It has been my experience that softening the front swaybar does little to fix a mid corner push. The same is true for springs.
I think your spring balance is not correct and that should have been the first thing you tried to improve. I would suggest that you start by increasing the rear spring rate and at the same time lower the rear ride height initially and then play with chassis rake until you get the car the best it can be with that new spring setup. Repeat the spring changes and rake adjustments until you optimize the car. Then play with the swaybars and after that work with the shocks.
Using the shocks to fix a setup issue is not optimal. Playing with the shocks to optimize an otherwise good setup is much more rewarding.
The last post by Steve Lathrop is spot on with my thought based on your numbers. With the Historic cars on harder treaded tires I have always started with the wheel rates being around 60-70% of the corner weight and within 5% of each other front to rear. Obviously just a stating point. So by my quick calc seems your front wheel rate to corner wt ratio is around 59% so in the ball park. Rear is around 44 % so that is where I would increase to start. Once you get springs closer as Steve mentions rake has a huge influence on front/rear balance due to the typically large roll center and CG changes with ride height in the older cars.
FWIW I work to get the balance right with springs and rake then do very minor tuning with bars (which typically are much too small to be of much good on the older cars- especially rear), and never touch the shocks (basically set them to some desired % of critical damping on leave them). Personally haven’t found using shocks as a tuning tool that effective on the older chassis and way more subtle and prone to error on my adjusting then my sensitivity and driving ability. I think if you get the car pretty well balanced with springs and reasonable rake you will be very happy.
There are a couple of guys on the west coast who runs Hawkes and are pretty fast. Maybe they will chime in.Originally Posted by Hawke
Steve, I put about 1" rake. I measured up the front roll centre over Xmas, by drawing it up half sized. However, my son thought that would be a good bit of paper to use to wrap his presents in, so the exact measurements are gone. None the less, the roll center now in lean, is close to the centre line. It was outside the track before.
It handles well in the braking zone, and does not turn in as well as I would like in slow corners. You can just wind on more lock, and not much happens.
Working on shocks is pretty easy for me, as I have my own dyno, and access to all the pistons and shims I need, so I tend to do it a lot.
Thanks for that. I'll get some new springs to come up to those numbers.
I will add one little bit of info from my experience. Every tine you change a spring, you need to recalculate the ride height so that the ratio of the ride height to the wheel rate of the springs remains constant. I will let you figure out the math. But what you winding up with is that as you increase the spring rate, you need to lower the car a proportional amount so that the ride height to spring rate remains constant.
In your case, as you increase the rear spring rate relative to the front, you will see a change in how the car behaves in the entry, mid corner and exit of a corner. If you optimize the rake with some spring spread, you can change one end of he car and get a different balance through out the corner.
Say I have a relative soft rear spring rates, as you are starting with, I would expect that the car would be a bit loose on entrance but by mid corner it would change to a push and them push more as you apply power on the exit. This is due to the rear being too low as the car loads up. Now as you increase the rear spring rate relative to the front, you have to lower the rear of the car or it will just get looser on entrance and maybe balance better mid corner to exit but by that time you will likely be in the grass from the over steer on corner entrance. But get the rake right and the car will be more balanced throughout the corner. Raise the re spring a bit more and you can get the car to go loose as you apply the power.
Question?
As you are changing rake, be it front or rear, do you reset camber each time, or just let it change?
john f
John,
I reset the camber every time.
Therefore changing the ride height and rake. At what point does one say "good enough"?
When the change is less than one's confidence in the accuracy of measurement.
Peter Olivola
(polivola@gmail.com)
Or when a change in either direction makes the handling worse. At that point you have optimized the current setup and it is time to try something different.
Yep.
I usually questioned my order of operations on a new-to-me vehicle because some adjustment put something else out of adjustment. So, just started big and worked towards small.
Yes, I was speaking more or less tongue-in-cheek, knowing that if you set the ride height exactly where you want it and then change camber, the ride height is no longer exactly where you set it. If you set the camber exactly where you want it and then change ride height, camber is no longer exactly where you set it. Obviously the more less camber gain/loss through travel the less noticeable this is.
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