Suspension setup

I am in the position of setting my RF GT40 onto its wheels
for the first time and I am interested in hearing from finished projects on where they initally set their spring rates and shock absorber settings. I understand that no two cars will perform to the same standards with identical settings but would like to have a road setting to start with. :
Also would like advise on sucessful methods that have been achieved to correct understear or oversteer.

Glayne

Glayne,
Although mine isn't an RF Forty, (scratch built monocoque), and therefore
will use different spring rates and settings, the process of tuning the suspension
will have similarities. Like you, I was interested in road settings as a track set-up
is considerably different. I started with fairly soft springs, 200lbs/in front;
250 in the rear based on my calculations for equalising roll resistance front and rear.
I could have gone to much stiffer springs front and rear but didn't want to feel
every bump and crack in the road. I use 12 position Leda shocks which
I set to mid position to start. With this set up, the initial behaviour was
noticable understeer. I then connected up the front 5/8" anti-roll bar and this transformed
the handling (improved turn-in and in introduced a touch of oversteer).
I then connected up the rear 3/4" rear bar. This appeared to give better traction
out of corners. Although I was pretty happy with the improvements in handling, I noticed
a slight pendulum effect when overtaking cars (sharp left-right-right-left manoeuvres).
At first I thought this could be due to shocks being too soft. I increased settings to
near max stiffness and tried again. Although this improved things a bit, the basic
feeling was still there. At that point I went back and checked the rear
suspension alignment. Rear toe-in was set to 0.1deg at nominal ride height. I then
measured the rear wheel roll-steer by removing the rear springs and jacking up
the suspension. I found that toe out increased with suspension compression to a point
where there was 0.1 deg toe out. I lowered the front of the lower trailing arm slightly
until the roll steer was minimised. I tried again and was pleased to find that the pendulum
effect was gone.
Although this is my own specific experience, it proved to me how important suspension
set-up is, and how small changes can greatly effect handling. Considering that things like
rear tyre size, nominal ground clearance can vary from customer to customer it is
important to take the time and check things carefully.

Trevor - Thank you for your reply. - You have answered all aspects of my question with the information I need to initally complete the first setup. To get the set up to as one would like, I guess is trail and error. I will set the spring rates and shock absorbers all in a mid range position and try out from there.
I am still interested in learning from others as to counter the efects of oversteer and understeer if I do have this problem. By stiffning the front or rear spring rates, what effect will this do to the handling.
As you can tell I am a novice with suspensions and chassis balancing and wondered if someone could advise on a technical manual or book on this subject
Regards Glayne

I would highly recommend any and all of Carrol Smiths books, the "to win" series. They have a wealth of information for race car preparation and tuning.

Another one is How to Make Your Car Handle by Fred Puhn.

In general, adding stiffness to the front end will result in more understeer. However, with cars that have a lot of camber change as a result of body roll, adding a front bar can actually decrease understeer. Stiffening up the rear reduces understerr/promotes oversteer. Toe-in at the front or rear improves high-speed stability but will tend to cause understeer. Toe-out is the opposite: high-speed instability and oversteer during transitional cornering...

glane,
I might just add two more comments to Steve's good advice.

Firstly, there is a section on wheel alignment under the "Suspension" chapter of the RF manual. Its a good place to start.

Secondly, make sure that you make one change at a time and record the change and the effect. That way you will know what change had what effect and you can go back to the "better" setting if you end up getting lost.

[ QUOTE ]
I found that toe out increased with suspension compression to a point where there was 0.1 deg toe out. I lowered the front of the lower trailing arm slightly
until the roll steer was minimised. I tried again and was pleased to find that the pendulum effect was gone.

[/ QUOTE ]

Bump-steering any track-driven car with an independent rear is pretty important. If that outside rear starts toe-ing out as it loads up in a corner, it totally upsets the car. Then you start thinking 'oversteer' and start chasing shocks, springs and bars, or changing static toe to compensate, which introduces more scrub. If you can't get rid of bumpsteer completely, you're looking for a wee bit of toe-in. They make affordable guages in the $100+ range with dial indicators which makes it pretty easy to do once you get the process down. We shoot for <=.01" of toe-in at 1" of bump. I wrote an excel spreadsheet that lets me input the readings on the guages, and excel decides which number to subtract from which. Then I paste the data for each setting change to another worksheet which does the graphs. I can email this if anyone cares to use it.

Also, with spring choice, you can't go too far wrong if you select springs in a ratio to your front/rear weight distribution.

How stiff you go with the bars depends a bit on how much static camber you're running. If the bars are too stiff, you won't get as much roll, and you won't be able to use lots of negative camber if you have it.

My advice is to take tire temps religiously after every session, and use a pyrometer with a probe, not an infrared one. The three measurements across the tread definitely tell you how efficiently you're heating the tires across the contact patch. I also believe if you add up the three measurements for each tire, and compare the four tires, you will see patterns of how hard you're working one corner or one end of the car, and this will help you decide on what to attack when making changes.

My $0.02. Have fun.

Geoff

Geoff,

One other point that some people may not appreciate is that for the reverse A-arm, radius arm type of geometry used, it does not allow for perfect roll-steer control over the complete range of suspension movement, and therefore minimization of roll-steer can only be achieved over a portion of the suspension travel.
I decided to minimize roll-steer from static to 2" suspension compression (mearured at the lower outboard pickup point). I wrote a computer program to analyse the 3D movement of the rear suspension, and found a lot of subtleties.
Certainly any changes to rear tire diameter and adjustments to ground clearance meant that the roll-steer "sweet spot" can move relative to the static position. Resetting the static toe-in for the new ride height does not solve the whole problem. This requires, for example, vertical adjustment at the front of the lower trailing (radius) arm, although there are a number of other ways rear roll steer can be adjusted.
After making these adjustments, I found roll-steer measurements were in good agreement with the computer predictions, and testing proved that the transitional over-steer or 'pendulum effect' I had experienced was now gone.
The good and bad thing about the reversed A-arm design is that this design will allow you a range of toe-in/out curves, but it requires careful set-up and measurement to make sure you are not chasing your tail when setting up the car handling. /ubbthreads/images/graemlins/grin.gif

I am thoroughly impressed at the level of knowledge of you two! I feel like I should pack it in and go watch the t.v.!

GeoffP

I would like to have a copy of this spreadsheet. E-mail to cavgt40_at_cox.net

Thanks
Gary

Geoff,

That spreadsheet does sound good. I would love a copy! /ubbthreads/images/graemlins/grin.gif

email: [email protected]

Thanks in advance,

Regards,

J.P

I'll send the file along tonight. I'll leave the data from my last round in there as an example.

Hi Geoff
Please include me in your posting too.

[email protected]

many thanks /ubbthreads/images/graemlins/smile.gif

Geoff,

Please send me a copy of your spread sheet. [email protected] Thank You

I put the file up on my ftp space for anyone who is interested:

http://users.rcn.com/gpinello/bumpsteer.xls

Geoff,

What units are you using in the chart?

Bob,
They are units of .001" read off of the dial indicators-Geoff

Just re-reading this post, and wondered if anyone had considered parallel link arms at the lower rear instead of the more typical A arm. I read in Motorsprt recently that this was used on some cars as it completely removed rear wheel toe in issues under suspension travel movements although a static setting is easily possible. I then happened to see it on a car and again the thought occurred to me whether this could be useable on a 40. The GTD has a weak area in rear wheel steering if not setup accurately. Could this be used to overcome it? What do people think?

Malcolm

Hi Malcolm, I have that type of set up on my Lola, and it reputedly does just that. I imagine that applying that type of system to the GTD would not be that difficult, though it would nee some chassis revision.

regards, Mike

Given a free rein, this is what I designed for our 427SC.



We stuck with the original design for our GT, with some minor changes for better street behavior.