Adjusting Corner Weights ? 5

[Spimage]

I need to set the corner weights on a Cobra replica in order to optimise the handling of the car. Any tips on the proceedure to achieve such, would be appreciated. (The car has coil over adjustable shocks and adjustable torsion bars, both front and rear.

 

[patprimmer]

Put a set of scales with appropriate weight range under each wheel.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[NormPeterson]

If by torsion bars' you mean 'anti-roll bars', disconnect them first. In general, work with all four coilovers a little bit each to adjust the corner weights rather than trying to find a solution by adjusting one or two corners only. Diagonally opposite corners get adjusted in the same direction. Reconnect the anti-roll bars, noting that the endlink lengths will need to be adjusted slightly to avoid screwing up the adjustment that you've just completed. Note that while you can make the adjustments with the bars connected, that requires greater adjustment (the "excess adjustment" that's required only serves to preload the bars).

BTW, you should simulate the driver's weight with ballast in the driver seat. Distribute a little to the footwell, if it's a light car, if you're tuning it for a large driver, or even if you're just being picky. Iron free weight (barbell) plates work well for this (you know their weight, they sit in flat stacks that can be pinned, and they can't rip and leak stuff all over the place).

Norm

 

[sreid]

All of the above answers are good but assume you have scales you can use to measure the corner weights. It might be less expensive to have a race shop do this setup for you.

 

[Spimage]

Hi Guys,
Thanks to all of you for the prompt response, In answer to questions, 1. By 'Torsion Bars' I do mean 'anti-roll bars', 2.Whilst I don't have corner scales, I do have access to such and really would like to do the set-up myself.

 

[MintJulep]

And while you are at it. While you have the car on the scales, don't limit yourself to just setting up what you think is optimum right now. Optimum changes for each track, different tires, etc.

Take the time to document what change has what effect on corner weights. Set yourself up a bunch of nice tables refering to something easily measurable for a reference. That way when you are at the track and don't have time to put the car on scales you can at least make a reasonably well informed adjustment.

 

[sreid]

Since you have access to scales;

1) Make certain the floor is level(this means the floor is a plane). Shims (thin boards, etc.) can be put under the scales to make this so. To check level, tape a, say, 12 foot length of clear tubing to two yard sticks. Fill the tube with colored water (food coloring from your local market-get some junk food while you're at it)to a height that the level can be read on the yard stick. How to use this "tool" is probably self evident.

2) Make ramps to the scales with a flat area of a few feet before the scales. This is because it is advisable to roll/bounce the car after adjustments have been made. There is considerable stiction in the suspension joints.

3) This is probably obvious but set the tire pressures before setting the corner weights.

4) For street/road racing cars, the corner weight are usually set equal side to side. Circle track cars play with "weight jacking".

 

[NormPeterson]

Another reason for #2 above is because most independent suspension arrangements involve lateral scrub of the tires as the ride height varies (that tends to hold its corner of the car at its initial ride height unless permitted to slip). Rolling the car a sufficient distance obviously works, and bouncing works better if you improvise some 'turn plates' with pairs of plastic sheets/sheet metal panels/etc. separated by a thin layer of grease and let the lateral slippage occur there.

Norm

 

[sprintcar]

One other point - Ride height (floor to frame rail) should be equal left to right to maintain suspensioin geometry. There should be dimensions in the builder's manual. This should also tell you the initial heights for front and rear to get the proper rake angle in the chassis. Perfectly equal L-R weight is second to perfect chassis stance, since things like steering gear, etc could offset the mass of the vehicle. Then remember about the 'mass' that will go in the driver's seat!

Oval track race cars are set up by 'blocking' or measuring the frame height first until they are dead on. Once that is done, then small adjustments are made to bias the car for left turns.

Keep the wheels on the ground
Bob
showshine@aol.com