Corner weighting?

[singlecoil]

Just wondering if anyone here might have anything to say on the interesting topic of cornerweighting? Probably in the context of a kit car that does a track day every now and then.

[Milky Bar Kid]

If I were to get my car corner weighted, I would do it in conjunction with getting the camber, toe and castor set up also. To me it's silly to get the corner weights right if the rest is out. I say this even though my car has only been set very, very roughly by eye when I could be bothered. It would probably handle better/be more settled if it were done properly.

[CNHSS1]

it must be done as part of a full setup to be worthwhile. The difference will be well worth it, and it makes all areas of the car work more cohesively. Best money i spend each season

[Jerkins]

My 7-alike's handling was originally pretty good, but I always had a suspicion that all was not quite right - it never felt 'settled', a bit like having 1 tyre a few pounds under pressure. I borrowed a corner weighing gauge from a friend and spent an hour or so getting the weights and ride heights as good as I could manage.

The difference was startling - it now feels a lot more secure on twisty roads.

A decent pro setup shop could get it much, much better I'm sure - but it still feels pretty good!

[Furyous]

Chris Seaman set up my Fury, including corner weighting, the car was transformed from good to truly excellent.

I would say its a must have for any track car.

[gtmdriver]

It's of debatable value in a car where the driver sits off centre and there is the possibility of passengers and luggage.

It may be worth doing if you're having the rest of the suspension set up too.

[Sam_68]

A few really basic points (apologies if these are all obvious, but may be worth pointing out for anyone who googles this thread in future):

• Get the corner weights done with the driver (or equivalent weight) in the driving seat. There's no point in carefully optimising your corner weights without, then adding 90 kilos you haven't accounted for...
• Get your spring rates & free lengths checked before you have it done. There's some production tolerance, so you might find that you have quite a difference in rates between pairs of supposedly identical springs. Most race set-up specialists have the equipment to do this in minutes and will charge a nominal sum, if you remove the springs yourself and take them to be checked.
• Similarly, make sure your ARB's are not twisted or pre-loaded in any way; they shouldn't be offering any resistance at all when the car is static and level, so you can disconnect them while you are doing the actual corner weight checks, but when re-connecting them you should make sure that they are not twisted before re-fitting, then ensure that the lever arm lengths are set absolutely identical and the droplinks adjusted so that there is no pre-load when the car is sitting level (ie. you should be able to re-connect the droplink on one side, then find that the other side lines up exactly when you reconnect it, without having the strain it up or down even slightly).
• You should definitely get a full geo check/set-up at the same time, but do it after the corner weights; no point in setting the static geo then throwing it out of alignment by screwing a corner up or down when you do the weights.
• You will never get perfectly equal weights, front and rear, on a 2-seater, because of the offset driving position.. what you should be aiming for (in most peopple's opinion) is diagonally opposite corners adding up to equal total weight; ie. Front Left + Rear Right = Front Right + Rear Left.
• Beware of friction in suspension joints and bushes, or torsional loading of metalastic bushes, which can introduce false results. 'Bounce' the car between adjustments, to encourage it to 'settle' properly, and ideally you want to slacken off and grease the bolts on any metalastic bushes while you do the checks, to ensure they're not loaded (not forgetting to tighten them back up once you have the corner weights set!).
• It's an iterative process, and there's a bit of a knack to doing it quickly (which I've never managed to acquire, personally, but I've seen people at work who have!), but try to make adjustments on diagonally opposite pairs of wheels at the same time; ie. to increase the corner weight on the Left Rear tyre, you need to screw the spring platform UP on the Left Rear, and DOWN on the Front Right by equal amounts at the same time.

[Sam_68]

gtmdriver said:

It's of debatable value in a car where the driver sits off centre and there is the possibility of passengers and luggage.

There's some truth in that, to be sure, but I've always taken the view that it is immature and irresponsible to be driving at ten-tenths with a passenger, since you are putting the other person at risk in a situation they have no control over; I therefore optimise the corner weights for me as sole driver, and accept that when I'm driving with passengers and luggage I won't be going fast enough for the slight imbalance to matter much.

[singlecoil]

Sam_68 said:

Lots of good clear stuff about corner weighting

Thanks Sam

[blueg33]

My Ginetta G33 was corner weighted just for me. Corner weighting and ride height should be done before geometry, but so full geometry immediately after.

it made a reasonable difference to the ginetta on the track and a small difference in spirited on road driving. When driving on road with a passenger it was still ok but not quite as sharp (mind you he was 17 stone of ex pro rugby player

[Woznaldo]

I've read an article on the net of American origin on corner weighting before attempting to set my own on my R5 GT Turbo, however there were a few issues. In the article it stated that there will be high weight diagonal i.e. if FR is heavy, so to will be RL? When a put my car in a level aircraft hanger at work, zero'd the scales and rolled the car on, the FR was the heavy side and the RR was also heavy. This is with me out of the my RHD car? Is this possible or is there something wrong?

The scales are aircraft weigh scales capable of a max of 25,000lb and may not be accurate enough for what I need? Also, after reading Sam's post above, I did not loosen off any bolts on my rubber bushed wishbones, nor did I disconnect the front roll bar. Could a twist roll bar give me these weird readings?

Obviously, if the scales are out (not accurate enough) then there could be nothing wrong at all. The figures I did get were:

FL: 257 FR: 269

RL: 138 RR: 139

Any help/guidance appreciated,

Woz

[Sam_68]

Woznaldo said:

Is this possible or is there something wrong?

It's certainly possible; if you screwed up the spring abutments on both wheels at one side of the car, it would cause this (think in terms of lifting one side of the car on a trolley jack: the weight would 'lean' onto the other side, simple as that), or if the springs had settled/lost temper on the more heavily laden side of the car over the years.

This is the reason that accurate corner weighting can be so valuable; time to get your car checked on an accurate set of corner weight gauges (yes, scales with that working range are not likely to be deadly accurate at the lower end of the range) and corner weight the car properly, assuming it has adjustable spring seats.

Regarding diagonals; there's an error in my advice above - I should have added that you need to 'weight' the diagonal split according to front:rear weight distribution. I'll add the calculations to show how to do this tonight - I'm just on my way to Mallory Park for the 750 Club meeting at the moment so I'm in a bit of a rush!

Edited by Sam_68 on Sunday 4th April 10:52

[Woznaldo]

Many thanks Sam.

[Sam_68]

Sorry... late back last night.

Further advice on setting diagonals:

First, do a check on the existing corner weights of the car, then you have two options:

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OPTION 1 : The 'intuitive' approach:

As suggested above, simply try to equalise the diagonals.

Lets say you had a car that gave the following figures on first check:
LF = 300 kilos
RF = 360 kilos
LR = 225 kilos
RR = 215 kilos

This car has a heavy diagonal running from RL to FR, with the front being particularly heavy (it helps to draw a quick sketch of the car on plan and write the numbers on to understand this). So in this instance you'd screw diagonally opposite spring seats up/down in pairs to place more weight on the RR--FL diagonal, but bias the adustment slightly to take a bit more of the weight from the front right and move it to the back.

This is the 'intuitive' technique, but you'll rarely get it spot on and it's a bugger to adjust the corner weights and still end up with something like the right ride heights; some people can do it, but you need to develop a 'feel' for when you've got the best compromeise between corner weights and ride heights. Doing it well is akin to being one of those guys who can tune a triple Weber carburettor installation using a bit of rubber hose and a screwdriver...

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Option 2: Calculate target weights:
Do a first check on the corner weights, as above, then;

• Calculate the % weights on the front & rear as decimals (ie. total weight on front axle 60% of weight on front would equal 0.60)
• Calculate % weight on left and right hand side of car as decimals.

For the example weights above, the front/rear decimal %'s would be 0.60 and 0.40; the left/right decimal %'s would be 0.477 and 0.523.

• You can then calculate biased target weights as follows:

Target weight LF = (Total weight) x (decimal % front axle weight) x (decimal % left side weight)
Target weight RF = (total weight) x (decimal % front axle weight) x (decimal % right side weight)
Target weight LR = (Total weight) x (decimal % rear axle weight) x (decimal % left side weight)
Target weight RR = (Total weight) x (decimal % rear axle weight) x (decimal % right side weight)

This method will tend to give you one slightly heavy diagonal, but will put you closer to a flat platform in terms of ride heights first time.

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In terms of disconnecting/adjusting ARB's, it hopefully goes without saying that you can't adjust an ARB to ensure that it has no pre-load unless it has adjustable length drop links. If you don't have this facility, then you're best off leaving the ARB's connected when you do the corner weighting, so that the pre-load is taken into account.

Similarly, you may be limited by other non-adjustable features on your car; for eample, I'm not that familiar with the Renault 5 GTT, but doesn't it have a torsion beam rear axle? If so, then you're partially stuffed, 'cos the torsion beam will act like a bloody great ARB with non-adjustable drop links, which will be 'fighting' your corner weight adjustments. And if the torsion beam is itself twisted, you're really stuffed and will never get the corner weights anywhere near working right.

[Woznaldo]

Thanks sam, that's more info than I could of hoped for and it's greatly appreciated.

Your memory serves you well, the GTT does indeed have a torsion bar spring rear beam so I know it's not the best base to start from for this sort of thing but, I'd like to get it to best place I can. I think that I could knock up a set of adjustable drop links for the front end and as there is almost nothing available, off the shelf, to increase the rear spring rate, I might switch to a lightly sprung coilover housed in some fab'd turrets?

For the time being I will be sticking with a front end only adjustable set up. Unfortunately I'll be away on a course for the next 5-6 weeks so won't be able to give any feedback either.

[Woznaldo]

Forgot to add that I've found an iPhone app for recording the corner weights called 'Cross Weight'.