Corner weighting....

[mike]

hey folks,
id like to have my car corner balanced/ corner weighted.
a friend of mine can do this and he asked me a few questions...

he asked the weight ratio front to back (spf mk2), and total weight of car....im guessing around 2300lbs engine and fluids. (aluminum block)
i already know my weight...(and it isnt good, lol)

anyone have any info and any tips to setting this car up?

thanks

 

[roaldin]

I don't have any numbers in front of me ATM but I remember my car (RF) was 57% rear 43% front.
(Alloy engine also)

These things tend to become apparent when corner weighting the car though...

Tim.

 

[DaveM]

My SPF MkII ended up at 41% front, 59% rear. Gas in the tanks will alter this. Weight with no gas or driver 2452 lbs. Figure 40 lbs. less for an aluminum block. My car has an accusump and luggage bin so you may be under 2400 lbs. I got my corner weights within 1 lb. front and 13 lbs. rear without driver. With driver I'm 16 lbs off front and 8 lbs off rear. Goal is get the fronts within 5 lbs, not so important rear. Make sure to roll the car back and forth several times after spring adjustments to settle the car.

 

[Mike Pass]

Hi Mike,
One issue you will have is that you do not sit in the centre like in a single seater. Therefore when you get in you will unbalance the car side to side. Also then when a passenger gets in it changes again. Also the fuel in the tanks L and R unbalance the car.
It is therefore a bit pointless to corner weight then car unoccupied. I would suggests that you set it up in the configuration it will mostly be used in. In my case with one heavy driver!
As the car will be heavier on the driver's side this will show up when you read off the weights left and right. It will also be heavier total rear to total front. I would set it up so that the ratios across and f to rear are in the proportion of the totals. Split the weights on each side by proportioning the total for that side in the ratio of the ratio total front to total rear. Therefore although the corner weights cannot be the same the suspension is carrying the loads in proportion to the out of balance loads l to r and f to r. When setting up a car solo I use bags of gravel equal to the driver weight (a lot in my case) placed in the driving seat to accurately represent the driver.
I assume you have set the ride heights first - usually slightly higher (1/2" to 3/4" ) at the rear.
Hope this helps.
Cheers
Mike

 

[Randy V]

Mike - One tip I can give you is to go out and purchase sand bags that will total up to your weight. We can buy it here in 70# tubes.

The way I set my car up is as follows;

* 3 sand bags - two upright in the seat and the third laying across the front of the seat as though it were a leg - or other similarly sized appendage ((ahem..))

* Fuel cell(s) at 50% fuel load.

* Air pressure in tires at max sidewall pressure.

* Sway bars disconnected.

You won't want to skip any of these steps.

When ever you change the spring preload on any corner, make sure that you bounce the chassis a couple of times in that corner in order to get any preload or friction within the shocks to settle out to neutral.

 

[Cliff Beer]

a lot of great advice here. Couple thoughts...

You can't really affect the front to rear balance with this process. Kinda obvious but thought I'd state the obvious.

There's some debate on whether to undo the sway bars, after all, you're going to be driving the car with them attached.

Corner weighting isn't actually affecting how the weight sits in the car, rather, you're just loading and unloading the suspension to equally load the tire patch. Again kinda obvious. Doing so places different spring loads on each spring. With progressive rate springs this can present problems when it gets too extreme.

 

[mike]

thanks guys...helps a bunch.
i have a friend who will actually do it, but i want to know the process so i make sure hes not a lunatic or something.

one question....
ill be switching tires soon....he said it wont matter much as long as we adjust all the heights proportionally after switching tires.
true?

thanks

 

[Dutton]

Mike,

If you're truly serious about the corner weights, you'll probably want to roll the car onto the scales after you later adjust the ride height for the new tires. You'd be surprised how even the smallest of differences will affect your set up.

If you're looking for a little something to read on the topic, pick up a copy of Tune to Win by Carroll Smith. An excellent book written by Shelby's former crew chief. Everything the guys have mentioned is in the book, plus maybe a couple hundred other little details.

Best,

T.

 

[Randy V]

Cliff - Have you ever tried adjusting the corner weights / percentages in a car with the sway bars connected?
They didn't brew enough beer for me to make it through a session like that...

Yes - rolling onto/off of scales after changing the springs or settings is a good idea. My particular scales (Intercomp) don't seem to care as I've tried it both ways (weighs?)... Why some scales bind and others do not remains a mystery to me but I'm certain it has something to do with the mechanical actions within the scale pads.. I do roll the car fore/aft just a bit (I use tire stops to keep from rolling off the pads.. This gets the suspension to settle out just a little... I suppose to the same degree rolling all the way on and off the pads would.

 

[Cliff Beer]

Randy, have another beer then.

The corner weighting I've done has been a first session with the sway bars disconnected to rough it in and then a second session with the sway bars connected and adjusted accordingly. That's pretty standard and a good approach.

Cliff - Have you ever tried adjusting the corner weights / percentages in a car with the sway bars connected?
They didn't brew enough beer for me to make it through a session like that...

Yes - rolling onto/off of scales after changing the springs or settings is a good idea. My particular scales (Intercomp) don't seem to care as I've tried it both ways (weighs?)... Why some scales bind and others do not remains a mystery to me but I'm certain it has something to do with the mechanical actions within the scale pads.. I do roll the car fore/aft just a bit (I use tire stops to keep from rolling off the pads.. This gets the suspension to settle out just a little... I suppose to the same degree rolling all the way on and off the pads would.

 

[Cliff Beer]

Randy, apologies, I did a poor job of explaining what I meant. Of course, you're absolutely right, reattaching sway bars is the last step in corner weighting.

What I meant to say (before my brain melted into mush) was that the second session with sway bars attached is an alignment session setting camber, castor, toe, etc. Corner weighting occurs with sway bars disconnected until completed, just as you rightly indicate.

 

[Dave]

I/ We corner weighted 4 cars on the weekend, my Coupe a DRB GT40 and 2 Cobra's. It was surprising to see that all of the Cars where very close to right and only needed minor adjustments, mostly to compensate diagonal loads upset by the driver sitting in the car. We have a spread sheet of all the cars so if there is interest I'll post it.

What we did was set the car up on a 2 post hoist and lower it down onto the scales, we then took a measurement and added shims or wedges under the tyres to change wheel loads before we adjusted spring platforms. Seemed to work ok and lets you know you're adjusting the right way. Doing it on a hoist makes life easy...but don't forget to give the chassis a shack before you read the measurements.

The scale set used was the Long Acre system.....

 

[Randy V]

No worries Cliff..
:thumbsup:

 

[Ross Nicol]

Thanks for the tip using the hoist Dave, I had not thought of that and i'll definitely use it in future.
Cheers
Ross

 

[Dave]

The hoist made life easy....more so for the Cobras where you have no acces to the shocks. But what's best is it unloads the coil overs so you can adjust them easier.

 

[Trevor Booth]

after all this corner weighting no one seems to be checking transverse ride heights mmmmmmmmmmmm

 

[Trevor Booth]

Could not buy a decent set of scales so made my own flexing beam scales, no mechanical linkages, all stainless steel , 0.5% accurate, 0-600 kg (each one) , 4 window DRO, calibrate with laptop (for hot and cold tyres and varying ambient), battery powered, aerospace quality,no stuffing around GENTLY rolling car on and off etcetera
(top plate is 100x125mm onto which sits a greased alum plate)
go to have the right tools to do the right job