308 GTB Corner Weighting Data and Question.

Philip, how "consistent" are the results? I'm thinking particularly of damper & bushing friction affecting the readings.

If you take one set of measurements & then push/pull each corner up & down a bit & then remeasure, do you get a repeatable result (say within +/- 3%)? If yes then that should give you confidence in the process. If you get big differences after bouncing the corners I'd first work on getting a truly repeatable procedure. Personally I'd like to be able to reproduce the "standard" weight results a number of times before continuing.

By repeating the measurement without changing anything but moving the car will give you a better idea about how much variation is significant. ie, if the numbers vary by +/- 10% for the same setup then this level of "error" will also apply to any future setups. Taking the mean of a number of sets of measurements for each unique setup may help if the scatter is large.

 

philip,

Interesting data, keep it coming!

 

Phillip,
I have done this a thousand times. Your numbers look pretty good. I always strived to get them dead nuts on with the ride height being level side to side although the ride height factor is much more critical with a high downforce car with underbody aero aid. It looks like you are running quite a bit of rake (wedge for the nednecks on ovals). 1/2 to 3/4 inch should be plenty.

Another note, the scales you are using are not incredibly acurate. I have used them before and always found them to be somewhat unreliable although it is better than nothing. If you have access to any, Longacre compu scales seem to work best.

Tips-
ALWAYS bounce the car front and rear after making adjustments.
Distribute the simulated driver weight to resemble your position in the car.
Dump some gas in the tanks and weigh with half tanks.
Raising a spring perch adds weight to that corner and its diagonal in a lesser degree and takes from the other cross.

Trick of the day--
Raise the left rear 1 turn and lower the right rear one turn will swap weight without affecting ride height. Dont ask how!!

Have fun. By your data above, the car is certainly not out to lunch. Besides, you need to be very experienced to feel 10 or 20 pounds with seat of the pants.
Check bump steer also. Being as low as you are, I bet you have some adverse angles with the tie rod arms in relation to the control arms. Easy to correct by using rod ends for outside tie rods and shimming to the correct height. Recommend shoulder bolts to attach if you go this route in the future.
Email me if you want some other ideas.

John

 

Dear Philip,

Great information. I love the way you’ve explained in great detail, very helpful, very scientific.

I agree that your spring and lever arm scale system has much more error than an electronic load cell system, hence, like geekstreet points out, your results are only as accurate as the method allows.

Here’s my advice:
Don’t focus on pounds, use percentage. For road racing in general, shoot for 50% cross weight. Your first data set is (assuming the scales are accurate enough) outstanding at 49.4% RF to LR. Within 1% with load cell scales on a heavy car like yours is perfect. Frankly, I’ve been happy with 2% occasionally, especially a road car with so much stiction, bushing deflection, lack of bending and torsional stiffness, etc.

Your second data set is then 52.5%, not too bad, but not great. Fine for a road car.

Random thoughts:
1 ½” of rake is common on heavy closed-wheel racecars. That said, I’d not run more than that, and for 348/F355 typically had less than 1”. Roll center, anti-dive, anti-squat and CG information is helpful here, but one can still do it by the seat of the pants and track time.

I’m assuming you’re disconnecting ARBs and then reconnecting them post-adjustment.

Agree with spang308 regarding spring collar adjustments. Ideally when dialing out corner weight, assuming you’re level already, you’d turn all four collars simultaneously; perhaps only ½ turn each, i.e. LF – RR up ½ turn, RF – LR down ½ turn. Even more accurate is using motion ratios, as the above assumes they’re the same front and rear (they are not).

Getting too accurate is wasted time though, as said, there’s too much error built into both the chassis and scales.

On load cell scales, I roll the car back and forth about 3 feet, then bounce the front and rear bumpers, then take a reading. This is very repeatable.

Highly recommend $1000 spend on some Longacre or Intercomp scales.

As for front vs rear weight distribution (noted your obsession re: alternator), you have 58% now. Experience shows 54% +/- is close to ideal (note how the heat exchangers are up front on 360s).

Best regards,

Rob Schermerhorn

Finally food for though: Newman-Haas places their ChampCar on a platten for corner weighting. Only the rear wheels rest on scales. Hmmm.

 

You won't shift any significant weight F/R by adjusting rake (well actually it WILL affect downforce on-the-go, but that's not the issue here). The total rear weight as measured (LR+RR) is determined by the mass of the rear of the car. You can move some from one side to another (eg LR->RR), but only at the expense of shifting it the opposite direction at the other end of the car (eg RF->LF).

You can only lighten the rear IN TOTAL by moving the CG forward. Adjusting the rake can do this but only in proportion to the wheelbase length, so you won't see any change by lifting 1/2" over a wheelbase of 100+".

 

!!!!! Diagonal cross weight needs to be exactly 50%. Then try and get LF to RF within 50 lbs as well as LR to RR under 50 lbs.

I've never "ever" sent a car out of my shop without the diagonals spot on 50/50.

Not having them 50/50 is like getting a shopping cart at home depot with 2 wheels on the ground and two wheels off. Turns great in one direction and sucks in the other.

Having the oil tank on the left side behind the driver is not helping.

How much driver weight do you have in the car? If you are setting up for competition, don't forget your helmet and complete driver gear....along with coolsuit and radio if you run them. 50% of the fuel you plan on using in competition is paramount too. i.e. if you run full tank, cornerweigh at 1/2 tank. If you run 1/2, then 1/4.

As far as driving and settling......NO. Roll the car back and forth the distance of the perimeter of tire and jump up and down on each rocker at same time, then roll back. If you made any spring perch adjustments that had you lift the car, make sure they get back on the perches. If you changed the camber, remember it changes the ride height and thus the weight on that corner and it's diagonal opposite.

Get the scales on a platform that is leveled. The thickness of a shop rag "in height difference" can cause 50-75 lbs of incorrect weight measurement. That goes for crap on your tires too.

 

Philip,

Absolutely must level the chassis side-to-side. This is the difficult part of corner weighting and aligning a production car. Sometimes difficult compromises must be made. I’ve had sessions with a 348 Challenge car go for hours mainly because the chassis was ever so slightly not perfect and the ARB drop links had to be the factory parts, non-adjustable so they would be pre-loaded if the chassis was level, but with the bars at zero pre-load, the chassis was slightly tilted. Settled for slight tilt with negligible pre-load on the ARBs. (Interesting aside: this chassis mysteriously settled in after a season of racing and never had the issue again).

Also, building a GT racecar is a huge packaging compromise. One places the mass as best possible, but mass distribution never comes out perfect. Go for level chassis side-to-side, rake that makes the roll centers vs. CG happy with an easy to balance chassis (while driving), and 50/50 (or nearly) cross weights (remember the error built into the measuring device).

Agreed that rake and tilt changes produce negligible weight distribution changes.

Don’t forget to put some simulated leg weight in the pedal box, this does matter.

Best regards,

Rob Schermehorn

 

Phillip,

After many, many threads on shock replacement, springs and improving handling, I think this thread will really help establish some specific targets for ride height and cornerweighting guidelinesfor the 308.



From my meager understanding of Carroll Smith's recommendations for race car preperation:

Getting it as low as you can without scraping the bottom - for minimal fore and aft weight transfer must be tempered with the ability of the suspension to "favorably" camber the tire to the chasis need for traction.

First, you have to keep the car from grinding itself to bits.
Next, you have to have aerodynamic stability for speeds in excess of 100 mph, that is, some rake, or the car is going to become unstable.

Both the front and rear suspension have their own seperate camber/ride-height change curves. First these have to be reconciled with the tires best performing camber curve/height change, both under acceleation, braking and cornering. some tires are camber tolerant and some are less so, (this is pretty much general theory, but, generaly, lower profile tires are more camber sensitive).

the 308 platform is a relatively balanced, (low cg & good f/r weight distribution), and I am really interested in improving the handling via favorable ride height, tires and bar settings.

The 308 is heavy, it flexes more than a race car, so, imho, is more forgiving of slight variations in corner weight. With a road car and a left or right side driver, the chasis weight is not inherently balanced, so we must accept some variation, or go crazy trying to make it so.

I'm going out on a limb here, but without having a graph of the front and rear camber curves, I would go with the stock height +- the same amount f/r, that is, assume the factory got it right and lower the car the same amount front and rear, or have a good reason to do so.

Those of you with a better understanding of current, (low profile), tire camber requirements, for best traction, can chime in.

Clearly, stronger springs and stiffer bars are going to minimize the height and thus camber change with movement.

I am pretty much out in the dark here but i would be semi willing to post as bond either a check for $500, or my left nut for the specified loan of a set of scales, (longacre or Intercomp). I'd pay $100 for the loan and all shipping charges, since no one has any hereabouts.

Just got notice form Il Duca re the new QA-i buchings,

ciao, gotta go,
chris

 

I respectfully disagree with the side to side chassis leveling. This is a 308 we are talking about. Air under the car is not a huge factor with concerns to aero. I have not had one in the wind tunnel, but they probably produce very little downforce and may in fact produce lift at speed.
I do however agree with the above advise concerning cross weight. Shoot for 50-50 cross and 1% side to side. I would be willing to accept up to 1/4 inch of ride height difference to achieve this, but ideally try to keep it within 1/8 inch side to side measured at the rocker. I still feel you should be under 1 inch of rake Fr. to rear.

As rexrcr stated, be sure to disconnect the ARBs and zero the links once done on the scales providing they are adjustable.

Dont split hairs or turn this into more of a science project than it needs to be. This wont be the last time you scale the car or change spring rates for that matter. Once you develop as a driver, you will find yourself setting the car up different for different tracks. Example, I have always found most cars dislike a lot of rake at tracks with lots of elevation change. Fast smooth tracks require more spring rate, etc. etc. etc.

I have used the above guidelines and won LOTS of races in all types of cars.
My personal preference has always been to run just enough spring to keep the chassis off the track and run lots of sway bar, sometimes to the point that I was running 50% the spring rate of competitors. The same competitors I was smoking in brake zones and hooking up out of twisty tight stuff. Especially on street circuits. You run Elkhart, and yes I have been there. Lots of elevation change, fast sweeping turns, and not all that bumpy the last time I was there (its been awhile). Hence modest rake and spring rate, and lots of bar.

Throw a set up on that bad boy and get out to the track. Take some tire temps, gather some data and evaluate the situation. It probably handles awesome on jackstands but thats not the point!!

We havent even got to shock valving 101 yet!!
Whos up??

Have fun,
John

P.S. Have you bump steered the thing yet?

 

Not concerned much with aero either (you're assuming ). Plot your geometry, then simulate cassis tilt, see what happens...

Best regards,

Rob

BTW, they do produce lift in the rear.

 

Depends upon the chassis. Some of them are not very "square". I've seen a few have ride heights varying over 10mm side to side, even more if the car has been hit.

You have to remember, the tires are the ONLY thing making contact with the road. It is extremely important for them to be firmly planted on each corner, i.e. in general terms: you don't want one diagonal "flat footed" while the other diagonal is on it's tippy toes.....

I typically get the car pretty square on heights (and refer to factory specs) and then do my preliminary rough in on camber and caster. DON'T FORGET CASTER!!!!! See were the weights are and then compare to the heights. For example, if your weights are light on the RF/LR and the ride height is a little low on one or both of those corners, you will know to jack that corner first to try to get closer. Could also be done by lowering the opposite diagonal corners.

Ideally you will want to run close to Euro specs on ride height, if you run lower, then you will run into the need to bump steer/shim the car

 

make sure you leave the weight in the car on the scales to install the droplinks. Make sure they are neutral, otherwise it will jack weight into your chassis.

 

Good job Phillip. That is very balanced. How do the ride heights look?

John