Corner weights on CAV

David Briggs · 10-10-05, 10:37 AM

I am preparing to install a Tilton aluminum pedal assembly, dual brake masters w/cable bias adjustment. That will allow me to shed the master/ booster combo and the vacuum pump that I have up front, losing a little weight, creating a little luggage space plus the pedals are very cool. The master cylinder selection process involves determining the all up weight of the car along with how the weight is distributed. The car had 15 gallons of fuel, 5 in the left and 10 in the right, driver is 160 lbs. Total weight was 2840! Pretty porky!
Left front 614 Right front 607
Left rear 817 Right rear 802
On the track I usually am running with fuel only on the right tank so the corner weights are pretty good.
I think that I will net - 25 when I am done and will post the results then.

Tim Kay · 10-10-05, 10:48 AM

Hi Dave,

If you don't mind, please provide which Tilton pedal assembly your installing (or do they only have one aluminum design?)

Looking forward to seeing the install.

Bob Childress · 10-10-05, 12:31 PM

Hi David,

Thanks for providing numbers. Specifics are always very helpful for other CAV owners and I am sure of interest to anyone setting up their car.

My numbers are very similar to yours. You may have already done this, but if not, you can shift your weight distribution about 1.2 % to the front by moving your battery from inside the rear of the fuel cell pod to the front of the car under the nostrils. Here are the comparisons:

--------Battery Rear----Battery Front----Your Car
Front-----41.6 %-----------42.8 %-----------43 %
Rear-----58.4 %-----------57.2 %-----------57 %

As you can see, my weight distribution went to almost equal to yours with only 6 gallons of gas and no driver. I presume that the weight of additional gas and the driver would make my car something like 44/56 or 45/55. My battery, enclosure, brackets, and cables weighted about 30 pounds. I did a post on this a while back pointing out safety advantages but for sure it is not good for luggage space.

Good luck with your modifications and I look forward to your posts.

(P.S. The battery in the picture is anchored with grade-8 bolts and a crossover steel strap, only the plastic lid is held by the webbed belt.)

David York · 10-10-05, 02:27 PM

Hey Guys,

What are you trying to achieve with the corner weights? When we set up our caterhams the goal was to get the weight across the fronts as equal as possible. This was until a friend of mine who is a suspension designer working at Ricardo spent a morning and pot of coffee explaining what was happening with the dynamics of the car when racing. He suggested an approach that made the cross weights equal. In other words the sum of the right rear and left front equal to the right front left rear. That centralized the mass in the car so it would react the same during transition. I just about understood it on the day but now am a little hazy on the details. Anyway the point is that the car felt much better to me in this configuration to when I had it equal across the front. Of course this is with the car as it would run, fuel load, driver dressed and in the seat etc.

What is the standard practice on 40's?

Great info on moving the battery to the front!

Russ Noble · 10-10-05, 06:13 PM

This is an interesting concept Dave.

I have (unsuccessfully) been trying to get my mind around the niceties and practical application of cornerweighting a two seater as opposed to a single seater for a while now! Out of interest could you please post your cornerweights for the Catheram as initially set up equal front and also the later set up, equal diagonal sum. That should give food for thought.

Can some of our resident suspension experts comment and maybe recommend some reference books etc. that refer to the diagonal setup.

Regards

Ron Earp · 10-10-05, 06:35 PM

Hi David,

Nice motor in that car! And it is porky no doubt, despite being a mono, but I thought the SS monos were known to be a little heavy. I bet it is a hoot on track, have any video or footage?

Dave and Russ, I am not chassis engineer but I do fool around with Spec Miatas, the hottest SCCA race class here in the US at the moment. It is a highly contested group and since nothing much can be done to the motor, chassis setup is key. Folks seem to aim at getting the cross weights correct since that has more of an effect on the cars' handling than having equal weights on each side. Reason being is that cornering and loading/brakeing has a much larger effect on handling than just about anything else (within reason).

One quick guide that a lot of folks use for SM setup, and might be handy in some cases here, can be found here:

http://www.specmiata.com/setupguide/...ing%20Tips.pdf

The setup is simple and a baseline, but it provides the concept and then one can wedge or dewedge a corner as one sees fit for a particular course. I'm just going through this procedure with our second car and am trying to dial it in at the race this weekend before we run the car in the 13 hour enduro (and have Malcolm and Julian over to run with us!). Might be some useful info there, certainly was for me coming at it from a beginners' standpoint.
R

Andy Besic · 10-10-05, 06:36 PM

All,

To further bend our brains....
I agree with Dave York's concept, the problem I ran into was when to know when actual weight needed to be transfered (ie battery, fuel etc.) to perfect measurments and when was a twist of the coil-over spring pre-load OK.

Russ Noble · 10-10-05, 07:46 PM

Thanks Ron.

Some good info there.

Regards

TimOz40 · 10-10-05, 09:51 PM

Definately, the diagonal sums should be equal. What we do is: get ride height to where you want it, then put the car on scales (we have some nice Intercomp digital ones that do all the calcs for you). The adjustments are done by turning the spring platforms up or down as required. Remove the car from scales and settle it, before repeating, as necessary.
Big pre-requisite: HAVE THE DRIVER IN THE CAR WHILE YOU"RE DOING IT. Otherwise it's all a waste of time.

Gary Gibbs · 10-10-05, 10:14 PM

Sorry if this sounds brief but I don't have a lot of time. A given car without modification has a fixed left side weight, right side weight, front weight and rear weight. It however does not have a fixed cross weight (sum of RF and LR divided by total). This can be adjusted by preloading each spring more or less. Adding a half turn to the RF and LR and removing a half turn from the LF and RR will result in nominally the SAME ride height at all four corner but will increase the cross weight and make the car tend toward "push" in left hand turns and "loose" in right hand turns.

In a road race car you are nominally trying to have handle symmetrically. Thus you strive for 50% left weight, etc. You put the same springs in LF and RF. Same in LR and RR. Have your sway bars "zeroed" at static ride conditions (disconnect for cross weighting and zero and reconnect at end). All suspension settings would be symmetric (camber, caster, toe, etc.). If your car set as it is to be driven (gas, driver(s), etc.) is relatively close to 50% left weight then you want to shoot for 50% cross weight . If the car when driven has a small bit of asymmetry in terms of handling at the limit (under steer one direction and over steer the other direction) then this can be fine tuned with cross weight changes. More cross weight will make the car "push" in left hand turns and "loose" in right hand turns.

NASCAR is quite different in that the entire car is asymmetric: Wheelbase, track, cg, camber, caster, springs are all different, etc because they are only turning one direction and optimizing for one direction. This can be done in a road race car to suit a particular track, etc. But not recommended for the average joe tweaking the street car. Good luck.

TimOz40 · 10-10-05, 10:44 PM

Good point about the sway bars Gary. I forgot to mention that. [img]/ubbthreads/images/graemlins/crazy.gif[/img]

David Briggs · 10-10-05, 11:36 PM

Thanks for all the input on this topic. To me,the cross weights are what is important as is the nuetrality of the sway bars. I am trying to get some room up front so the battery relo won't work for me. you can compensate for the front /rear differential in several ways, stiffness of the bars, shock settings, brake bias, some minor aero mods. I didn't concern myself too much with the way the rear wheels fit in the openings but after seeing all the posts related to that subject I have come to realize further modification is in order!
There is an abundance of good information here, and that is why I enjoy this site.