Scales and corner weighting.

[xromad]

Ok, I'll piss in the wind here and see what gets wet....

 

Concerning Scales and corner weighting:

 

You do not need $1500 worth of scales to do corner weighting on a Seven.

 

Here is my take: you can do one of two things:

 

1. Ignore corner weights and concentrate on other things like balancing your brakes/etc.

 

a. Unless you are willing to either do massive re-arrangement of components, or add uncecessary weight you probably have little or no control over what your corner weights are. You can jack your diagonals to redistribute some of the weight, but you can't actually change the weights themselves.

 

b. Unless you are driving in a class where you NEED to add weight you probably are not benefited by adding weight just to balance the corners.

 

c. Here is the real sticker. Unless you are really balancing the weight by moving the weight around (not just jacking the suspension) you can actually introduce some very BAD behavior to your car. Example: if you try to match the diagonals (like is taught in some books) you can actually cause one of your front tires to have considerably less weight on it. Our cars transfer little weight under braking (compared to a normal sedan), so this can result in you locking that tire even in mild braking. Bad braking severely compromises your ability to go fast....

 

2. Go ahead and do the weights but without $1500 worth of equipment.

 

a. My car weighs ~1500lbs with me in it. I use bathroom scales with wood ramps across pairs of scales. I.E. 2 scales per tire and one 2X6 that the tire sits on and distributes the load to both scales. 1500lbs / 8scales = 187.5 lbs per scale. That weight is easily within the limit of standard (cheap) bathroom scales. I went down to Wal-Mart and found the scales on sale for $10 each. Remember, the weight for that tire/corner is the sum of the weights of the two scales that it is sitting on.

 

b. If I were going to go to nationals I would have to add weight to my car to meet minimums (I refuse to do it for local events). So, I would then add weight in a way that balances all four corners without any spring jacking.

 

c. But since I am NOT going to nationals. What I did was use spring jacking to make sure both my front tires had equal weight and that they both locked up at the same time under EXTREME braking. I then ignored my rear weights and just set the front/rear brake balance so that under EXTREME braking the fronts lock up just slightly before either of the rear brakes.

 

[slngsht]

Ok, I'll piss in the wind here and see what gets wet....

 

Concerning Scales and corner weighting:

 

You do not need $1500 worth of scales to do corner weighting on a Seven.

 

Here is my take: you can do one of two things:

 

1. Ignore corner weights and concentrate on other things like balancing your brakes/etc.

 

a. Unless you are willing to either do massive re-arrangement of components, or add uncecessary weight you probably have little or no control over what your corner weights are. You can jack your diagonals to redistribute some of the weight, but you can't actually change the weights themselves.

 

b. Unless you are driving in a class where you NEED to add weight you probably are not benefited by adding weight just to balance the corners.

 

c. Here is the real sticker. Unless you are really balancing the weight by moving the weight around (not just jacking the suspension) you can actually introduce some very BAD behavior to your car. Example: if you try to match the diagonals (like is taught in some books) you can actually cause one of your front tires to have considerably less weight on it. Our cars transfer little weight under braking (compared to a normal sedan), so this can result in you locking that tire even in mild braking. Bad braking severely compromises your ability to go fast....

 

2. Go ahead and do the weights but without $1500 worth of equipment.

 

a. My car weighs ~1500lbs with me in it. I use bathroom scales with wood ramps across pairs of scales. I.E. 2 scales per tire and one 2X6 that the tire sits on and distributes the load to both scales. 1500lbs / 8scales = 187.5 lbs per scale. That weight is easily within the limit of standard (cheap) bathroom scales. I went down to Wal-Mart and found the scales on sale for $10 each. Remember, the weight for that tire/corner is the sum of the weights of the two scales that it is sitting on.

 

b. If I were going to go to nationals I would have to add weight to my car to meet minimums (I refuse to do it for local events). So, I would then add weight in a way that balances all four corners without any spring jacking.

 

c. But since I am NOT going to nationals. What I did was use spring jacking to make sure both my front tires had equal weight and that they both locked up at the same time under EXTREME braking. I then ignored my rear weights and just set the front/rear brake balance so that under EXTREME braking the fronts lock up just slightly before either of the rear brakes.

 

Makes alot of sense.

[locostv8]

Just to clarify I thought you wanted the rear to lock just before the front??

[BobDrye]

I remember when the bathroom scales adaptors became popular 20 some years ago. Like a lot of other racers, it seemed like a great deal! I even purchased a set. However the problem was, and still is, that the readings are not repeatable. That means that they are not accurate. If you're looking for total weight they are usually close enough. To prove my point, if you weigh yourself 10 times on a set of bathroom scales, how many different reading are you going to get? Most racers don't spend more than they have to for an item, as the budget is usually tight. Most serious racers that I know use electronic scales. Before the electronic scales became available we used (4) grain scales.

 

My offer of $200.00 for a set of scales to be used in the DC area is still open.

[RDROCKT]

I guess my last try to reply didn't make it!

 

I would NEVER want the rear to lockup first! ..... that is, unless I was in a doughnut contest.

 

Always the fronts do the major work and the rears do what they can....no more, no less.

 

Chuck said that...

[slngsht]

Just to clarify I thought you wanted the rear to lock just before the front??

 

Other way around... lock the fronts going into a corner, the car starts going straight - you take your foot off the brakes and you got steering again.

 

Lock the rears going into a corner, and it quickly becomes unrecoverable.

[xromad]

I remember when the bathroom scales adaptors became popular 20 some years ago. Like a lot of other racers, it seemed like a great deal! I even purchased a set. However the problem was, and still is, that the readings are not repeatable. That means that they are not accurate. If you're looking for total weight they are usually close enough. To prove my point, if you weigh yourself 10 times on a set of bathroom scales, how many different reading are you going to get? Most racers don't spend more than they have to for an item, as the budget is usually tight. Most serious racers that I know use electronic scales. Before the electronic scales became available we used (4) grain scales.

 

My offer of $200.00 for a set of scales to be used in the DC area is still open.

 

Hi Bob, :seeya:

 

I'll amend my above statement to add this....

 

For a professional (or national level amateur) racer who has the budget to afford them, the means to meaningfully affect the actual weights, and the skill to obtain repeatable results on the track: Sure, buy the best set of scales you can afford. If you use them correctly, I guarantee they will help you go faster and win more races. :thumbs:

 

However, Here are my caveats:

 

Remember I wasn't going for balanced corner weights, I was going for balanced braking on a car whos actual corner weights I had very little control over.

 

1. There are ways to do it very very accurately without using a single scale, digital or otherwise, balance bars come to mind...No idea what the weight is, but guaranteed that everything is exacly balanced....

 

2. If I rememember correctly, by bouncing my car a repeated number of times per corner in a repeated pattern, to settle in the springs, I was able to get a minimum variation of around 6 pounds per corner. Which is about 2% of the total weight on that wheel (assuming about 375 lbs per wheel). I then averaged out 3 repeated measurements (bouncing inbetween) to take my reading after each change.

 

3. Corner weighting is a static process. Race car driving is a dynamic process: Accelerating, Braking, turning, bouncing, heating, consuming fuel, sloshing fuel, tire wear, size of lunch, etc....

 

#3 is Obvious, but it leads to the next series of questions, for which I don't have an answer. (Maybe someone on the forum does)..

4. Does +-1 grain (1.00000 Grain = 0.00014 Pounds) accuracy really buy you a repeatable change in lap time?

5. Does +-1 Pound accuracy really buy you a repeatable change in lap time?

6. Does +-10 pound accuracy really buy a repeatable change in lap time?

At some point the answer is an obvious yes. That much weight has a proveable change. But where? So many other dynamic factors are involved in a moving car that I suspect it takes a surprisingly large number to make a repeatable change... Remember, by repeatable change, I'm not saying "It felt loose" or even saying "all the other race teams are using that level of accuracy", I'm saying, play with the weight behind the drivers back and measure his times, then prove statisticly that he is slower. Better yet, do it with 100 drivers and proove they are all slower.

 

So, after all that, the real question is: Michael Schumacher notwithstanding, how much accuracy is realy meaningful?

 

7. "readings are not repeatable. That means that they are not accurate." Need to be careful with this one. Statistics and an understanding of accuracy vs precision come into play here. "My god, the statistics...." or "Lies, Dambed Lies and Statistics".... I don't want to go into statistics right now, I'd have to break out all my college books and relearn most of it. But, as far as accuracy and precision goes:

 

Precision vs Accuracy- http://en.wikipedia.org/wiki/Accuracy

 

I just went out to my garage and used all 8 of my scales. I tested zeroing the scale and then placing a 30Lb weight (excercise weight, best standard I have at the moment) on the scale in as close to the same position each time as possible. I tried it 10 times on each scale. In 8

[Dave W]

A 6 lbs variation is as good if not better then most measurement systems.

 

A LOOONG time ago I was involve in a test problem where we measured the vehicle on a machined bed plate using four precsion load cells. The vehicle wt was about 4000 lbs and we where only repeatable from 8 to 12lbs with no changes. Just raising and lower the vehicle to verified the added wt between set-ups. There is that much friction and histresis in a production suspension system. I'm sure it would be a little better W/O rubber bushings, but you are not going to get much better then your current results.

Dave W

[Davemk1]

I'm running a Birkin and I found that having it properly corner weighted really helped in autocross. I can feel it handling more consistently in slaloms.........the right and lefts are the same. For me it was worth the trouble.

 

I made my own corner weight set up with cheap bathroom scales and a lever of sorts. The hard part about using a lever is that the exact placement of the tire on the lever is hard and if it's off it will change the reading a great deal. So I added a pivoting pad on top of the lever. The tire sits on the pad which in turn sits on the lever. This allows for a precise location of the weight on the lever. The underside of the pad has a small inverted "V" on it to allow it to pivot to allow for tire camber. This way a tire with camber won't change the leverage ratio.

 

I used them and they seemed absolutely repeatable even with the cheap scales I used (less than $10 a piece at Target). Months after I did this a good friend bought some Longacre scales and I checked the set up going back and forth between set ups and they were very, very close. I was getting less than a 5 pound variance between the two set ups. The digital Longacre set up was over $1000 and mine target set up was less than $75 all materials included.

 

It's well worth the time and effort and the car will work better at the limit.

 

Davehttp://www.usa7s.com/aspnetforum/upload/556173090_DSC_6960.jpghttp://www.usa7s.com/aspnetforum/upload/1620524268_DSC_6961.jpghttp://www.usa7s.com/aspnetforum/upload/939962813_DSC_6962.jpg

[xromad]

Wow, Very nice!

 

I also used target lines and blocks (but not v blocks) under my 2X6 pads. I hadn't thought of the second pad and V to allow for camber. I just used 2 scales.

 

However, I did also use a second perpendicular line to make sure everything was centered as close as possible in two dimensions.

 

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[xromad]

A 6 lbs variation is as good if not better then most measurement systems.

 

Dave W

 

My relatively accurate readings were probably helped somewhat by the fact that I didn't remove the car from the scales between weighings.

1. I set it up once.

2. I bounced the corners until I felt I had a repeatable pattern

3. I adjusted the coil-overs

4. I bounced the corners according to the pattern

5. I repeated 3 and 4 until I was happy with the results.

 

Brent

[Kitcat]

When I had my car weighed recently on professional scales, by a racing shop, it was nearly perfect as-is, at all 4 corners when empty. When I was added as a passenger, the front driver's side was about 40 lbs more than passenger front (So, that's why the battery is on the far right front side), rears were still almost even. I don't auto-x and track days are just for fun so perfection is not essential. Bedsides, with a Crossflow engine, it's not especially competitive in class anyhow.

[slngsht]

 

My relatively accurate readings were probably helped somewhat by the fact that I didn't remove the car from the scales between weighings.

1. I set it up once.

2. I bounced the corners until I felt I had a repeatable pattern

3. I adjusted the coil-overs

4. I bounced the corners according to the pattern

5. I repeated 3 and 4 until I was happy with the results.

 

Brent

 

Brent, do you set your shocks to their softest settings for this exercise?

[xromad]

 

Brent, do you set your shocks to their softest settings for this exercise?

 

Yes, I do set them down. Not to the bottom, but to the first setting above softest.

[BirkinBernie]

Brent,

 

Great setup, and I'm with you until you start talking about the distribution of weights that you are targeting. You are absolutely correct that, since the driver is offset to the left, the left side of the car will be heavy unless you can move enough stuff around or use required ballast in the most advantageous places. But you certainly wouldn't want to ballast unless you had to.

 

Remember that the car has four legs, like a chair. And given that you cannot change the percentage weight on each wheel by moving the spring perches up or down, what you have done by moving the right front spring perch up until it carries the same amount of weight as the left front, is add a bunch of weight to the left rear, and take weight off of the left front and right rear. If you took a chair and put a shim under the right front leg, you will see the same thing happen. What you have done is put a bunch of "wedge" in the car, as the roundy-round racers do to make their cars turn left really well.

 

Rather than thinking about cross-weight, think of it this way. The car has a static front-rear weight distribution. Nothing you do with the spring perches will change that. Let's say it's 52% rear, 48% front. What you want to do is get the corner weights such that the distribution of the left side total weight is 52% rear, 48% front. And the distribution of right side total weight is 52% rear, 48% front. If you do this, it happens that the cross weights will be 50-50, but this is really what you are shooting for.

 

Given that the car will always be heavy on the left side, the best compromise for balanced handling is to get the cross weights to 50%. Anything else you do will compromise the chassis for turning one way or the other.

 

Here are a couple of good articles on the subject:

 

http://www.grmotorsports.com/news/012005/understanding-corner-weights.php

 

http://www.hotrod.com/howto/113_0309_corner_weights_correction/crossweight.html

 

I hope this helps!

 

Regards,

Bernie

[BobDrye]

Ok bro's:

I need to confess that I went to Kmart and spent $45.00 for (4) cheap bathroom scales. The neat part about a seven is that if it's light enough you can just set it directly on the scales. My car weighs 1130 lbs without the radiator and intercooler. I will weight it again when it's back together next week at a friend’s shop that has an electronic set. I can A/B them at the same time. I will report the results.

 

[xromad]

Brent,

 

Given that the car will always be heavy on the left side, the best compromise for balanced handling is to get the cross weights to 50%. Anything else you do will compromise the chassis for turning one way or the other.

 

Here are a couple of good articles on the subject:

 

http://www.grmotorsports.com/news/012005/understanding-corner-weights.php

 

http://www.hotrod.com/howto/113_0309_corner_weights_correction/crossweight.html

 

I hope this helps!

 

Regards,

Bernie

 

Hi Bernie, I agree completely with the theory behind what you said, and behind the articles that you indicated. Excelent articles, Thanks! When I set mine up, I was looking at Carrol Smith's books. So the confusion is probably more a matter my poor wording than a misunderstanding of the cornerweighting theory.

 

I also agree with the "wedge" analysis of what I did to my car by making the front wheels have the same weight with my weight in the drivers seat. But I don't agree with the reason why. The roundy-rounds set up a wedge to gain an advantage in one direction cornering. I set up a wedge for almost exactly the opposite reason: Improved Braking.

 

I have tried making the diagonals 50% twice. Both times there was a major problem. Not because it corners poorly, but because it then severely compromises my front brakes. When I set the cross weights to 50% with my weight in the drivers seat, the right front tire winds up carrying significantly less weight than the left front. This results in my right front tire locking up considerably sooner than any of my other tires. This happens under even moderate braking. My analysis of this was that my car transfers relatively little weight (compared to the sedans talked about in those articles). They can get away with one front tire being relatively light (and 50% cross weights) because they transfer a lot of weight when they hit the brakes. They also aren't puting almost the entire drivers weight right beside the left rear tire. I felt that I confirmed this analysis (and proved that it wasn't a braking issue by setting the two front weights the same and the braking problem went away).

 

So, I probably Should have made my original post a little more clear. This is the compromise I have found that sacrifices some (left turn/right turn) cornering balance for a huge gain in braking. This is also based on the fact that I don't want to have to add a considerable amount of balast in order to get good diagonals without causing this issue. Not until I learn to drive a heck of a lot better and then go to Nationals... :-)

 

I do learn more every time I go through the exercise though and I plan to do it all again very soon. Maybe I'll discover I was doing something realy stupid and I won't need the compromise anymore. I'm going to redo my scale setup to incorporate the good ideas I see used by Davemk1.

[BirkinBernie]

Brent,

 

Your braking problem is puzzling. My car is set up with the crossweights at 50% - the fronts are about 50 lbs different. And my brakes are great. I have a Birkin with a stock engine, and use Hoosier slicks on the track. From 105 or so, it stops straight as an arrow. Same on street tires. No tendency to lock the right front. And I have never heard of anybody having this problem. Gonna have to think about this one!

 

Regards,

Bernie

[Davemk1]

Brent,

 

Your braking problem is puzzling. My car is set up with the crossweights at 50% - the fronts are about 50 lbs different. And my brakes are great. I have a Birkin with a stock engine, and use Hoosier slicks on the track. From 105 or so, it stops straight as an arrow. Same on street tires. No tendency to lock the right front. And I have never heard of anybody having this problem. Gonna have to think about this one!

 

Regards,

Bernie

 

I'm in the same boat. Stock Birkin with 50% cross weights and both front tires lock at the same time. Stops on a dime.

 

Dave

[slngsht]

Interesting... If your weights are not the same on the 2 front wheels, wouldn't the expected behavior be for the wheel with more weight to lock up after the one with less weight?