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Author Topic:   Wedge, Cross wieght, Math
leapinlizard
Dirt Freak

Total posts: 402
posted August 30, 2004 12:35 PM  
Can someone explain to me in detail Cross Weights? I know by preloading right front adds weight to left rear as well. I also knows it removes weight from the other two corners. I also have a basic understanding why cross weight’s help a racecar turn left by either decreasing or increasing the amount of preload pertaining to the correct wheels. I can't wrap my mind around about exactly this phenomenon works. I know what it does, but not how it works. I can picture in my head that lowering a corner would lift the opposite corner while slightly lifting the two adjacent corners. Perhaps if I could get a mathematical formula to describe the physics then I can finally understand and begin using it to my advantage.

ratracer10
Dirt Maniac

Total posts: 141
posted August 30, 2004 02:22 PM  
Try not to think so analytical about it. If you realize what wheels are gaining or losing weight your half way there. There is a kind of simplified way of looking at it that helps a lot with setup. Look at the wheel you are changing the weight on the think what the car will do if it is the only one driving or braking. For example if you add weight to the left rear, when that wheel is driving it will pull the rear to the left (tighten) when it is braking it will pivot around it and pull the rear to the right (loosen). You can apply this to each wheel at each point on the track to help with making setup changes.

------------------
- When in doubt, hit the gas.

leapinlizard
Dirt Freak

Total posts: 402
posted August 30, 2004 03:16 PM  
Quote: Try not to think so analytical about it.

I really aprecciate your input. I've alway's been a man of science(my occupation)and not knowing the physics of the geometry's doesn't allow me to understand it. If someone were to ask,

How can that be? you would be bending the car in the wrong way.

I can't answer them. It's alway's been a belief of mine that if you can't explian it to others in a way they would understand it then you must not understand yourself. Sorta like the old phrase: If you teach someone to fish...I feel I need to understand the theory completely so that I may be able to have a advantage above a fellow racer who doesn't.

Kromulous
Dirt Forum Champ
Total posts: 796
posted August 30, 2004 03:47 PM  
Maybe a chassis dynamics book or something may help. There are a few out there i think that may cover what you are looking for.

Cross weight, or wedge i call it, is a line from either corner to the other end's (front to rear) opposite corner, LR to RF etc. So if the LR and RF are the heaviest respectively, you have some cross weight.

High wedge setups or alot of cross weight in my mind allows the car to hinge kinda over this line. High wedge LR to RF will have alot of LR bite in them and usually are quiet loose getting in the corner due to the RR being so light. Very loose cars to drive in, and hook hard coming out, hopefully.

Thats my understanding of cross weight, but its alot to be desired. Looking forward to reading more posts about this topic myself.

leapinlizard
Dirt Freak

Total posts: 402
posted August 30, 2004 05:39 PM  
I love books!

I have a few already and none of them really explain either. I am waiting for another now though I'm beginning to give up and research my own theory of operation. It's easy for me to see the reason for needing cross weight. Drag racers even use it to some extent. They often refer it as "preloading the right rear spring to counteract the forces of rotational twist (torque) upon the drive shaft unloading the right rear as well as counterbalancing the unloading of the left front upon inertia of acceleration of the engine and forward movement. Circle trackers apply the same technique with the exception of they are already in motion and the coefficient of friction is to a lesser degree. But again, it's what it does, not how it works. Maybe I am trying to pick it apart too much. Can't think "inside" the box. Anyone else have a comment or suggestion? All are welcome.

cd2
Dirt Maniac

Total posts: 161
posted August 30, 2004 07:37 PM  
leapinglizard,don't know if this will help or not.You got it right about the drag car,if the rear tires are the same size you can use preload to make the car drive straight. On the dirt car lets say we have a even weight on each wheel & all the tires are the same size & the rear end is locked.As we go down the straight we are good but as we start to turn in we let off the gas an weight shifts to the front& to the right.Now this is where we have to pretend a little.Our front & rear springs work out so they have the correct wheel rates so when the weight transfers to the right it will load the R.F.& the R.R.the same.With the locked rear end the car does not want to turn,it is to tight or pushes.Now we can stiffen the R.F.spring or soften the R.R.& the car will get more weight to the R.F.& less to the R.R.The car will try to turn because the traction on the R.F.& L.R. increases & decreases on the L.F.& R.R.Now we will stiffen the L.R.also & the car will carry more crossweight& will try to turn in easier because the traction has been increased more on R.F.& L.R.& decreased on L.F.& R.R. If you soften the L.F. spring it will loosen the car even more because the weight will transfer to the stronger R.F.& will increase the crossweight more.The crossweight can be changed with jack bolt adjusters on the springs also without changing the springs.Now that we have the car turning in with crossweight or wheel traction changes we have to fix the car so it will try to keep turning as we go around the corner & off.This is where we use stagger,the R.R.tire larger than the L.R.With no stagger after entry the car would push & not turn the middle & drive off the corner.Crossweight & stagger work together to get the car in ,around & off the corner.So what crossweight really does is make a teeter totter out on the car so the R.F.& L.R.has more traction on entry to help the car turn & more L.R.drive to get off the corner.Also as you add left side the car will turn in better & get off the corner better.Let's stop here an not get into the other dozen or so things that go with this.If you get a dozen answers they will all be diffrent.You can call Walter Stanley @ pro shocks & he can get into this a good bit farther.Hope this helps. cd2

MikeN
Dirt Full Roller

Total posts: 27
posted August 31, 2004 09:17 AM  
Sooo... WHat you're saying is...
I'm still confused?

leapinlizard
Dirt Freak

Total posts: 402
posted August 31, 2004 09:24 AM  
These are really great answers on how to use wedge but I think I'm not asking the right question.

How is it that screwing the right front weight jack or preloading the right front spring can add weight to the left rear as and take weight off the the adjacent corners.

ratracer10
Dirt Maniac

Total posts: 141
posted August 31, 2004 09:48 AM  
I can get into as much detail as you want, but to save writing a book a few more questions are in order. Are you only concerned about static cross at this point or dynamic? The second gets really hairy, but I believe key. Are you concerned with how each adjustment changes cross or what the effect of the change in cross dose to handling?

I see you are an engineer as am I, and believe me I’m not trying to talk down to you just not sure what you are really after (I assume all of the above though). I have spent a lot of time doing calculations and simulations and some of the setup still comes back “feel” not numbers. However, I do believe I have gained more “feel” for what to do by playing with the numbers so to speak. I will be glad to answer your question to the best of my knowledge if you like. So I guess my real question is what part first?

Kromulous
Dirt Forum Champ
Total posts: 796
posted August 31, 2004 09:53 AM  
Equal & oppisite reaction deal there.

Screw the RF jack bolt in, and your adding more pressure to that corner, loosen it = less etc. Adjusting one wont nessessarily give you any cross weight, but it may under alot of load, ill handling but possable i guess.

Adding cross weight corner to corner, you would need to add turns to the RF and LR or vice versa.

If you had a car with equal springs and weights on each corner all the corner weights would be the same on a scale (not likely but this is just an example) then if you screwed in the RF and LR jack bolts 5 turns you would make the RF and LR heavier than the LF and RR, equal & opposite reaction. Push down on the spring and it pushes down on the the A-arm etc etc on thru to the ground. Your scales would now show some cross weight, LR to RF.

Thats the way i understand it, but i may be wrong myself. Like you i need to fully understand something before i can begin to manipulate it, so i'm right there with you on the no books out there that covers cross weights etc, wish there was.

Ratracer, write that book. Would love to see you cover both of your questions in great detail.

[This message has been edited by Kromulous (edited August 31, 2004).]

MikeN
Dirt Full Roller

Total posts: 27
posted August 31, 2004 10:31 AM  
Leapin'

When you put weight in, you're screwing in the weight jack, not pushing down the chassis. If you where pushing down the chassis, it would do exactly what you're picturing in your mind. When you screw down the weight jack onto the spring,
it is putting upward force against the chassis on that corner. (physically lifting the chassis up the screw) That upward force becomes a downward force at the opposite corner, putting in weight there too. The only way I can picture it unloading the other corners, is if the chassis moves around a point that is not in fact at the center of the car. I would assume the the front and rear suspension work together to put that "pivot point" along where two lines connecting the corners of the car would cross each other, and behind their intersection.

Rat,
If I'm wrong, correct me so nobody goes along believing the wrong thing.
Also, please, write a book. on both static,
and dynamic cross.

MikeN
Dirt Full Roller

Total posts: 27
posted August 31, 2004 11:06 AM  
Or, the "pivot" behind the line intersection may have nothing do with the suspensions, and have everything to do with the fact that the chassis is longer behing the rear spring than in front of the front springs, which puts the physically center of the chassis behind that intersection.
O.K. so maybe it does "pivot" around the center of the chassis. So kill me.
I spaced on the fact that, in this case, the table top is offset from the legs.

[This message has been edited by MikeN (edited August 31, 2004).]

leapinlizard
Dirt Freak

Total posts: 402
posted August 31, 2004 11:15 AM  
That's exactly what I can't get my mind wrapped around. My theory states if you lift the right front by whatever means, then the left rear must go down if the chassis is rigid. That's Newton's law. If you lift the right front then you take weight off the corner and add weight to the left rear. How is it you can add weight to the right front AND right rear, defying the physics law, which has stood for eternity? As long as geometry has been existed anyway.

Am I trying to hard here or what?

leapinlizard
Dirt Freak

Total posts: 402
posted August 31, 2004 11:23 AM  
Are you saying that the weight jack apply's pressure to the top of the spring and that force is transmitted through the bottom of the spring then again transferring to the control arm and wheel effectively pushing down the wheel which will cause a rise (since the ground is by all general intent's and purposes an immovable object) in the chassis then, the chassis pivot's at a point intersecting it's center of gravity from the X, Y, axis?

It makes a lot more sense to me.

Kromulous
Dirt Forum Champ
Total posts: 796
posted August 31, 2004 12:30 PM  
Thats the way i understand it Leapinlizard, finding that "pivot point" may take some doing but thats the idea as i understand it.

MikeN
Dirt Full Roller

Total posts: 27
posted August 31, 2004 12:36 PM  
Bingo.
When you lift a corner, the reason you are putting weight on it is because it is taking the weight from the opposite
axis.(Lifting the top up off the legs. That's why the four legged table analogy works so well. You're unloading the opposite corners. You can see it pick up
the legs. You can't on the car because of the spring keeping the tire on the ground.)
On the car, you're picking up the chassis.
The part you forgot about is the screw jack. It does not raise with the chassis when you screw it down. They do on the opposite axis though.

[This message has been edited by MikeN (edited August 31, 2004).]

leapinlizard
Dirt Freak

Total posts: 402
posted August 31, 2004 12:51 PM  
So preloading the spring is really a misnomer because preloading would require a force input on either end of the spring to compress it. By movind a wieght jack screw, that's attached to the frame, all you are really doing is changing the location of the respective corner's distance from the ground and not compressing the spring. Hence the need to adjust all four corners to the same to maintain not the same hieght's all around but to keep the hieght percentages in relation to one another. Inconclusion, ride height itself isn't as important as the relationship percentages (not to be confused with cross percentages) between the respective corners.

Duhhh,

I should have thought that through better.

ratracer10
Dirt Maniac

Total posts: 141
posted August 31, 2004 01:12 PM  
I think I now see how I misinterpreted the question and it looks like you are on the right track.

Yes that is exactly what the weight jack dose, it moves the top chassis mount down and compresses the spring. So if you move this mount lower on the chassis (lets say right front) it reacts on the chassis with an increased force (spring force = rate * compressed distance). Seams how a good race chassis is ridged when this corner raises the chassis will try and rotate around an axis thru the two adjacent wheels (left front and right rear). This will force the opposite diagonal corner of the chassis down (left rear) and compress the spring.

To make the remaining reactions easier to explain lets assume that all four corners have the same suspension (this eliminates the wheel rate calculations), same rate spring and the car is balanced with its cg in the center.

The right front of the chassis will gain the same amount of weight as the left rear and at the same time the left front and right rear will loose the same amount. The sum of the forces must remain constant and total chassis weight dose not change. So once again with a balance car and equal wheel rates while the left rear of the chassis drops, say 1”, the left front and right rear raise 1”. Now back to the right front. If you look how the ridged chassis is now setting the right front has raised by 3” so you have had to lower the weight jack 4” to get the same 1” of compression of the suspension. This goes to show why it is recommended to adjust all four weight jacks to change cross (i.e. RF and LR down 1 turn with LF and RR up 1 turn).

Of course if you really want to do it right you have to calculate each wheel rate and adjust each corner by the ratio of the wheel rates.

leapinlizard
Dirt Freak

Total posts: 402
posted August 31, 2004 02:35 PM  
Yeah,

What I said.

It's exactly what I needed.

Thanks Rat!!

I'm gonna stop talking bad about ya!!

ratracer10
Dirt Maniac

Total posts: 141
posted August 31, 2004 02:43 PM  
Yea I see I missed a couple of posts while I was typing. If I would have check back a few minutes later I could have saved on the carpel tunnel. lol

leapinlizard
Dirt Freak

Total posts: 402
posted August 31, 2004 02:59 PM  
I'd like to hear what you have to say concerning Dynamic Cross Percentages V.S static cross percentages and how velocity inertia affect the static settings. If of course, the carpel tunnel ever clears up.

MikeN
Dirt Full Roller

Total posts: 27
posted August 31, 2004 03:08 PM  
I'm with the 'liz on that one.
Only I would say it like this. "What the H-E-double-hockey-stick happens to the weight when the car is turning"
I would think that it has a lot to do with
spring rates, tire presure, speed, how fast you're turning (rate of yaw)track banking, and a dozen other things.
If there's a few basics one should keep in mind, I would sure like to know.

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 01, 2004 04:13 PM  
spring rates and tire pressures determinethe how the cross changes in the corner, and side note teh cars are not rigid objects they are also a big spring.

Alltel
Dirt Maniac

Total posts: 180
posted September 01, 2004 07:40 PM  
Being an educated man, I can honestly say the best thing to do is twist on the girl till she flies, write everything down, and then don't worry about it.

[This message has been edited by Alltel (edited September 01, 2004).]

x13jc
Dirt Freak

Total posts: 390
posted September 01, 2004 07:53 PM  
You set the chassis with perload (weight) in the left rear so when the chassis goes into the corner and the weight rolls over there is the weight you want on each rear tire. If you started equal on the rear when the chassis rolls the left rear would unload and the right rear would overload past the traction ability of the tires contact patch and you would spinout.

leapinlizard
Dirt Freak

Total posts: 402
posted September 01, 2004 08:29 PM  
So by increasing the cross percentage you thighten entry by making the car have more weight on the left rear then the right rear after the car noses over to the right front.

Decreasing cross will allow more down ward movement in the right front and will lift the left rear and making the car drive off the right rear. Adding rear brake will loosen the car as well but not on corner entry unless you brake all the way in (I don't). If you were to raise the left rear lower bar and cause the left rear to rise under accelleration you would also be adding cross and tightening the car. Many other adjustment's are made for fine tuning of when the car gains cross and tightens the car. Either at corner entry, or middle or exit depending on the track efficcy of traction, banking, (causes car to roll less) and horsepower levels (torque lift's left front and rotates rear pinion up and raises right rear causing a reduction in cross wieght to drive the left rear.


Right?

IowaFuzzy1
Dirt Maniac

Total posts: 136
posted September 01, 2004 10:11 PM  
You guys are missing the whole deal here.The reason it defies all theory and rules of logic is because it'a RACE CAR !!!so you do like Alltel said.just twist her till she flys and go racin!!That's just another symptom of this disease called RACING that we all have...

RACECAR spelled backwards is still RACECAR

Ego Racing
Dirt Forum Champ
Total posts: 724
posted September 02, 2004 02:31 PM  
By GEORGE I THINK HE GOT IT! We try to get our SLM to go into the corner and cause the weight dist to equal out on the rear. Upon application of the go pedal the left rear goes onto the bars and adds the wedge we want to tighten it up comming out of the corner.
Left, rear and cross %'s are important but the cross or wedge is also dictated by the amount of weight transfer to the right rear, the track and as you noted the driving style. We have run heavy shot springs and 60# of bite in the rear and the car flew when I was in it, the owner couldn't drive the car. We lowered the rear spring weights, lowered the rear % (softer springs = faster weight transfer) and went to 165#'s of bite and he is VERY fast but it took me a while to get upto speed. He also likes to run it on 3 wheel brakes and I like 4 wheel. He uses the brakes to put it into the corner and I throw it in on 4 wheel brakes and it works good.

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