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Author Topic:   Rear % ?
DEEDDUDE
Dirt Maniac

Total posts: 111
posted March 01, 2002 02:44 PM
I need some help with a question or two about rear %. On dirt StreetStock cars it is recommended to run between 52% and 54% rear weight. I have a metric frame SS and it seems to work better with about 50.5% rear, anymore and the car is loose in, middle and out. Why is this? I guess my question is "why are these high amounts of rear % needed?"
I've seen cars with +300 lbs. of lead bolted to the leftrear bumper to get the % up. Metric frame,(same as mine) and they fly. Some say it has a pendulum and others say it has no effects.???
I've heard to increase rear % to loosen and also increase rear to tighen. Which one is correct? In my case I decreased rear % and it tighen the car up.
Any comments are welcomed, I'm trying to learn the reason(s) for the large % of rear weight. Sorry for rambling but I've never had a clear answer to these questions. I posted on an other forum that suppose to have chassis gurus on and they never answer any of these type questions. Are they loaded? There are more than a few on this forum that I do believe know more that these so called gurus know, because you guys are racers like myself. I just don't have the experience yet. Thanks for your time.

jdukes74
Dirt Full Roller

Total posts: 37
posted March 01, 2002 03:09 PM
I had a 76 malibu and it liked 50 or 51 percent rear. I know this ---with less rear it was loose(the rearend) and with more it pushed(couldn't turn). so to me it is all about the weight on the tires ,,,in other words a tire with more weight is going to stick to the ground better. so you are achieving the balance that gets all 4 corners working together and working with your driving style. I had guys tell me you had to have 56% reAR weight ,,,all i had to say about that is (that %^&^%* wall hurt!) . This is just some personnel experience some of you better learned guys need to help this boy out! J-2 JDukes

RangeRover
Dirt Maniac

Total posts: 174
posted March 01, 2002 06:16 PM
We all know that what works for one car, or driver for that matter, may not work the same on another. However, there are pretty good rules in this race car set-up game that one can use to improve. Generally, adding rear % will tighten the car and lowering rear % will loosen the car up like jdukes stated. That's pretty simple, but then we throw in the ton of other variables that can come into play and you can see why one car is so different from another. For example, let's say we have your car with 50% rear weight and 50% cross and it works good. Now we have your buddy with a very similar car except, whether he knows it or not, he has 3/16" RR trail in the rear end. Let's throw in some roll oversteer in his set-up, and maybe his rear shocks are a tad too stiff for a dry feature. We could even play with his rear wheel offsets moving the tires to the right, but let's hold off on that. OK, I think you can see that he is going to need at least 54% rear and maybe more cross just to keep from spinning out in front of the pack when the track get's a little slick! I didn't even mention stagger or air pressures, but what I'm getting at is that your car needs to have all the bases covered (at least close) and then the rules of thumb will help you tune the car better or help you adjust to changing conditions.

There are many different ways to get to the same place (good handling), just know where you're at first. I didn't even mention driver induced handling problems- creating one problem by trying to compensate for another, but that's another chapter.

DEEDDUDE
Dirt Maniac

Total posts: 111
posted March 01, 2002 09:04 PM
Thanks for the replys. I would still like to hear others point of view.

RR I really don't have any handling problems, last time out the car was really hooked up and had more forward bite than ever. This was after I moved most of my ballast forward. The car got tighter. I still need to tighten exit a little, but I wont be moving the weight. Just put a round or two in the left rear. Thanks agin for the replys.

juni
Dirt Full Roller

Total posts: 73
posted March 02, 2002 09:14 AM
RangeROVER is correct there is more things that have to be considered .I have ran as much as 61% rear on tracks that are black dry slick but i changed my rear lower arm angles to create rear steer on entry when the body rolls . But my car weighed a ton to get that much rear . WE won with this set up but then a guy shows up and has about 200 lbs added weight total and his car is 1000 lbs lighter then our car has almost a full gear point less , smaller motor then ours and he passed us and beat the rest buy a half lap . So you have to take a look at the whole picture and think it all over. Thats what I did and am still sorting things out. The higher rear can make a car tight if your cross weight is high and your rear wedge is high . If you put your weight way back in the trunk and have very little wedge the car should be loose but as you put wedge in the rear the car should get tighter . Thats my two cents worth . ASK awkwardjeff or snowman lets hear from them.

bship
Dirt Roller

Total posts: 7
posted March 02, 2002 06:41 PM
adding rear percentage will loosen a car in pure cornering,but will increase forward bite under power. more rear percentage is needed to tighten exit. a car with a strong engine will want more rear percentage. to make more rear percentage work;other adjustments are needed to dial out the loosening effect of more rear weight.normally the cross weight should be increased when rear weight is increased.

dirtracer17
Dirt Forum Racer

Total posts: 88
posted March 05, 2002 07:17 PM
You need to talk to owracer. He has built several in the past year and he will be glad to help.

YAZ
Dirt Forum Racer

Total posts: 77
posted March 06, 2002 09:32 PM
DEEDDUDE,
I THINK THAT IF YOUR CAR IS HANDLING GOOD
AND HAS GOOD FORWARD BITE YOU SHOULD LEAVE IT THAT WAY AND LIKE YOU SAID DO A MINOR
ADJUSTMENT TO THE WEIGHT JACKS FOR CORNER EXIT. IT THAT'S WORKING FOR YOU WHO CARES WHAT IS WORKING FOR THE OTHER GUYS. LIKE RANGEROVER SAID WHAT WORKS FOR ONE CAR OR DRIVER DOESN'T WORK FOR ALL.

snowman
Dirt Maniac

Total posts: 140
posted March 12, 2002 12:42 PM
Juni, Thanks for the compliment. However, You could be SO wrong about my opinion being worth hearing....lol...Thanks

RangeRover, I think You made a really great post about a difficult subject.

DeedDude, I'm no guru or expert on the subject. What I am is,...willing to help if I can.I will try to answer(or at least give my opinion) to all of your questions. The first thing I learned when I began to study race cars was how little I actually knew.

The first question I would like to deal with is " are these questions loaded". Yes,they are. Loaded, because there are few absolutes. If you study long enough,You will find the various authors and experts contradicting each other. So who is right? Sometimes both. No one wants to be wrong,but no matter what I say,someone will be able to say (correctly) that I am wrong.This is because this is a package deal,changing one thing changes everything.Sometimes making a change in the "wrong" direction will improve your car.

"why are these high rear percents needed ?"
The "circle of traction theory" basicly says that four equally loaded tires will provide the maximum cornering abillity. This is true.
But,it's a long way from telling the whole story. The whole story,(even if I knew it,Which I don't) couldn't be told in a 500 page book,much less a post here.But hang with me,I'm trying.

You need to know that adding weight to a tire increases that tires traction. As the car is in motion,weight(and traction) is constantly changing. As you accelerate, weight is transfered to the rear tires,and when you brake,weight is transfered to the front tires.

So, by starting with the rear heavy,You increase the grip of the rear tires which helps you accelerate. And... during braking,the weight is transfered to the front tires... where we are trying achieve four equally loaded tires. Same reason we start with the left side heavy...as we corner the weight will transfer to the right.
So, all this static left and rear weight that we are puting on the car is to create four equally loaded tires during cornering.
Thats the theory, and it's true,as far as it goes.

When was the last time you saw a dirt late model go around a corner with four equally loaded tires? Again,this is a package deal.
Mathmaticly, the late model with little or no weight on his left front has lost cornering power.So why is he faster? He is faster because he has increased his rear grip,and he is steering with the rear tires.
I include this about the late model to demonstrate the difference in theory and practice.

About the "pendulum effect", What people are talking about is the polar momment of inertia. This theory says that the further away the weight from the center of the car,the harder it becomes to change direction of the car,and,once the change of direction has began,the harder it becomes to stop. Basicly,having a lower polar momment will create a more responsive car. Polar momment is important. I have heard people say that it isn't.I think one of Steve Smith's books said something to the effect that this is important for an F1 type car and not important for a stockcar. I would disagree. Now before any Steve Smith fans start throwing tomatoes at me (I don't mind the tomatoes so much,but golf balls hurt),Let me clarify,polar momment is polar momment.It will effect different cars in the same manner.It is no where as important on a stockcar doing 60 in a corner as it is in a F1 car doing 250. SO,the effect of a high polar momment is the same,but the importance is not.

This is one of the areas where a step in the "wrong" direction may improve a car. Adding the weight on the bumper increases the polar momment and should be considered "wrong",But,depending on thier combination,people have done this and improved thier car.I'm not reccomending this,I'm just pointing out how difficult the "right" answer can be to find.

About adding weight to the rear to loosen or tighten the car.... In general, a front heavy car will tend to understeer,and a rear heavy car will tend to oversteer.I'm talking about in the dynamic state. Remember,by making the rear 52-54% in the static state,we are trying to achieve a balance of front to rear weight while cornering.

Adding rear weight will generally tighten the car.But it depends on what you have to begin with.You can go to far and actually loosen the car,Put that weight on the bumber,and as the rear comes around,it wants to keep coming around(higher polar moment)and you spin out. So,when one guy tells you more will tighten and another says it will loosen, They are both right,and both wrong,it really depends. Package deal.

I hope something in all this helps.I know that you already know much of this,but I couldn't explain importance of rear percent without mentioning some basics first.


snowman
Dirt Maniac

Total posts: 140
posted March 12, 2002 03:31 PM
I see one area in my post that I should clairify, When I said "adding rear weight will generally tighten the car", what I mean is adding rear weight will reduce understeer,thereby allowing the car to turn tighter. (you can keep reducing understeer to the point where you have oversteer)

Some people use the terms "tight or tighten" to mean " push or understeer",I should have only used the terms oversteer and understeer for clairity. Sorry about that.

When you start out with a car with no added weight, You have a front heavy car that will understeer.(push) As you add rear percent,you will reduce understeer. Reduce understeer enough and the car will become neutral. Any futher additions of weight past neutral will create oversteer (loose).

Of course,there are many factors that contribute to over/understeer. I am only speeking of the role of weight in the package.

SLEEPY GOMEZ
Dirt Maniac

Total posts: 199
posted March 12, 2002 10:18 PM
quote:
Originally posted by snowman:
I see one area in my post that I should clairify, When I said "adding rear weight will generally tighten the car", what I mean is adding rear weight will reduce understeer,thereby allowing the car to turn tighter. (you can keep reducing understeer to the point where you have oversteer)

Some people use the terms "tight or tighten" to mean " push or understeer",I should have only used the terms oversteer and understeer for clairity. Sorry about that.

When you start out with a car with no added weight, You have a front heavy car that will understeer.(push) As you add rear percent,you will reduce understeer. Reduce understeer enough and the car will become neutral. Any futher additions of weight past neutral will create oversteer (loose).

Of course,there are many factors that contribute to over/understeer. I am only speeking of the role of weight in the package.


I believe you guys have it mostly right everywhere. But let me add a little. The polar moment of inertia has the same effect like snowman says. He is also right that its importance is not the same in different weight and type of cars.

My IMCA Stock, metric frame car runs with 240 lbs of steel plate st the rear bumper. I get 54% rear. This much increases rear traction. The car also has 55% left.
Build the car heavy on the rear and left keeping the total in line. My I-stock weighs 3160 with driver and fuel.

Loose, tight etc can now be adjusted by springs and cross weight. Wheel travel determines springs. Go soft enough to get close to 4" bump travel at each wheel. Don't let ANYTHING bottom out in the suspension. Shocks need to match up with springs reasonably close. Use what ever cross weight the car wants to get around the corner.

Maybe I could come and take lessons from Snowman. SLEEPY

snowman
Dirt Maniac

Total posts: 140
posted March 14, 2002 03:08 AM
Sleepy, Hi. First, I would like to begin by making an apology to You. I do believe I have offended You by my comments concerning polar momment,But it was unintentional.

I have been away from home the last week and a half. While I'm away from home,my wife sometimes checks the streetstock forum,and makes a note of any questions that I may have been asked. That way,I know which to read first, it takes a while to catch up if you are gone very long.

So,at the time of my post,I thought I may be marginally disputing Steve Smith. I was not aware that You had joined the Forum or had said anything about polar momment.

Granted,I'm not the smartest guy around,but had I seen posts made by a guy named Sleepy Gomez giving out racecar advice, I probably could have made the connection between You and that guy from Circle Track.

It just so happens,that a couple of weeks ago, a friend and I were talking about polar momment.He had been talking to a friend of his,who had told him what Smith had to say. So, I assumed the question was brought about by Smith's book.

After taking time to read all the posts, I find You have not only joined the forum,but You have talked about this very thing.Futhermore, by making my post, I have put You in a position where You feel you had to defend Yours.

I would not intentionally do that. I hope by explaining this You will understand how it came to be,and accept my apology. I believe that a good measure of a man is not the mistakes he makes, (Lord knows I make my share), But what he does to make it right afterward. Now I have done all I can do.

I only post on this forum to help, and to interact with people who share my intrests. If it were possible for me to absolutely,positively prove someone wrong who disagreed with me on this forum,I would not do it. How can I feel good about helping someone if doing so requires me to embarrass or hurt someone else's feelings? No way.
It is far more important to me to be kind to others,than it is to be considered "right". But, I may make an exception to this on safety concerns.

------------------------------------------

I really don't like to answer questions like these because there is no difinitive "right" answer. Not in Winston Cup and not at the local level. We will not agree on everything,but I hope you don't take it personally, I won't. In fact, it's only when someone dissagrees with me that I have an opportunity to learn something new. I think I will certainly learn something from You,and I am not being sarcastic when I say that.

On this PMI deal, We are not as far apart as you may think. When I think of the forces involved with weight in motion,I cannot consider it insignificant,But I will agree that it is not of major concern.

I think your rear weight works for you because you have tuned the package for it.

I said earlier that increasing rear weight past neutral will increase oversteer. In practice,with the package most guys run, it seems to be true. I think You have discovered that with the right combination,You can't create oversteer with weight alone.(at least with the weight required to meet minimum weight) Van Valkenburg went as far as to say, ""rear weight bias causes oversteer" is a total myth,considering all the others factors that have an overwhelming influence on control and stability."

I would ask one favor of you, Since you are an accomplished builder and write for Circle Track, would you please include your complete set-up for your car. I think many people will seek your advice,and if they run your weight without the rest of your set-up,they just may have a problem.

In closing I apologize again, and I join with everyone else in welcoming you to the Forum.


By the way, Does anybody know which Steve Smith book that guy was talking about? I guess I ought to read it and find out what he actually said about the deal before I run my big mouth any further. Steve will get the last laugh when he puts my money in his pocket.....lol (I do like Steve's books)

DEEDDUDE
Dirt Maniac

Total posts: 111
posted March 15, 2002 01:26 PM
Thanks for the help guys. I apperciate everyones replies. I guess I'll try to get more weight on the rear and see how she works. I've noticed cars at the track with lots of weight on the rear frame rail, and it seems to work for them. Thanks again. Deed

SLEEPY GOMEZ
Dirt Maniac

Total posts: 199
posted March 18, 2002 01:08 PM
quote:
Originally posted by snowman:
Sleepy, Hi. First, I would like to begin by making an apology to You. I do believe I have offended You by my comments concerning polar momment,But it was unintentional.

I have been away from home the last week and a half. While I'm away from home,my wife sometimes checks the streetstock forum,and makes a note of any questions that I may have been asked. That way,I know which to read first, it takes a while to catch up if you are gone very long.

So,at the time of my post,I thought I may be marginally disputing Steve Smith. I was not aware that You had joined the Forum or had said anything about polar momment.

Granted,I'm not the smartest guy around,but had I seen posts made by a guy named Sleepy Gomez giving out racecar advice, I probably could have made the connection between You and that guy from Circle Track.

It just so happens,that a couple of weeks ago, a friend and I were talking about polar momment.He had been talking to a friend of his,who had told him what Smith had to say. So, I assumed the question was brought about by Smith's book.

After taking time to read all the posts, I find You have not only joined the forum,but You have talked about this very thing.Futhermore, by making my post, I have put You in a position where You feel you had to defend Yours.

I would not intentionally do that. I hope by explaining this You will understand how it came to be,and accept my apology. I believe that a good measure of a man is not the mistakes he makes, (Lord knows I make my share), But what he does to make it right afterward. Now I have done all I can do.

I only post on this forum to help, and to interact with people who share my intrests. If it were possible for me to absolutely,positively prove someone wrong who disagreed with me on this forum,I would not do it. How can I feel good about helping someone if doing so requires me to embarrass or hurt someone else's feelings? No way.
It is far more important to me to be kind to others,than it is to be considered "right". But, I may make an exception to this on safety concerns.


Hey snowman, thanks for the kind words. You are absolutely right about a "package" in making a car handle. Also about the book. I have considered writing one but can't find a publisher who will allow me 10,000 pages. That was a joke!

So, right wrong or indifferent here is the set up I use on a metric frame I Stock.
ALL TIRES ARE IMCA Mc Creary G-60 15
GEARING IS: HIGH GEAR THROUGH A POWERGLIDE (1:1) with 6.50:1 in 9" Ford
FRONT LB 1472 46.4%
LEFT FRONT RIGHT FRONT
LB 856 %27 LB 616 % 19.4
SPRING 950 SPRING 1150
SHOCK #5 VALVE SHOCK #5 VALVE
AIR 14 AIR 18
CASTER 5 DEG CASTER 9 DEG
CAMBER POS 1 DEG CAMBER NEG 2.5 DEG
TOE SETTING 3/8" OUT

LEFT LB 1724 %54.3
CROSS WEIGHT 1484 %46.7
LEFT REAR RIGHT REAR
LB 868 %27.3 LB 836 %26.3
SPRING 225 SPRING 200
SHOCK #5 VALVE SHOCK # 5 VALVE
AIR 14 AIR 15
REAR LB 1704 %53
TOTAL WEIGHT ON THE TRACK DRIVER AND FUEL
LB 3176
NOTES: empty weight is 2756 no driver, no fuel NO BALLAST. TOTAL includes 210 pounds STEEL PLATES inside the body as far to the lerft as possible and against the rear bumper.

The setup as shown is what we run on a 1/4 mile bull ring. Good clay, wide track low to med bank but it is usually dry by feature time. Yes, I make adjustments at the track. After I get the car suitably ******* up I go the shop, put it back on the scales and return to the original setup. I then promise myself to only make small adjustments in the future.

I think the rear springs could be change to 200L and 175 R. Remember the front shocks are mounted to the top ball joint on the A frame, therefore they can be this light.

I know this is not the only setup which works but it works for me on my track. I hope some of this helps. SLEEPY

c21
Dirt Freak

Total posts: 346
posted March 18, 2002 01:18 PM
toe'd out 3/8"?

SLEEPY GOMEZ
Dirt Maniac

Total posts: 199
posted March 18, 2002 10:02 PM
quote:
Originally posted by SLEEPY GOMEZ:
Hey snowman, thanks for the kind words. You are absolutely right about a "package" in making a car handle. Also about the book. I have considered writing one but can't find a publisher who will allow me 10,000 pages. That was a joke!

So, right wrong or indifferent here is the set up I use on a metric frame I Stock.
ALL TIRES ARE IMCA Mc Creary G-60 15
GEARING IS: HIGH GEAR THROUGH A POWERGLIDE (1:1) with 6.50:1 in 9" Ford
FRONT LB 1472 46.4%
LEFT FRONT RIGHT FRONT
LB 856 %27 LB 616 % 19.4
SPRING 950 SPRING 1150
SHOCK #5 VALVE SHOCK #5 VALVE
AIR 14 AIR 18
CASTER 5 DEG CASTER 9 DEG
CAMBER POS 1 DEG CAMBER NEG 2.5 DEG
TOE SETTING 3/8" OUT

LEFT LB 1724 %54.3
CROSS WEIGHT 1484 %46.7
LEFT REAR RIGHT REAR
LB 868 %27.3 LB 836 %26.3
SPRING 225 SPRING 200
SHOCK #5 VALVE SHOCK # 5 VALVE
AIR 14 AIR 15
REAR LB 1704 %53
TOTAL WEIGHT ON THE TRACK DRIVER AND FUEL
LB 3176
NOTES: empty weight is 2756 no driver, no fuel NO BALLAST. TOTAL includes 210 pounds STEEL PLATES inside the body as far to the lerft as possible and against the rear bumper.

The setup as shown is what we run on a 1/4 mile bull ring. Good clay, wide track low to med bank but it is usually dry by feature time. Yes, I make adjustments at the track. After I get the car suitably ******* up I go the shop, put it back on the scales and return to the original setup. I then promise myself to only make small adjustments in the future.

I think the rear springs could be change to 200L and 175 R. Remember the front shocks are mounted to the top ball joint on the A frame, therefore they can be this light.

I know this is not the only setup which works but it works for me on my track. I hope some of this helps. SLEEPY


Sorry about the format. I typed all of the specs in like a chart. The computer didn't read it that way. I'll try to correct it tomorrow. And yes, I DO USE 3/8" TOE OUT. Don't worry about drag in the straight, it is too short. Get the wheels pointed properly in the turn, whatever that is. Check toe setting with the wheels turned 15 deg. It takes two LONG straight edges and a carpenters square. SLEEPY

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