Stiffen rear springs for more traction

[XxrusianxX]

Hey guys, I am going to the track this friday and i need a way to keep my rear springs from comprassing in order to get more traction for the front wheels. This can, or should, be a temp. thing so that I could take it off after the day at the track...ANY IDEAS?????

 

[DG-FNR]

We talking autocross, road racing, or drag racing here?

DG

 

[aves911]

My guess would definitely be drag racing.

-Jacob

 

[XxrusianxX]

your guess is correct, drag racing it is

 

[DG-FNR]

OK, drag racing it is.

I'm also going to assume we're talking about FWD here rather than AWD. The same stuff we'll discuss here applies to both driveline types, but it's a bigger effect with a FWD.

Whenever you accelerate, weight is transferred off the front wheels onto the rear wheels. How MUCH weight is a function of the height of the CG, the length of the wheelbase, and how hard the car is accelerating - AND THAT'S IT.

The amount of grip a tire produces is a function of a lot of variables built into the construction of the tire (that we can kinda all roll into one and call it "tire type") the weight pressing down on the tire, its temperature, the angles of the contact patch (camber and toe) relative to the road surface and direction of motion, and a measure of the dynamics of the tire that we'll call "contact patch stability" for the moment.

Of these, the one that we'll be most concerned about for the moment is the weight pressing down on the tire.

Now A FWD has a problem. Launch hard (high acceleration) and weight transfers rearward (as it must). But that takes weight off the drive tires, and eventually som much weight is taken off the tire that it can no longer maintain that level of forward acceleration and the tire starts slipping. But that drops how hard the car is accelerating, which sends weight back forward again, whch gives the tire more grip, so now the tire hooks up and the car accelerates harder which sends weight rearward which unloads the tire and around and around we go.

That's why FWDs make that "chirp chirp chirp chirp" sound at launch - that's the tire loading and unloading, gripping and regripping, until finally engine power is no longer enough to produce enough acceleration to unload the tire and the tire stays gripped.

With me so far?

Now here's the kicker - THE SPRINGS ON THE CAR HAVE NOTHING TO DO WITH THIS.

Changing springs will NOT change the amount of weight transfer. Weight transfer is wheelbase, CG height, and acceleration NOTHING ELSE. Replace the springs with coat hanger wire, or replace them with solid metal tubes, and weight transfer *will not change*

What WILL change is the *pitch angle* that the sprung mass adopts in response to the amount of weight transfer. Strictly speaking, pitch angle is a function of the "pitch resistance" (which is primarily a function of spring rate) and the "anti" geometry built into the car (anti-dive, anti-squat) but we can squint at it and pretend it's just the springs - because you have no control over any of the other variables so it's a wash.

As the car pitches, it drags the suspension pickup points with it. As that happens, it is possible to change the camber and toe angles that the tire sees, though a combination of camber compensation and bump steer. That means that the contact patch is being pried and/or twisted around as the car pitches - that's what I call "contact patch stability"

If there's a lot of this going on , then the contact patch is to some degree unstable, and that is unlikely to make the tire happy. If there is none of this going on, then the tire is more likely to be happy. A happy tire is a grippy tire, so it's worth making the effort to keep it happy.

There are two ways to fix the problem - either fix the geometry so that the pitch motion doesn't twist & pry on the contact patch, or reduce the pitch motion so the pickup points aren't moved around so much.

It's worth looking at the first option. It's more work, but cheap to fix, and once fixed, stays fixed with no side effects. Do a bump steer check (probably an inch in bump and 2 inches in droop) and then reduce bump steer as much as possible in this range. Camber compensation you can't fix (its a good thing for cornering) but measure it and get an idea for how bad it is.

For the second option, you have two choices:

Adding rear spring will reduce the pitch angle. It will also make the car more tail-happy, and taken to extremes it can make the car a handful to drive if it ever does anything other than go in a straight line. Coilovers make spring changes easiest, but you're still looking at pulling the shock and spring assembly out of the car to make the change. Doable, but a pain in the ass.

The other option is to put stiffer shocks on it. In particular, using a lot of front rebound force will hold the nose down and reduce the pitch angle. It won't change the MAXIMUM pitch angle (the car will eventually get there) but it will slow down how QUICKLY the car reaches max pitch angle - and in a way, that *rate of change* of the contact patch effective angle is more important than what the ultimate angle is. A slow twist is less likely to break the tire loose than a hard twist - even if the hard twist doesn't go quite as far.

Happily, off-the-shelf Koni shocks have a LOT of rebound adjustment in them, and adjusting them is as simple as twisting a knob on the top of the shock.

So my recommendation to you is to do the following:

1) Have your local race shop or alignment shop check your front bump steer, and do your best to eliminate it.

2) Buy a set of Koni Yellow "Sport" shocks for the car, put the rears on full soft, put the fronts on full soft, and then lowly stiffen the front shocks until your 60' time peaks (you want them as soft as possible) Then mark down that setting, and that's your race setting.

DG

Far North Racing Team Home Page

 

[robvas]

On my old FWD with stock suspension, I couldn't hook at all. Every time I got on the gas the car would sink WAY back (hello 130,000 mile stock suspension) and the weight would come off the front tires, and I'd get a ton of wheelspin.

I added plain old GR2's and Eibach pro-kit springs. No more squat, and it hooked dead every time. I'd get a light chirp from clicking over to 'high boost' but no more spin/hop.

 

[XxrusianxX]

In responce to DG-FNR... ok rear springs first of all have a lot to do with the traction of the front tires. The first thing you siad which is wrong is that the weight on the tires makes a difference...if you knew physics then you would know that weight is not a factor of friction so having more weight on a tire does not provide more friction. What does provide more friction is more of the tire in contact with the road...one way of woing this is to keep the front of the car lower allowing more of the tire to touch the ground. If you lock the rear springs then the car can not sit back and will remain almost in a flat position... and that is what you want for that added traction.

 

[jtmcinder]

The first thing you siad which is wrong is that the weight on the tires makes a difference...if you knew physics then you would know that weight is not a factor of friction so having more weight on a tire does not provide more friction.

This is really pathetic. Step away from the keyboard. Now.

- Jtoby

 

[DiamondStarM]

In responce to DG-FNR... ok rear springs first of all have a lot to do with the traction of the front tires. The first thing you siad which is wrong is that the weight on the tires makes a difference...if you knew physics then you would know that weight is not a factor of friction so having more weight on a tire does not provide more friction. What does provide more friction is more of the tire in contact with the road...one way of woing this is to keep the front of the car lower allowing more of the tire to touch the ground. If you lock the rear springs then the car can not sit back and will remain almost in a flat position... and that is what you want for that added traction.

How can you make a solid argument against someone after asking for help, and then be wrong on top of that? Yes surface area makes a difference, hence why FWD drag cars have wide front slicks and RWD's have wide rear slicks, but making the front of the car lower doesnt make anymore tire touch the road. Now as for the weight vs. friction part, I'd say your wrong there too. Do a physics experiment and make a 3x3x3 balsa wood block and a 3x3x3 lead block. Now put them on the table in front of you and push them at the same time. Go again and push the lead weight with your right hand if it helps but that lead weight is harder to move because the extra weight creates more friction on the table. You have to break that initial "Static Friction" of both objects and the lead block has more because of its weight.

Schools out.

 

[defrag010]

I had a friend with a badass turbo lebaron, and he would bolt in "shock bars" onto his rear suspension so it had absolutely zero travel at all. With an open diff and some 225 tires, he could cut 1.7/1.8 60' times consistantly.

I'm sure you could get some welder/fabricator to take an old set of blown rear struts and weld the piston to the shock body at the statid ride height of the rear of your car. Just spend 5 minutes and bolt 'em in before you go to the track, but be careful on the way there. You could always just park your car at the track and do it there.

 

[jtmcinder]

Close.

The interface between a tire and the pavement is not the same as a block of wood sliding on a table. Tires "grip" the road; they do not -- unless you have done something very wrong -- slide across it. Most of all, the function relating weight to grip is not linear (which makes it very different from the function relating weight [or normal force] to sliding force). The function relating weight to grip is sub-linear in particular, which is a very important concept to absorb since much of the negative effects of weight transfer starts from this fact.

- Jtoby

 

[jtmcinder]

I'm sure you could get some welder/fabricator to take an old set of blown rear struts and weld the piston to the shock body at the statid ride height of the rear of your car.

You need to take a keyboard break, too. The idea of running a car that has no (effective) suspension is almost as whacked as the previous.

Defiant - Time to put this puppy to bed.

- Jtoby

 

[DiamondStarM]

Close.

The interface between a tire and the pavement is not the same as a block of wood sliding on a table. Tires "grip" the road; they do not -- unless you have done something very wrong -- slide across it. Most of all, the function relating weight to grip is not linear (which makes it very different from the function relating weight [or normal force] to sliding force). The function relating weight to grip is sub-linear in particular, which is a very important concept to absorb since much of the negative effects of weight transfer starts from this fact.

- Jtoby

I wasnt trying to make a direct comparison between the two, just a simple way to show weight does effect friction, but thanks for the clarification. Maybe it will help others to understand. Although like you say, the thread has gone to hell, the information is really helpful if you read though it enough to get a good understanding of it. Maybe cut out the good stuff and throw it in the tech pages for handling and suspension so people can try to understand how all this works a little better?

 

[DG-FNR]

Come on guys, there is no reason to jump all over him like this.

That misunderstanding about how friction and grip works is pervasive throughout the world. It's the end result of the way the subject is taught in schools. Give the guy a break; he's obviously new enough to the game that he hasn't had the difference explained to him yet.

Now then, let's look at friction for a moment.

The defininion of "friction" given in high school (and for that matter, even undergrad) physics is the Newtonian one, and yes, in the Newtonian world, frictional force is directly proportional to normal force (which is a fancy way of saying "the weight pressing down on a thing")

The Newtonian model assumes rigid, incompressible bodies interacting with each other. When you *have* rigid, imcompressible bodies - as you do in the science labs designed to illustrate this phenomenon in schools - the law holds just fine.

But a tire is neither rigid nor incompressible. It is rubber filled with air. It is springy. It is squishy. And there is a degree of molecular *adhesion* going on, if it is a race tire or if there is VHT on the track (it is "tacky")

As such, a tire does NOT behave as a Newtonian body. Typically, we use the word "grip" instead of "friction" to differentiate what is going on at the tire/road interface road from Newtonian friction.

Now an interesting point about the way that tires produce grip. The more you force down on a tire, the more grip you get. But the AMOUNT of grip gained per unit normal force decreases with increasing normal force. So start with a tire, add 100 lbs of force onto it, and maybe your get 100lbs of grip. Add another 100lbs, maybe you get 80 lbs more grip. Add another 100lbs, now you get 50 lbs more grip. Total grip is always increasing (100+80+50) but you get less and less of it per unit load as load is added.

This is called "load sensitivity" and is an important part of setting up a race car. It means that any set of tires always produces maximum grip while equally loaded.

It is also important to note that the characteristic grip vs load curve for any given tire changes with camber angle and with tire pressure.

So then, on a FWD car, given that rearward weight transfer is a function of wheelbase, CG height, and acceleration, the ONLY way to reduce rearward weight transfer and increase grip is to lower the CG or lengthen the wheelbase.

That's why those pro FWD cars run wheelie bars - when the wheelie bars are in contact with the ground, the wheelbase is longer, and the weight transfer is less. That's also why they are as low as they are - lower GC means less weight transfer.

That's also why there is a rule that limits how much static forward weight distribution they are allowed to run. If you must weigh, say, 2000 lbs, and you get 500 lbs of weight transfer, then you will get better grip if the car weighs 1500/500 (a net 1000 lbs on the front wheels at launch) than if it weighs 1000/1000 (a net 500 lbs on the front wheels at launch) For a while, the FWD guys were running super nose-heavy, like 90% nose heavy.

Unfortunately, you cannot sustain the acceleration G that you get at launch on the big end of the track, and that means that as the car went down the track, weight moves forward, and eventually the rears get so unloaded that any slight disturbance gets them to let go, and the car goes out of control. A car with 90% front static weight distribution is unsafe to drive, and I think they are now limited to something like 80% (which is still really high!)

OK then, so if springs don't change weight transfer, why is there so much empirical evidence of people increasing rear spring rate and gaining grip?

That comes back to what I said in my first post about the suspension points being dragged around as the chassis pitches. If that movement is causing a toe change (bump steer) or a camber change (camber compensation) in the tire, then the tire's contact patch is being wrenched around and that will tend to reduce grip.

If the front wheels were mounted on a live axle (which cannot have a toe or camber change if the vehicle pitches- well, mostly) then you could run super soft springs and the car could make huge pitch changes without changing grip.

There, now you have enough information to re-read that first post and understand everything in it. If you have any more questions, feel free to ask.

DG

 

[jtmcinder]

The defininion of "friction" given in high school (and for that matter, even undergrad) physics is the Newtonian one, and yes, in the Newtonian world, frictional force is independant of normal force (which is a fancy way of saying "the weight pressing down on a thing")

All of you: Step away from the keyboard. Go read a high school physics text. Or if you refuse to put the computer away: http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Dynamics/Forces/FrictionalForce.html

Frictional force equals the coefficient of friction times the normal force.

- Jtoby

 

[XxrusianxX]

Wait now I am lost...so how do you stiffen the rear springs temperarely

 

[jtmcinder]

Well, you can use those inserts that hold the coils apart. They are mostly used on station wagons and such when you overload the rear. Problem is: they will raise the rear as well as stiffen it. If you think about it, anything that stiffens the rear while not moving the perch and/or hat will also raise the rear. (Hm. Given the level of physics knowledge displayed so far in this thread, maybe people shouldn't think about it; maybe they should just take my word for it.) In any event, you don't want to raise the rear, since it will increase weight transfer. That's a RWD trick.

So your only real option for temporarily stiffening the rear is a set of shocks with way too much compression damping, such as cranked-up AGXs.

- Jtoby

 

[defrag010]

You need to take a keyboard break, too. The idea of running a car that has no (effective) suspension is almost as whacked as the previous.

Defiant - Time to put this puppy to bed.

- Jtoby

Sure thing, Daddy!!

Listen, people have been using "shock bars" on drag race tracks since the creation of FWD cars. I don't care what sounds good on paper, on the keyboard, or in physics class - people have been doing that for quite a while now.

I can't help it that I've seen people cut AWD times on stock open-diff FWD cars with these tricks.

A quick google search yielded this: http://www.thedodgegarage.com/traction.html . He even has a picture of his little setup of the same thing I'm talking about. He says he doesn't run his best times, but he lists it on his page for improving FWD traction.

 

[DG-FNR]

You'll note, however, that he also states that they work OK on a "glass smooth track" but are not recommended for anything else.

That's consistant with the behaviour of a car that has a really, really high rear spring rate. With that bar in place, the spring rate is whatever the rate is of that bar in compression... and I'll bet that's pretty high. Like near-infinate high.

Yes, all this tomfoolery with the rear springs CAN and DOES increase front grip - but it is important to understand WHY. And the "why" has everything to do with the front suspension geometry wiggling the contact patches around as the chassis pitches and NOTHING to do with weight transfer (save, of course, that it is weight transfer that is causing that pitch to begin with)

Now one you grok the "why" of why stiffer rear springs work, that can lead you down other paths that effectively do the same thing without all the hassles of changing springs (which is the only way to increase spring RATE) or without the higher-CG tradeoff of raising spring PRELOAD (which increases the amount of force needed to start the spring compressing, but does no increase the rate once the compression has started)

And the easiest thing to do is change the front shocks, because a lot of front rebound force, from a launch perspective, acts the same as a lot of rear spring.

Have a look at this picture:

http://www.teamrip.com/images/Dg50 side shot.jpg

That is a car in the middle of a 1.4G launch - an AWD to be sure, but the same principles apply. It has stiffer rear springs than stock, but it also has a lot of front rebound in the shocks (and in this particular picture, a lot of rear compression as well)

See how flat it is? If it wasn't for the dust being thrown up by the tires, you'd never know it was even moving by looking at the chassis.

A pair of front Konis get you rebound adjustment via a knob, and it is the nature of Konis to have a LOT of rebound force when the shock is at or near full hard. You don't even need to do the full-bore coilover kit (although it is recommended, as it has other advantages) - just a pair of Konis should do the trick.

DG

Far North Racing Team Home Page

 

[JimiHO 81]

Im gonna go with DG-FNR on this one. Things behave very differently in the real world than in theory due to all of the cofounding variables. But hey, just my 2 cents...im only a senior in high school. BTW, where did you learn allthis from? Are there some good books on this stuff or something?

 

[DG-FNR]

In order to get a really good understanding of all this stuff, you need to do 3 things:

1) Read all you can on the subject. See http://autox.dsm.org/books.html for a good list, linked to Amazon.com for your shopping convenience. Then once you've got the books, do the math yourself. Try the stuff the books talk about, and see what's right and what's wrong.

2) Get what training can be found. There's precious little out there, but I did one of these ttp://www.optimumg.com/seminars.htm plus I've had the opportunity to talk to a lot of my peers in great detail about a lot of this stuff. I got a precious few hours of Caroll Smith's time before he passed on that turned out to be invaluable, for example.

3) How do you get to Carnigie Hall? Practice. Compete as often as you can at the highest level you can afford - and in the autocross world, that essentially means you must do National Tours and ProSolos, as many as you possibly can. It also means attending private test sessions when they can be found.

I cannot possibly stress this enough - without the benchmark of running against the best driven, best prepared cars in the sport, you have little to no worthwhile feedback as to how well you are progressing, both as a driver and an engineer.

When I first started driving in competition, I got to the point where I was a regular 3rd-4th place finisher in my region, running the OEM tires against cars that were on race rubber. I got to thinking pretty quickly that I was hot shit, and that all I needed were race tires to start winning. Then I went to a Divisional race, and discovered that there was a whole 'nother level of talent out there. A good enough performance Regionally to put me in the trophies was a solid backmarker performance Divisionally and damn near DFL Nationally.

Once I got over the shock and started applying myself to the new challenge, the quality of my game rose dramatically - again, both as driver and as engineer.

Same deal with drag racing. I drag raced for nearly 10 years before I bought a DSM and started autocrossing. Accordingly, I had enough experience that when I did some drag racing with a DSM, I started off fast. In fact, I won the first ever drag race I entered with a DSM, the '97 Buschur Racing Shootout Bracket 2 class.

Experience is the best teacher.

Beware people who talk a lot, but DO nothing.

DG