Wanted: Rear Traction!

[nomadtrash]

I had much success with the KA24DE swap. The motor runs really strong. I tried the new motor out at the autocross last week using last years old slicks. The slicks wore down to the cord within a couple of runs. I think that they should have had more life left on them. I believe that they wore out so fast because the car has a tendency to spin the rear wheels. I would like to increase rear traction without doing a complete re-design of the rear suspension. Right now I have comp rear speings, a panhard bar, limited slip, and the stock torque arm. I've noticed some of the road race guys have a rake to the car with higher rear ride height.

Can I make minor changes to the suspension to get more traction?

I was thinking about raising or lowering the ride height, changing out the torque arm for a heim jointed piece, moving the mounting points for the torque arm up or down, flipping the shackles, moving the panhard bar up or down, moving the rear springs inboard a few inches in the rear, or some other change. I don't have the time or money right now to change over to a three link with coil-overs.

Thanks,

[dbrick]

There is a book called "how to make your car handle" written by Fred Puhn.
It gives you all the formulas to do the math to find out what will work.
Gives me a headache, but his info is really well presented and gives the downside of every change.

[nomadtrash]

I have Fred's book. I have read it several times. He explains a lot of stuff in there. I am looking for some Roadster specific setup information. I was hoping someone would tell how they got improvements.

I think I am going to try flipping my rear shackles which will raise the rear ride height about 1-1/2". This will make the axle farther away from the frame and effectively raise the frame mounting point of the torque arm. I will then have to build a longer torque arm and will probably use spreical rod ends for it instead of the rubber bushings.

[TR]

I thought that the torque strut's main function was to decrease wheel hop? Does it actually provide traction except for trying to keep the rear tires from hopping?

Does lifting the back end increase traction?

Hmmm....TR

[toolsnob]

Flipping your rear shackles is about the worst idea out of all you could do. By flipping the shackles you will make the car handle terribly, especially in autocross.

What size rear tires are you running? you could bump them up to 205's to help. Rear bar is for axle hop which does help you keep the tires down but is not a real traction bar. If your axle wraps up it creates hop which reduces traction.

You could always strap traction bars on the bottom of your springs like camaro's/mustang's

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[nomadtrash]

I hope to hear from Steve about his experience with flipping the shackles. The Roadster has it's springs set angled in at the front and also with a lower mounting point in the front. This causes the axle to go up and forward when it hits a bump on one side. This movement causes the rear to steer a little bit which can cause problems. Many of the racers have flipped the shackles which eliminates most of the height difference between the front and rear mounting points. They use lowering blocks to get the ride height back down.

There are several ways to keep an axle from twisting under acceleration and braking. One of the worst is to use slapper bars like a 70's vintage Nova might have. I thought about using slapper bars on my Roadster but lack the mullet to go along with them. Slapper bars raise the frame and lower the axle. This causes the rear geometry to change significantly under acceleration. They are actually a little bit effective in a straight line when set up properly but do horrible things when going around corners and one side is loaded more than the other.

A three link suspension uses a third link from teh top of the axle tube forward. This link is adjustable for height on the frnt end. You can raise and lower it to tune in more traction or anti-dive. The link does not lift on the frame like slapper bars. It pushes and pulls forwards and aft. By raising the front mounting point it allows for a tiny bit of frame lift. This lift is adjustable based on how high the bar is set.

The torque arm on the Roadster works in this manner. The bushing is soft rubber to allow for the axle to move freely in most directions and dampens the spring wrap up under acceleration. My thought was to replace the torque arm with a heim jointed third link. This would still allow free movement but eliminate the soft rubber bushing. If it were adjustable then it might work like the third link in a three link setup.

I have not tried new slicks with the new motor. They will be installed on Tuesday. My concern was that last years slicks with the U20 motor wore down faster than I expected. I attribute the fast wear in part to wheelspin. The car has never been able to hook up as well as I think it should.

The motor swap has sucked most of the fun money up. It has taken most of my wife's patience also. I don't want to do the right thing and install a three link with coil overs right now. I was hoping for a bandaid till next season.

Next season I plan to hack the frame off from the step up to the package shelf back. This should save a lot of weight. I will build a minimalist tube frame that is just enough to support the rear three link, coil overs, and sheet metal. My goal is to get down to 1450 pounds. Right now I'm at 1650 so I've got a long way to go.

205's are a huge step in the wrong direction. I've got 20x9.5x13 slicks right now and plan on running 23x13x13 next year.

Thanks,

[S Allen]

I ran my car hard on the track at Thunderhill that way before I put the competition springs on the rear. Granted it was only a 1600 motor with a bit of a racy cam. I did not experience any problems what so ever. I hit turn 1 at full throttle and ran up on the back of some anxious Miata's. Besides it is not all about the horsepower. Slow in fast out and hitting the apex correctly can make you fast . I would say give it a try. I gotta say though it looks funny but if it works-who cares? It sounds like you have plenty of torque.

Steve

[dbrick]

I flipped the shackles on my 1600, didn't notice any bad handling, but that was 20 years ago. I did it to get the rear up a bit, hate the ass dragging look. It also got the spring closer to level, which, according to Mr Puhn would reduce roll steer. I don't know if it did, but looked a whole lot better with the rear end up a bit.
On GM G bodys, Regal, Malibu etc, they sell anti hop bars which raise the mounting point of the two inner/upper control arms on the rear by almost 5 inchesand are reputed to work well. I do know stiffer springs and a clamp on the leafs will help. In autocrossI think slapper bars might unbalance the car: as you increase throttle because the will cause the rear to try to squat, and may load the tires unevenly.
If a 4 link is in the future, maybe just wait?

[oilleak]

Andy,
Whats the break-away torque on the LSD? With slicks and that engine you should probabley be at at least 150lbs. In CSP trim, I'm running 85lbs and I've been told by those who know that I could run more.

[nomadtrash]

I don't really know the break away. I'm pretty sure that it is very tight. It will chirp one wheel when going around slow corners much like a spool would do. It always leaves two black lines behind it. I've been meaning to measure it but haven't gotten around to it. I don't really want to remove the pumpkin. I thought that I might strap the car on the trailer really tight, Jack up one side of the axle so that only one tire is on the trailer bed, wrap a strap around the tire several times like a yoyo, and then use a come-along and a load cell (from work) to measure how much force it takes to move the one wheel. I could measure the radius of the tire to determine the foot pounds of torque.

I had also thought about using a torque wrench and an old axle with a nut welded on it.

We'll try racing with new slicks next Sunday and see how we compare to the same cars as last week.

[oilleak]

I had also thought about using a torque wrench and an old axle with a nut welded on it.

That's what I do. You could also make a 4x4.5 bolt circle peice of flat stock with a 1/2 inch square for a torque wrench drive directly in the center and bolt it to the axle hub so you can take a torque reading directly.
I think the new slicks will solve most of the problem though

[mgslayer]

I flipped the shackles on my "Beast" 10 + years ago to give me more room for the tires so they wouldn't scrape the inside of the rear fenders while cornering, and didn't notice any negative handling problems while Auto-Xing, actually it handled alot better! But to be fair I went from tired rear springs with worn out black stock Nissan shocks one weekend to flipped shackles with re-arched springs with an extra leaf added, all new rubber bushings, the white KYB shocks all around and upgraded to the 4.11 rear the next weekend. Right now the rear is stiff as the proverbial board, so I went from one extreme to the other. Give it a shot, might work for you, might not. Relatively easy to swap back if you don't like it. I don't think flipping the shackles in and of itself will do anything to help your traction issues, but it might allow for a wider tire to be placed back there. As for the way it makes the roadsters look: some like it , some don't , and some don't care , depends on your taste.

Happy Wrenching and Good Luck

[nomadtrash]

I took apart the LSD today because there was still a lot of wheelspin with the new tires. I also was able to spin one tire by hand with the other stationary. It couldn't have been more than about 50 pounds of force to spin. When I got it apart I found that it was the Belleville type LSD and the plates and discs measured 1.73mm. I think that 1.75mm is the thinnest that they come in so mine were worn down a little. I don't have any way to check preload adn don't have the time to build anything right now so I just added some shims and will see how it works out. The Bob Sharp manual says to add between .025 and .035 on the shims to get 230 pounds of preload. I added .035 by taking apart one of my other differentials and pulling the shims from behind the pinion bearing adn putting the shims between the last Belleville plate and the case end. I got everything back together and can no longer turn one wheel. I will see how it does on the track on the 2nd and 3rd in Houston at the SCCA National Tour. I still got a few bugs to work out on the car before then but I should be ready in time.

Later,

[dbrick]

Does the 68 have the upper torque link? If so, maybe urethane bushings, If not, time to get out the welder.
Ignorant questions...Is a tight LSD a good thing in autocross? Wouldn't it make the car extremely tail-happy? I know it helps with accelleration on the (short) straight parts.
I used to drive a ridiculosly overpowered (340 dodge shortbed 4 speed)pickup truck. I found the tight LSD to be a problem in turns, both wheels breaking loose at once with 275/60's. Went back to an open rear, lost off the line traction, but kept at least one rear wheel rolling with traction if I overpowered it.
High speed pickup truck handling is at best a touchy thing, usually involving trees and tow chains, but Damn that thing was fun.
Dodge has a 402 Magnum now, 4 inch stroke .060 over 360,and the engine in my Ramcharger is getting a little tired........