Datsun Roadster Forum-311s.org

Mike Unger wrote:Folks,

You don't want the front of the spring lower than the rear (whoops not front sorry for the typo)! This induces bump steer. Don't go there, this is not a good idea...

Mike

Don't all rear leaf spring suspensions have bump steer? If the spring has an arc, its going to get longer the more it compresses, if its flat or reverse arced its going to get shorter.

The front mount is pretty much fixed. If you lower the rear, don't you increase the Moment arm by lowering the Moment center? Do you want more roll in the rear?

I'm interested in a car that handles best (as close to neutral as possible) in all conditions of travel. Drag racers and circle track racers have different goals. Circle track racers will actually use bump steer to help the car turn in at a corner. You want the axle to move front to rear as little as possible during travel. To accomplish this you want the spring level. Look at the arc that the axle must go through during spring movement. When you go around a corner one side compresses and the other side extends, think about what happens to axle placement when this occurs. I prefer as little movement as possible.

Mike

If you do a search for Pinto, you can go to page 93. No pics, but the text is there. Explains how a level spring induces less roll steer.http://www.amazon.com/How-Make-Your-Car ... 0912656468

Mike Unger wrote:I'm interested in a car that handles best (as close to neutral as possible) in all conditions of travel. Drag racers and circle track racers have different goals. Circle track racers will actually use bump steer to help the car turn in at a corner. You want the axle to move front to rear as little as possible during travel. To accomplish this you want the spring level. Look at the arc that the axle must go through during spring movement. When you go around a corner one side compresses and the other side extends, think about what happens to axle placement when this occurs. I prefer as little movement as possible.

Mike

Mike,
I do not know that much about the suspensions.

Can you please tell us what you have found with respect to the rear suspension?

Do you think it would be best to run comp (flat) springs, 66 springs or some thing else?
Have you lowered the rear mounting location? Do you run lowering blocks?
How about "traction" bars? Lowers or uppers?
Do you run a Panhard Bar or watts linkage?. Where are you location points? Below the axle, inline with the axle. Is the bar level or angled.

Any Pictures?

Thanks,
Phil

"I do not know that much about the suspensions."

Check out the book that Dave mentioned. It's very good.

"Do you think it would be best to run comp (flat) springs, 66 springs or some thing else?"

The different springs will change the spring rate and ride height but not geometry much. It depends on your goals. Our street car has a completely stock rear suspension and Comp front springs. I like that for the street just fine. My SCCA race car has a coil over link suspension. The vintage race cars have Comp rear springs. Travel can get pretty limited with the comp springs because the axle will hit the frame. If your on the bump stop then that becomes your spring rate. That will induce snap oversteer as the spring rate goes towards infinity. I wouldn't put them on a street car.

"Have you lowered the rear mounting location? Do you run lowering blocks? "

Yes, for a race car it is a good idea to lower the rear mounting point. The lowering blocks just change ride height. See Dave's link.

"How about "traction" bars? Lowers or uppers? "

Not a big factor for a road race car with rolling starts, in my opinion. You have to be careful that these don't bind up the suspension at travel if you add them. The stock one on the later cars is OK if the mount is in good shape.

"Do you run a Panhard Bar or watts linkage?. Where are you location points? Below the axle, inline with the axle. Is the bar level or angled. "

Yes, a Panhard bar is a good idea on a race car. You want it level at rest and pretty low and very rigid and as long as possible. You want it level at rest so it reduces bump steer problems with travel. It moves through an arc and therefore steers the rear of the car. That's why you want it level at rest and long. The frame mount needs to be beefy and well braced because it will be long to get the bar low and therefore have some leverage on it. Make the mounting points adjustable so it can be changed to level with ride height changes or to tune handling. I have one chassis with a clever Watts link on it but never drove the car. A panhard bar is simpler and easier to adjust roll center.

F

As Mike mentioned, if you are cornering on the bumpstops, it can be very dangerous. Any further load on the suspension will transfer directly to the frame and suddenly unbalance the car. I run very low comp springs on the street with soft shocks, GR2's, but I don't push the car to extremes. I do need to get newer, shorter, softer and/or progressive bumpstops at both ends.

Just to be sure, you can use a tire crayon or chalk on the bumpstop or frame and see if you get transfer. The solution is smaller softer bump stops, more spring rate, or more ride height. you also need to make sure the rear shocks aren't too long and still have some travel at full bump.

Now oddly enough, my totally stock 2005 Dodge Ram has about 1 inch of clearance at the front bump stops and looks like it hits very regularly. I guess they didn't read the book.

I still need to get around to posting some photos of my car w/ the mono-leafs installed - and measure the hub to fender lip height.

Cheers
James

I have been following this thread with interest... I could not see this info on here, which I got from a Volvo forum.

Rear leaf springs from 1984;
340 part no. 3201759
360 (excluding GLT) no. 3205441
360 GLT no.3201761
360 (Heavy duty) no. 3284425 (standard Aus.)

You may have to drop the prefix 3, as it seams the rest of the number is stamped on, but not the 3.

I'm getting my car pretty well sorted out now. I added about 60lbs of lead as ballast to the trunk and with a full tank of fuel, the car is perfectly neutral on the track. It gets more tail happy as the fuel burns off, but its far better with the weight added back.

I rode in a street car with comp front springs, monoleafs in the back with Bilsteins (cut down), and it was amazing. The ride was smooth and controlled even on some bumpy back roads. The damping was a little soft for track use, but for the street it was spot on.

Will

What are the Volvo part numbers for? Are the Volvo rear leafs an interchange to the roadster?

Cheers
James

jamesw wrote:What are the Volvo part numbers for? Are the Volvo rear leafs an interchange to the roadster?

Cheers
James

The Volvo parts are what the rear leafs start as, they must be cut down and rerolled to work on a roadster. Its not a direct fit.

Will

I got a nice present last week in the form of 2 sets of lowering blocks and a new set of U bolts. The blocks are 33mm tall and 45mm, I went straight to the 45mm ones. I also found a broken U bolt, good thing I got a new set. All I can say is wow, what a difference it made to the way the car feels and handles. The 60lbs of lead in the trunk that could barely be felt is now VERY noticeable. I'll bet I'm taking some out. Even with my worn out street tires the back end feels like it sticks better. It feels more positive under braking and almost feels like it might understeer if pushed through a corner. (I haven't tried yet, just took it for a short run around the block.) If nothing else, it looks better, the stance is perhaps a little taller than with comp springs, but not as tall as stock springs.









Will

I got my composite springs recently and started the install today. Much less arch than the steel monoleafs and they weigh only 6 pounds each. That negates the weight I added with my new roll bar. Flex Form found an old die for me in one of their drawers, tested it, and it worked well. I may have to add lowering blocks but they'll be pretty thin. We'll see after I install one and sit it on the ground.

gboone wrote:I got my composite springs recently and started the install today. Much less arch than the steel monoleafs and they weigh only 6 pounds each. That negates the weight I added with my new roll bar. Flex Form found an old die for me in one of their drawers, tested it, and it worked well. I may have to add lowering blocks but they'll be pretty thin. We'll see after I install one and sit it on the ground.

I'll bet you have to add some weight to the back end. I'm guessing that I'll end up pulling 20 lbs out of the car, or running a half tank of fuel to get the balance right. It would be nice to loose more, but I don't think its going to happen. With your springs that are half the weight of mine, you will likely have to add more weight than I do to get it balanced right. Time will tell. I'd love to put my car on a set of corner weight scales, I just don't happen to have any.

Will