pivot points and bump steer

[j260z]

i've seen a few posts about moving the pivot points on the front arms up an inch or so to make up for the cars being lowered but when i was at a z club show and shine in Edmonton last week i also saw an lt1 conversion in which the owner spaced the whole crossmember down 1/2 an inch. his reasoning was two fold. firstly, of course, was to correct the steering geometry and secondly to lower the engine and center of gravity slightly. what are peoples thoughts on this solution. i'm thinking of doing a RB26DETT conversion and i know it would be nice to get the engine as low and back as far as possible.

[260DET]

Am I missing something? Moving the Xmember down in relation to the body will lower the suspension inner pivot points, not raise them?

[jeromio]

I don't see how cross member spacers affect steering geometry, bump-steer or otherwise. It will "adjust" the angle of the LCA (lower control arms) though. On a lowered car, the arms may point up a bit, which does not translate into good geometry. Instead of gaining camber in a turn, you lose it. Adding spacers makes this slightly worse.

 

The spacers are mostly for underhood clearance AFAIK.

 

Moving the pivot point up adjusts the angle of the LCAs to correct for lowered-ness. It also helps with bump steer by making the tie-rods more parallel with the LCAs. Stock, their not quite parallel so with susp. travel, the LCA and the tie-rod travel in different arcs, causing unwanted steering. When you move the holes up, yYou can also move them out to gain some camber.

 

I moved mine up by 13/16s and out by 1/4 inch. I wouldn't recommend this though since I now have lots of front camber which makes the car turn easily, but it's very, very twitchy. This mod does appear to have eliminated bump steer nicely.

 

I'd go with 3/4 or 11/16 inch up and maybe 1/8 out or none out.

 

Nothing scientific backing me up though, so take those figures for what they're worth.

[j260z]

thanks for showing me the fault in my logic. that's why i ask you guys questions before doing anything. i saves me a lot of trouble when i have a brain fart!!

i must have gotten something mixed up in my mind when i was talking to this person.

[johnc]

Uuuhhhh... the steering rack is bolted to the crossmember and the lower control arm attaches to the crossmember. How does lowering the crossmember change the relationship between those two components?

[pparaska]

Right John. All it does is change the position in the bumpsteer curve that the car sits at and drives through. By lowering the crossmember, it's like lowering the car with shorter springs. What it does is change the stationary, full droop and full bump extents of where the car rides through the bump steer curve. It moves them all to different points along the "stock height" bumpsteer curve, but the curve remains the same. The problem is that as you get away from the static position in the bumpsteer curve, the bumpsteer amount usually increases (more toe change) so that the static to bump portion is now shifted toward the full bump end of the OE curve. In fact, the new full bump position is beyond the OE full bump position.

 

Sample bumpsteer curve:

 

http://www.rallypartsireland.com/TechInfo/GartracescortbumpsteerABrev.pdf

[Zsane]

Lowering the car lowers the center of gravity, good! leaving the control arm pivot points in the stock location also lowers the roll center, a measured point, one line running 90 degrees from the top of the struts, intersecting with a line from the ball joint pivot through the control arm pivot, and the last running from the centerline of the tire to the two first lines intersection, where this last line crosses the vehicles vertical centerline is the roll center. To get more stability the point of the center of gravity and the roll center should be moved closer together, by lowering the car and leaving the control arm pivots alone, or not using bump steer spacers, this relationship remains the same but lower, by using the bump steer spacers, or raising the control arm pivot points, or both, the center of gravity is still lowered but the roll center is raised or brought closer to the center of gravity point. Think of the roll center as the center pivot point of the car as viewed from the front, side to side, and the center of gravity is the weight on the end of a rod, the distance between these two points is the rods length, or leverage, the shorter the distance the less leverage the weight of the vehicle has to induce roll! You need to lower the center of gravity to the roll center, not the ground, and if the roll center moves down with the center of gravity, the leverage remains the same. Good suspension geometry also reduces the movement of the roll center as the suspension goes through rebound and compression. You’ll have to take lots of measurements and make drawings to scale to map your set up on paper, make changes and see where things go. The exact center of gravity will be a bit more complicated to come up with. Have fun!

[TomoHawk]

If you put spacers between the crossmember and body, doesn't it lift the body? Te increased underhood clearance would be nice, but dthat means the C/G of the body would be higher, but not much.

[blueovalz]

Nope. The body height (ride height) is determined by the springs. The crossmember only lowers the inner pivot point (along with anything else attached to the crossmember), but the spindle height (which is determined by the wheel/tire combo), and hence, the springs sitting on the spindle, determine where the body sits.

[TomoHawk]

ride height is determined by the spindle height (which is determined by the wheel/tire combo), and hence, the springs sitting on the spindle...

I forgot about that. Thanks.

[pparaska]

Zsane, that's a pretty good explanation - I don't see that too often about roll center, center of Gravity, etc. If I read the first part of your post correctly, it jives pretty well with what I've learned from Carroll Smith's books, etc.

 

You’ll have to take lots of measurements and make drawings to scale to map your set up on paper, make changes and see where things go. The exact center of gravity will be a bit more complicated to come up with. Have fun!

 

Of course there's software out there to do this for you, once you have all the measurements. The packages I've seen are several hundred bucks. If anyone knows of a free or shareware version that does MacPherson and Chapman strut suspensions, I'd love to hear about it.

 

Measuring the center of gravity (CG) location isn't difficult, once you have access to a set of wheel scales. Measure the weights with the car level, and then raise one end a specified amount (as much as practical - by putting something sturdy under the scales at one end of the car) and then solving some equations. I seem to remember seeing it in Puhn's "How to Make Your Car Handle," but can't find it now. I did find this with a search:

http://www.geocities.com/MotorCity/4014/cgrav.html

[blueovalz]

I've often wondered, what with a Formula car's type of suspension, if the roll center couldn't be raised up to (or above) the car's already low center of gravity. Would this then completely eliminate any need for an anti-sway bar (and if above, cause the car to roll into the curve instead of away from the curve)? Is the CG, acting upon the roll center, what causes the body to roll? Hypothetical of course

[Zsane]

I read that raising the roll center to get it closer to the center of gravity has been known to cause a jacking effect and erratic suspension movements. It also can reduce camber gain, and that most succesful racecars have the roll center height between 1" below ground level and 3" above ground.