JMortensen Posted July 30, 2016 Share Posted July 30, 2016 (edited) Another product that is readily available now, is bump steer spacers. These correct the geometry of the A-Arm on lowered cars so it has a downwards slope again ( important for Camber gain during cornering, particularly on Mac struts ) and they also reduce Bump steer. It's a bit more elegant solution than moving the pivot location upwards. Relocating the pivot upwards corrects the Camber curve in roll, but does nothing to correct Bump steer. Something to consider. You've got this backwards. Moving the pivot changes the relationship between the LCA and the tie rod, so it actually changes the bumpsteer. Using a "bumpsteer spacer" doesn't change the angles of the control arm relative to the tie rod it moves them down together the same amount, so it's a roll center corrector, but doesn't do anything with regards to bumpsteer other than to move what part of the stock bumpsteer curve you're driving in most of the time. I made a similar mistake when writing the Bumpsteer FAQ post: http://forums.hybridz.org/topic/38615-bumpsteer-faq/ and BlackBeaut posted an image to clarify, but it is a dead link now. FWIW, when I was running a stock steering rack and front xmember, I had to move the LCA pivot up about 7/16" to minimize bumpsteer. Edited July 30, 2016 by JMortensen Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted July 30, 2016 Share Posted July 30, 2016 Bump steer spacers don't correct bumpsteer. At least not completely. Quote Link to comment Share on other sites More sharing options...
Chickenman Posted July 30, 2016 Share Posted July 30, 2016 (edited) Delete..see below. Edited July 30, 2016 by Chickenman Quote Link to comment Share on other sites More sharing options...
Chickenman Posted July 30, 2016 Share Posted July 30, 2016 (edited) You've got this backwards. Moving the pivot changes the relationship between the LCA and the tie rod, so it actually changes the bumpsteer. Using a "bumpsteer spacer" doesn't change the angles of the control arm relative to the tie rod it moves them down together the same amount, so it's a roll center corrector, but doesn't do anything with regards to bumpsteer other than to move what part of the stock bumpsteer curve you're driving in most of the time. I made a similar mistake when writing the Bumpsteer FAQ post: http://forums.hybridz.org/topic/38615-bumpsteer-faq/ and BlackBeaut posted an image to clarify, but it is a dead link now. FWIW, when I was running a stock steering rack and front xmember, I had to move the LCA pivot up about 7/16" to minimize bumpsteer. We'll have to discuss this further, but I'm rushing off to a Birthday party right now. Since it's a bunch Road Racers and Autocrossers, we will have a topic for discussion. However, as you state Jon ( and I've quickly read in the Bump Steer FAQ ) , the use of these " Bump Steer spacers does bring the Bump steer curve back into line with the stock curve. However perfect or imperfect it is. As the spacer goes between the strut and the steering arm, both the steering arm and the LCA outer end are moved downwards. Does that not alter BOTH the Roll center and the Bump Steer Curve ( after lowering a car ) . It has to does it not??? Discuss and I'll be back later... Edited July 30, 2016 by Chickenman Quote Link to comment Share on other sites More sharing options...
Chickenman Posted July 30, 2016 Share Posted July 30, 2016 Quote from Whitehead Performance: https://whiteheadperformance.com/1971-datsun-240z-restomod-front-suspension-and-wilwood-front-brakes/ Bumpsteer spacers were added to keep the front suspension geometry correct. The Datsun 240Z 260Z and 280Z all suffer from bumpsteer and lowering the car exaggerates the effect. Adding the aluminum bumpsteer spacers help return the amount of bumpsteer back to stock. Relocating the front control arm mounting hole on the front crossmember can further reduce bumpsteer below factory levels. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 31, 2016 Share Posted July 31, 2016 (edited) We'll have to discuss this further, but I'm rushing off to a Birthday party right now. Since it's a bunch Road Racers and Autocrossers, we will have a topic for discussion. However, as you state Jon ( and I've quickly read in the Bump Steer FAQ ) , the use of these " Bump Steer spacers does bring the Bump steer curve back into line with the stock curve. However perfect or imperfect it is. As the spacer goes between the strut and the steering arm, both the steering arm and the LCA outer end are moved downwards. Does that not alter BOTH the Roll center and the Bump Steer Curve ( after lowering a car ) . It has to does it not??? Discuss and I'll be back later... When you use bumpsteer spacers, the curve is exactly the same. What part of the curve you're driving in is different. Moving the LCA pivot changes the curve. Edited July 31, 2016 by JMortensen Quote Link to comment Share on other sites More sharing options...
gnosez Posted July 31, 2016 Share Posted July 31, 2016 Jon is 99.99% right (margin of error with a confidence level of 95+/- 4.9%) regarding spacer and moving the pivot point up. Before we starting running with the vintage folks, we had really nice turn-ins and almost no bump steer. Now, well back to stock and it's okay but I was tempted today to move it back. Tech at SVRA events is extensive and they have someone with a pretty good grasp of what can and can't be done with an S30 body Z. And yeah, they know that the machined Toyo calipers aren't really FIA versions. Quote Link to comment Share on other sites More sharing options...
Chickenman Posted July 31, 2016 Share Posted July 31, 2016 (edited) When you use bumpsteer spacers, the curve is exactly the same. What part of the curve you're driving in is different. Moving the LCA pivot changes the curve. I think we're saying the same thing, but it is a difference in semantics. On a lowered car with no other changes, you often see an increase in Bump steer " amount" because the geometry change has now moved the steering arm into a poor region of the curve. Using bump steer spacers does not change the curve shape as you say, but it does lower the steering arm, putting the bump steer plot point back on or close to the stock design point on the bump steer curve. So in that respect, it does improve the bump steer situation over a stock LCA positioning.....on a lowered car At the same time the Bump steer spacer will alter the roll center. Moving the inner pivot point upwards does not change the position of the steering arm point on the curve but rather changes the shape or axis of the bump curve line. Hopefully into a shape that is more advantageous. A change in either the inner pivot or outer pivot, will both affect the amount of bump steer. Either method can be used and either method affects the parallelism between the tie rod linkage, the LCA and the steering arm. Not saying one method is better than the other, just that both methods will affect the amount of bump steer generated. Are we agreed on that gentleman? Edited July 31, 2016 by Chickenman Quote Link to comment Share on other sites More sharing options...
Chickenman Posted July 31, 2016 Share Posted July 31, 2016 Either way you do things, by relocating the inner pivot point for the LCA by drilling the X-Member, or by relocating the outer LCA pivot point and steering arm with a spacer, the amount you re-locate either pivot point is going to affect the amount of bump steer experienced . You could use different thickness of spacer at the outer pivot point or a different height of hole drilled on the X-member and end up with different amounts of Bump steer generated. Are we agreed on that gentleman? That's why modern suspension point calculators are so handy these days. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted July 31, 2016 Share Posted July 31, 2016 (edited) I'd call this more of a friendly chat than a debate. Cause this weekend all this stuff takes a back seat to a set of book cases for my daughters freshman dorm. And we are all just a bunch of buddies having fun. But: Returning to the "stock" bumpsteer curve is probably not a good goal, if you want to eliminate bumpsteer. Now, I don't know jack about bumpsteer for road racing or autocross. I do know that my car kinda "hunts" under max traction straight-line braking (tail seems loose) on track days regardless of bias adjustment (I work with that too), and the tail feels loose on the transition from on throttle to off throttle and light braking at the top end, say 170-ish on a straight course. I think I may have some REAR end "steering" issues. I have these problems no matter what style of LCA bushing I use (rubber, poly or aluminum). Now, back to the front end. The stock geometry has lots of bumpsteer from the factory. Disclaimer: I don't know what a LOT is, I just know it looks like a lot. If you will text me and ask for it, I can send you a little video showing the bumpsteer of a buddy's '70z as we jack it up and lower it. 256-366-4685. But I dunno, for autocross, is it beneficial to get some dynamic toe-out on braking? To help you "turn in?" As for me, I want no bumpsteer in front or rear for two reasons: to keep car going straight as possible, and to be able to make quick ride height changes in the pits without having to change toe while I'm at it. I got one more day in the wood shop, then a trip to deliver my daughter to college, then it's gonna be a 2-month car only orgie in the shop. In order to go straighter, I'm even willing to limit steering travel (call me the Anti-drifter), to allow me to crank in more caster. With stock fender openings, I can't get more than 4.5 of caster due to fender clearance. Edited July 31, 2016 by RebekahsZ Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 31, 2016 Share Posted July 31, 2016 (edited) ^^^This. That's one of the few things I really disagreed with John Coffey about. There was a post here where we were discussing bumpsteer and his take was that he could use two different bumpsteer spacers to get the "best" bumpsteer for a particular track if it turned more left than right, etc. My comment was just about the same as Keith's, paraphrased: "I want the car to go where I steer it, not to steer itself because of bumps in the road." Stock Z has too much bumpsteer. There was a rule of thumb that Cary (tube80z) used, want to say its .010" toe change for 1" suspension movement. If you get under that and it's toeing out under compression, you've got it handled enough to move on to the next thing. I did it and then moved on, which is why I can't remember exactly. You cannot completely eliminate bumpsteer on a strut system because change in caster as the suspension moves with change the angle of the steer knuckle and that affects bumpsteer, but you can get it to where it is no longer an issue. Dan (74_5.0L_Z) has used some software to figure out the exact length of tie rod that gives the absolute minimum amount of bumpsteer, I know in his simulation the tie rod is slightly longer than the control arm, but can't remember exactly what the spec is. It's in his very technical suspension FAQ post that also deals with motion ratios, roll centers, camber gain, etc. The rear of a Z doesn't have much if any bumpsteer, but bushing compliance could get you a decent amount of toe change without gemoetric bumpsteer. Edited July 31, 2016 by JMortensen Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted August 1, 2016 Share Posted August 1, 2016 And speed magnifies everything. Little of this matters under about 100. I just picked that number out of the air. Quote Link to comment Share on other sites More sharing options...
clarkspeed Posted August 2, 2016 Share Posted August 2, 2016 If you want to minimize bump steer and are not bound by rules, a small 1/4" vertical slot, not slanted, in the cross member (combined with strut spacer if you have lowered the car) is usually all you need. Quick and easy adjustments. And it doesn't take a $150 gauge to get in the ball park. I use what amounts to 2 square pieces of wood connected with a piano hinge. I place weights on it to keep it from moving on the floor. The free side leans against a wheel with what amounts to 2 nails resting on the wheel outer shell about center height. Adjust it until you get both nails touching equally. Remove the springs and sway bars, jack the car up and down through normal range of movement, and watch how far one of the nails will move away from the wheel. It usually moves A LOT. Adjust inner pivot until you can keep both nails touching through the entire range. I think I mentioned before, 0 toe in the rear of my car really added to high speed stability. But your speeds are another deal. Quote Link to comment Share on other sites More sharing options...
grannyknot Posted August 2, 2016 Share Posted August 2, 2016 Clarkspeed, very interested in your plywood bumpsteer tester, do you have any pics? Also, I remember a pic that John Coffey posted many years ago of that vertical slot in the crossmember/inner LCA mounting point, wouldn't moving the LCA mounting point straight up pull the bottom of the tire in resulting in positive camber? Where as making that slot slanted or curved with the radius of the LCA ball joint would keep the camber where it was. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 2, 2016 Share Posted August 2, 2016 If you want to minimize bump steer and are not bound by rules, a small 1/4" vertical slot, not slanted, in the cross member (combined with strut spacer if you have lowered the car) is usually all you need. Quick and easy adjustments. And it doesn't take a $150 gauge to get in the ball park. I use what amounts to 2 square pieces of wood connected with a piano hinge. I place weights on it to keep it from moving on the floor. The free side leans against a wheel with what amounts to 2 nails resting on the wheel outer shell about center height. Adjust it until you get both nails touching equally. Remove the springs and sway bars, jack the car up and down through normal range of movement, and watch how far one of the nails will move away from the wheel. It usually moves A LOT. Adjust inner pivot until you can keep both nails touching through the entire range. I think I mentioned before, 0 toe in the rear of my car really added to high speed stability. But your speeds are another deal. In my case I moved up 7/16" to get rid of the bumpsteer. Used a similar homemade gauge, but I put a dial indicator on one side and nail on the other. Clarkspeed, very interested in your plywood bumpsteer tester, do you have any pics? Also, I remember a pic that John Coffey posted many years ago of that vertical slot in the crossmember/inner LCA mounting point, wouldn't moving the LCA mounting point straight up pull the bottom of the tire in resulting in positive camber? Where as making that slot slanted or curved with the radius of the LCA ball joint would keep the camber where it was. That was me, not John. I put a tall slot in the xmember. When I went to the Woodward rack, I raised the LCA pivots as high as possible and moved the rack up to match. Now with 2" ground clearance on the front of my splitter the control arms are close to level. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 2, 2016 Share Posted August 2, 2016 Here's a drawing of a similar gauge: Quote Link to comment Share on other sites More sharing options...
clarkspeed Posted August 2, 2016 Share Posted August 2, 2016 Please send the royalty payments to my paypal account. The answer to the camber question is it depends. If you are running a high roll center then technically it would pull out some camber that might be noticed. If your control arms are closer to parallel, then not so much. A 1/4" is not so much movement and most (all?) people who would attempt this already have some form of camber adjustment device to compensate. If you slant the slot to raise inner pivot and increase camber, you will end up with a nice roll center location, increased camber, and very poor bump steer curve. Everything is a compromise. Quote Link to comment Share on other sites More sharing options...
clarkspeed Posted August 2, 2016 Share Posted August 2, 2016 Here's a drawing of a similar gauge: Jon, that is not fair. You beat me to the post with a better drawing! Quote Link to comment Share on other sites More sharing options...
mtnickel Posted August 2, 2016 Author Share Posted August 2, 2016 (edited) Great thoughts and discussion guys. I'll add 'slotted Inner pivot' to my growing list of fabrication mods (LCA, and poor man rear toe adjuster). For repeatable placement, I'll likely also tack a eccentric washer onto the Inner LCA nut and weld a peg to the crossmember...then Adjusting Bumpsteer is as easy as loosening the bolt and rotating the nut (similar to many camber adjusters). Pics to come sometime (though if you check out when i started this thread, it could be years down the road). Edited August 2, 2016 by mtnickel Quote Link to comment Share on other sites More sharing options...
socorob Posted August 4, 2016 Share Posted August 4, 2016 Do you need to weld some type of washer around the slot, or is the metal in the crossmember thick enough to not wallow out? Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.