socorob Posted December 26, 2015 Share Posted December 26, 2015 I have seen the point be moved up an inch. Is that the most ideal place, or if it was moved outboard slightly also, would it help any by widening the track and adding some negative camber? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 26, 2015 Share Posted December 26, 2015 There are various recommendations. I think JTR says up 3/4 and out 1/4. I slotted the xmember and then moved the pivot up until the bumpsteer was minimized and that was more like 7/16" in my case. YMMV. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted December 26, 2015 Share Posted December 26, 2015 I think the correct answer is: "It depends." I don't know how much hurry you are in, but I'm slowly working on this issue with the goal of minimizing bump steer, and I can keep you posted on my findings. So far I have slotted the crossmember vertically 1". I have made doublers for the crossmember that now need to be welded into place before I start reassembling my front end. My TTT LCAs are a little longer than stock which automatically adds camber, so I did not move the pivot out any. Raising the pivot up 1" vertically essentially placed the pivot at the same height as the inner toe rod. But, the steering knuckles may effectively shorten the steering "linkage" relative to the length of the LCA. So it may prove that a compromise LCA pivot position may wind up being best, as Jon indicates. Quote Link to comment Share on other sites More sharing options...
socorob Posted December 27, 2015 Author Share Posted December 27, 2015 So does the steering knuckle look like it would need to be shorter or longer? Quote Link to comment Share on other sites More sharing options...
clarkspeed Posted December 27, 2015 Share Posted December 27, 2015 From my experience, large movement of the inner pivot makes the bump steer unmanagable. I prefer controlling the RC and camber by other methods. Then bumpsteer can be mostly eliminated with a very small vertical slot in the cross member of 1/4" or less. Quote Link to comment Share on other sites More sharing options...
socorob Posted December 27, 2015 Author Share Posted December 27, 2015 Do you know if anyone has had any problems with the control arm bolt walking when the crossmember was slotted? Quote Link to comment Share on other sites More sharing options...
socorob Posted December 27, 2015 Author Share Posted December 27, 2015 What other ways do you control bumpsteer? Is one of the ways by spacing the outer tie rod like this? Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted December 27, 2015 Share Posted December 27, 2015 Are you having trouble with your car, Rob? Or are you just curious? Seemed your car was rock solid at Memphis.... Quote Link to comment Share on other sites More sharing options...
socorob Posted December 27, 2015 Author Share Posted December 27, 2015 A little of both. My car gets a little squirrelly on this one road here where the road has depressions where the tires have sank the road a little, but the center and edges are higher. Since I will be getting the longer tie rods with spacers from TTT and will have to take some things apart and realign it, I thought it would be a good time to address that if needed. Right now I can only get just over 2 degrees of camber and I run out of tie rod, that why I want to get the longer ones, plus he said he has spacers to help get them more in line. My control arms are barely angles down, maybe just a few degrees. My think was is would be good to get some angle back to the arms for some camber gain, but not sure if I am looking at this correctly or not. I was thinking something like this picture from arizona z. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted December 27, 2015 Share Posted December 27, 2015 I think wide tires are gonna follow rutt-ed pavement no matter what. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 27, 2015 Share Posted December 27, 2015 s30 rod suspension.JPG What other ways do you control bumpsteer? Is one of the ways by spacing the outer tie rod like this? Yes. A third is to raise or lower the rack. I have a slotted xmember with the LCA moved as high as possible with an aftermarket rack that was raised 3/4" with spacers and the tie rods spaced down to minimize the bump steer. Quote Link to comment Share on other sites More sharing options...
tube80z Posted December 28, 2015 Share Posted December 28, 2015 A little of both. My car gets a little squirrelly on this one road here where the road has depressions where the tires have sank the road a little, but the center and edges are higher. Since I will be getting the longer tie rods with spacers from TTT and will have to take some things apart and realign it, I thought it would be a good time to address that if needed. Right now I can only get just over 2 degrees of camber and I run out of tie rod, that why I want to get the longer ones, plus he said he has spacers to help get them more in line. My control arms are barely angles down, maybe just a few degrees. My think was is would be good to get some angle back to the arms for some camber gain, but not sure if I am looking at this correctly or not. I was thinking something like this picture from arizona z.lca pivot raised.JPG Funny thing, I have that crossmember in my shop right now and it's only raised 7/8s of an inch, not the indicated amount. Cary Quote Link to comment Share on other sites More sharing options...
socorob Posted December 28, 2015 Author Share Posted December 28, 2015 That's clean looking. Did the 7/8'' make a big difference? Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted December 28, 2015 Share Posted December 28, 2015 (edited) "Make a big difference." Rob, for suspension behavior, I think we have to be REALLY specific in terms of what type of bad manners we are trying to correct. And based on the one time I got to do ride-alongs in your car, I'm not sure you have any problems to correct. I'm looking into slotting for a very specific car behavior problem. And, fortunately, landspeed racing is super sanitary: the car is accelerating at a fairly constant rate, accelerating in a straight line on a (relatively) smooth, concrete, non-rutted course. The only thing the suspension is doing is going up and down (theoretically) absorbing bumps. So, there are really only two forces that could cause my specific problem: dynamic toe changes (front or rear) and aero influences. The front tires have zero cornering forces (theoretically) since I'm going in straight line. Trying to tune a car for twisties must be so much more difficult because you have so many other variables: bump, compression, track surface, banking, off-camber changes, tire temp, body roll, tire roll, tire wear, chassis deflection/twist, throttle input, braking, ackerman angle, static and dynamic camber and caster, tire width, tire compound, spring rate split between front and rear, steering rate, driver factors (everybody's biggest problem): fast/slow entry, turning the steering too fast, not fast enough, braking too hard or not hard enough, accelerating too fast or not fast enough, or just bad driving line. "Bad or suboptimal handling" could come from any or all of those factors. And any single adjustment ANYWHERE in the system affects ALL the rest. I would bet that that 7/8" measurement reflects more the practicality of how difficult it is to do this mod with the car on jacks with the steering rack still installed, or maybe even the size of the rod-end bearing installed in the LCA. I would not try to extrapolate the necessary adjustment based on a part of dubious origins. What research went into this oft-quoted "JTR mod?" Did other mods were on the development vehicle? Roll center adjusters? Camber plates? And what were the other suspension set-up numbers on that car? We don't know and we can't know. I have the welder coming on Saturday to weld the doublers onto the crossmember, so I'm making (slow) progress. I will measure my bumpsteer with the LCA pivot at the stock location before I change it. Please don't take your sweet-handling car apart for suspension mods until I demonstrate any real improvements that slotting can make (if it makes any difference at all). Edited December 28, 2015 by RebekahsZ Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 28, 2015 Share Posted December 28, 2015 I would bet that that 7/8" measurement reflects more the practicality of how difficult it is to do this mod with the car on jacks with the steering rack still installed, or maybe even the size of the rod-end bearing installed in the LCA. I would not try to extrapolate the necessary adjustment based on a part of dubious origins. What research went into this oft-quoted "JTR mod?" Did other mods were on the development vehicle? Roll center adjusters? Camber plates? And what were the other suspension set-up numbers on that car? We don't know and we can't know. I have the welder coming on Saturday to weld the doublers onto the crossmember, so I'm making (slow) progress. I will measure my bumpsteer with the LCA pivot at the stock location before I change it. Please don't take your sweet-handling car apart for suspension mods until I demonstrate any real improvements that slotting can make (if it makes any difference at all). JTR is still around. Someone could just ask them how they came up with that 3/4" up and 1/4" out spec. I feel like I've repeated this here 100x, but for my car the difference before and after moving the pivot was night and day. There was one particular turn on one particular track that was making my car do an emergency lane change style swerve as I went over a bump. Also was sawing at the wheel a lot in sweepers. After moving the pivot up 7/16", the swerve was gone completely and the sawing at the wheel much diminished. It only did that swerve on that one corner, and the sawing at the wheel was only at the limit so this is one of those things I'd call a wasted of time for most people, but for racing was worth it. Quote Link to comment Share on other sites More sharing options...
socorob Posted December 29, 2015 Author Share Posted December 29, 2015 I want to do this for 2 reasons. First, to help improve camber gain. Right now my lca is almost level, so I don't think I'm getting the full potential of front grip. The second is some worn roads here. The valleys were made by trucks and suv's, which have a much wider track than me. My car is very jittery when driving on it with one wheel in the depression and another on top. It goes all over the place. My tie rods and my lca's are at very different angles. You can tell just by looking at them that they are off a bunch and things won't be moving in the Same plane as everything goes up and down. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 29, 2015 Share Posted December 29, 2015 Camber gain won't be helped. Look at 74_5.0L_Z's sticky in the BWSC forum and he graphs out the camber curve. It's really linear. Have no idea if the bumpsteer will help in the ruts. Suspect not. Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted December 29, 2015 Share Posted December 29, 2015 (edited) Actually, I have been doing a little deeper digging into suspension geometry lately and have discovered that I was not completely correct in some things. To explore the suspension geometry questions that I have had and that have arisen here lately, I created an accurate 3D kinematic model of my car's front suspension (in Pro/Engineer) which I could move and measure things like camber, caster, toe. The model was great, but making geometry changes was time consuming. So, when I was satisfied that I had an accurate model, I wrote a program using Visual Basic within Excel that given the same input as the Pro/engineer model gave the same output (camber, caster, toe). The nice thing about the program as opposed to the model is that I can change the geometry very quickly and see the results. The most difficult part was not making the Pro/engineer model nor was it writing the program, but was getting accurate dimensions of all the necessary components that make up the suspension. To model the kinematics accurately, you need to know the center of the attach points of the LCA, T/C rod, strut top, and inner tie rod relative to the chassis. You also need accurate geometry for the LCA assembly (which consists of the LCA, TC rod and ball joint), accurate geometry for the strut assembly (which includes the steering arm, spindle, strut insert, and spring), and accurate geometry for the rack and pinion. I have spent considerable time lately doing my best to quantify all of this, and I have been trying to find the time to post the measured geometry and the derivation of the model. I have spent hours cutting apart ball joints and disassembling steering racks and measuring all sorts of parts. I also plan to freely distribute the Excel Program to those who want it. Within the next couple of days I will begin a thread with my results. What I will ask in return is that those who have better measurements to provide them. Edited December 29, 2015 by 74_5.0L_Z Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted December 29, 2015 Share Posted December 29, 2015 (edited) I'm really out of my league here. I'm gonna sit on my hands and just listen for a while. Edit: I couldn't stand it! Rob, if you have any skinny front tires, swap those on and re-try that section of the road that has the ruts. I really just think you are "train-track-ing". Ruts and cracks act like train rails and steer your car along the ruts, just like if your car was a train on rails. Wide , low profile tires really make it worse. Slotting isn't gonna fix that. If I recall, you didn't have camber plates at ZCON, and yet your car was so much faster than anything I rode in (race rubber, a V8 and great attention to detail in the build). If you don't have camber plates up front, that's the ticket for adding camber. There's so much temptation to over-think our cars. Dave drove his car from NY, adjusted his camber plates, did not adjust toe (so I know his toe was off), ran his car hard for 2 days, re-adjusted his camber to his street setting, and drove back to NY. Camber plates are awesome! To slot your crossmember with it mounted on the car, you need to remove your tie rods from the rack, to include the inner tie rod. Then, if you crank the steering first to one side, then the other, the rack is out of the way to do the drilling. I wrapped the exposed rack sector with saran wrap to keep metal filings out. I used a 90-degree drill with a stepped bit after using a dremel tool to remove the spot welds that retain the washers. Then I used the dremel to connect the two drilled holes and convert them to a slot. Edited December 30, 2015 by RebekahsZ Quote Link to comment Share on other sites More sharing options...
tim.d Posted December 29, 2015 Share Posted December 29, 2015 love these technical threads! Quote Link to comment Share on other sites More sharing options...
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