namz7791 Posted March 12, 2004 Share Posted March 12, 2004 Well, got the 2.8L out and am beginning the JTR book prep for the V8. Was wondering how many of you do the bumpsteer mod where you re- drill the crossmember to relocate the control arm? I am not a very good welder and am afraid that I will ruIn my crossmember trying to close up the old hole and welding on the backing plate. Also, trying to align the holes perfectly with the equipment I have available is also scary. The car will have a stock suspension except I am upgrading to the energy suspension urethane bushings. Car will not be lowered and will not be road raced other than an occasional red light drag race. I like the way the car handled with the 6 and if the 8 does not make the bumpsteer worse than I can forgo this part of the conversion? Experts please put in your 2 cents. I did the search on bump steer but they all seemed to cover the spacers for when the cars are lowered. Thanks! Quote Link to comment Share on other sites More sharing options...
Mike kZ Posted March 12, 2004 Share Posted March 12, 2004 I did this on mine, but I lowered my car. I don't think you need to do this if you're leaving it at stock height. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 12, 2004 Share Posted March 12, 2004 Don't bother. Jon Quote Link to comment Share on other sites More sharing options...
Jumbo240ez Posted March 14, 2004 Share Posted March 14, 2004 yeah, dont bother. Quote Link to comment Share on other sites More sharing options...
auxilary Posted March 14, 2004 Share Posted March 14, 2004 bump steer is not caused by different weight of hte engine. Bump steer is the jittery feedback as a result of a lowered car, because the geometry alignment between the control arm and the tie rod has changed. To restore the steering feedback to its original state, the pivot point must be moved to make the tie rod parallel with the control arm again. or use a bump steer spacer (which doesn't really fix the issue, just moves it elsewhere) On a road/track car, this is not a problem as roads are smooth. Driving the car on the street, however, might result in negative feedback. In my case, I put bumpsteer spacers on and it worked out ok for me (my car was about 1.5" lower than before). I'm going to be taking those out and moving hte pivot point up about 3/4" Quote Link to comment Share on other sites More sharing options...
Owen Posted March 16, 2004 Share Posted March 16, 2004 For those who answered not to bother...my car is mainly street but lowered and definitely will see some parking lot/orange cone days. What do you think about using tie rod ends/heim joints with a spacer instead of doing the crossmember mod? Sounds way easier to me... Owen Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 16, 2004 Share Posted March 16, 2004 I said don't bother because of the way he plans to use the car. The problem with using a rod end and bump steer spacer kit is the tie rods. You'll have to make them. The inner tie rods are metric thread IIRC, and any rod end you get is probably going to be SAE. I was planning on doing this myself, using 1" hex aluminum stock and drilling and tapping the inside end to match the rack and the outside end to match the SAE rod end. You'll also need to bore out the holes in the steer knuckles, and you would STILL benefit from raising the inner pivot and raising the roll center. My plan is to get the inner pivot as high as possible, and do all of the above, still haven't figured out if I want to continue running bump steer spacers or not. Might be overkill, but if I'm going to go for it, might as well go for it, right? This is a LOT of work compared to just raising the pivot, so you have to weigh the ROI of doing all of this. If it is an occasional autoxer, probably not worth it. Bumpsteer spacers or raising the pivots or both might be better if you're not building a racecar. Jon Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 16, 2004 Share Posted March 16, 2004 For those who answered not to bother...my car is mainly street but lowered and definitely will see some parking lot/orange cone days. What do you think about using tie rod ends/heim joints with a spacer instead of doing the crossmember mod? Sounds way easier to me...Owen I feel a little awestruck answering a HybridZ Godfather, but here goes. The point of the crossmember mod is to relocate the pivot point so the lower control arm sits nearly level to the ground when the car is parked. Thus as the arm swings through an arc, the apparent length of the arm doesn’t change as much thereby limiting how much the wheels toe in and out. When you say heim joint, I think adjustable length control arm. I am not sure how this would affect bumpsteer since the arm will still be pivoting about the same point. BTW, even a stock height Z can benefit from the bumpsteer mod, since apparently Nissan put the pivot point where they did to compensate for all the rubber they used in mounting the front suspension. Urethane should take care of most of that. Quote Link to comment Share on other sites More sharing options...
wheelman Posted March 16, 2004 Share Posted March 16, 2004 Pop N Wood, I was under the impression that the bumpsteer problem was caused by the relative length differences in the control arm and the connecting rod. The difference in length comes from them not being parallel and/or having different pivot points. Am I wrong in this and if so how? When I look at the front of my Z it appears that the control arm and connecting rod are parallel but the control rod pivot point changes when the steering wheel is turned and they no longer remain parallel if the suspension is compressed. I thought the idea of moving the control arm pivot point on a lowered car was to make the arm and rod parallel again when the suspension is in it's natural static state. Again am I wrong in this idea? Wheelman Quote Link to comment Share on other sites More sharing options...
Owen Posted March 16, 2004 Share Posted March 16, 2004 I have the same impression as Wheelman, to get the two rods (control arm and steering tie rod) as parallel as possible. "The point of the crossmember mod is to relocate the pivot point so the lower control arm sits nearly level to the ground when the car is parked." ---Here, I am under the impression that the pivot point needs to be moved up, which would not allow for the control arm being parallel to the ground. Your next sentence after that tells me we are basically talking about the same thing tho. (and don't worry about me being a godfather, I may have started this whole thing but I'm still learning and willing to learn!) As for the metric pieces, I swear I've seen metric threaded tubes and the metric rod ends are pretty common. Anyone know the size and pitch of the stock pieces? Owen Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 16, 2004 Share Posted March 16, 2004 The point of the crossmember mod is to relocate the pivot point so the lower control arm sits nearly level to the ground when the car is parked. Thus as the arm swings through an arc, the apparent length of the arm doesn’t change as much thereby limiting how much the wheels toe in and out. The reason people shoot for "nearly level" is because that is what is possible without moving the pivots, and using bump steer spacers. This has turned into the target for many people, but it is not what is ideal for best handling. John Coffey described the ideal situation in another post: The camber gain (more negative camber) in bump on the front a 240Z is mostly a result of caster, although the control arm/strut design does give a little more negative camber in bump, but body roll tends to negate this. Once things start getting modified, then all bets are off. In all cases at static ride height your front lower control arm must at least be level with the ground or, better yet, pointing down from inboard to outboard to some degree. The more the better while still maintaining the ride height you desire. The highly prepared 510's at the autox I used to go to every month in CA had the inner control arm pivots moved WAAAY up (like 3"), and I think they ran bump steer spacers in addition. They fixed the bumpsteer with spacers at the tie rods like Owen described. The more the control arm points down, the more neg camber is gained when the body rolls or the suspension compresses. Wheelman and Owen have it right on bumpsteer. The spacers are better at correcting the camber curve than fixing the bumpsteer. EDIT for Pop--the spacer deal that Owen is describing uses shims between the steer knuckle and the tie rod to move the outer end of the tie rod down until it is parallel with the control arm. Jon Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 17, 2004 Share Posted March 17, 2004 My understanding of the bumpsteer problem was gleaned from reading posts by John C and the like. After all the information that went in my brain, that was my take away on it. So admittedly I could have missed something. Sounds like there are multiple factors at play. The control arms swing in an arc, so if they are level they are at the average ride height, then they will be swinging through the top of the arc causing a minimum change in horizontal distance between the two pivot points (like a piston at TDC). I should probably go back and read some old posts to get my thoughts straight first, but the quote you posted from John is talking primarily about camber changes. Obviously changing the length of the lower control arm directly affects camber. And I have to admit, I can see how keeping your tire flat on the ground is a good thing but I get a little lost when those guys start talking camber gain. But I thought the bumpsteer mod primarily has to do with toe in and out as the wheels go up and down. (As the name implies, if compressing the strut causes the wheel to turn, then bumps will tend to steer you in a different direction.) And I am not sure how changing the length of the control arm influences that. To support my memory of the toe problem, one old post talked about measuring the change in toe by attaching a couple of laser pointers onto the wheel hubs. This will project two dots on the front of the garage wall. If you trace the dots on the wall as the front wheels are moved through their full range of motion (obviously need to remove the springs and have the crossmember on blocks for this), then measuring the distance between the dots will give you the change in toe. I couple of guys were doing something similar to this to try and dial in inner control arm point exactly for their car. Seemed like a better idea than the blanket statement of "3/4 inch up and 1/4 inch out", or what ever JTR recommends. Quote Link to comment Share on other sites More sharing options...
wheelman Posted March 17, 2004 Share Posted March 17, 2004 Pop N Wood, My understanding of the change in toe when the strut is compressed or extend is caused by the change in the distance the wheel hub is from the center of the car as the control arm moves through it's arc. The distance increases and decreases, so if the control arm and tie rod are not parallel the distance of the end of each at a give point in the arc will be different. So if they aren't parallel and the strut is being compressed so the control arm is horizontally level the wheel hub will be at it's maximum distance from the center of the car but the tie rod end will be closer to the center which pulls the front of the tire toward the center of the car. As the strut either continues being compressed or extends the control arm will start to pull the hub toward the center of the car and the tie rod will either continue to pull the front of the tire in or start pushing it out depending on the part of it's arc the tie rod is moving through. This description is probably more confusing than not and a diagram would help but I'm not artist so I'll leave that to someone else. The camber changes are caused only by the control arm changing the distance the wheel hub is from the center of the car as it moves through it's arc. This is because the top of the strut acts as a pivot point and the top of the wheel hub is either pulled in or pushed out depending on which direction the control arm is moving the hub relative to the center of the car. At least this is my understanding so if I'm wrong on any of this stuff please correct me!!! Wheelman Quote Link to comment Share on other sites More sharing options...
Owen Posted March 17, 2004 Share Posted March 17, 2004 Maybe this will help? http://240z.jeromio.com/frontcross.html Pop N Wood, correct me if I am wrong but average ride height is a variable thing, especially with coilovers, therefore whether the control arm is level or not is also variable. But I think I know where you're going with this, you set a ride height and then measure bumpsteer, you don't like the results so you think about making a change somewhere. "But I thought the bumpsteer mod primarily has to do with toe in and out as the wheels go up and down. (As the name implies, if compressing the strut causes the wheel to turn, then bumps will tend to steer you in a different direction.) And I am not sure how changing the length of the control arm influences that. " I agree with you on this definition. But after reading Wheelman's post I am now more confused. It sounds to me like control arm length vs. tie rod length would also be a factor (vs. tie rod length on its own causing toe changes). I plan to have some adjustable front control arms in the future as a means of adjusting camber, but if it's also going to affect toe, I need to really research this. Owen Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 17, 2004 Share Posted March 17, 2004 When I changed my bumpsteer I used two dial indicators, one on the front of the rotor, and one on the rear. I pulled the front springs, and put jackstands under the ends of the control arms. I jacked the car up and down very slowly while measuring the change on the dial indicators at the rotors. Prior to this I had tried strapping a long stick to the hub and using a plumb bob to measure on my garage floor, that just didn't work for me. I don't think the idea of jacking the body would work for most people either, because bushings will deflect and throw off your readings, but since I have rod ends on the control arms and TC rods, I think it worked pretty well for me. The "standard" way of checking is to bolt a flat plate to the hub and jack the control arm up and down while measuring with dial indicators. To get adjustability I drilled a new control arm hole directly above the original hole and connected the two making a slot, and removed the spot welded washers. I would do my test, then loosen the control arm bolt, tap the control arm up a bit, then test again, until I got it right. It took me a couple of hours to adjust both sides, and it was my first time and I stopped for a coffee break. Modding the crossmember was also easy. The only "hard part" if you could call it that was putting the crossmember in. I use washers on both sides of the control arm, but I never welded them back onto the crossmember. I have not had them move over 4 or 5 years of racing like this, and I check the torque every so often and they have stayed tight. Owen, if you extend your control arms 3/4" and don't change the length of the tie rods, you're going to have some pretty serious toe in! The tie rod length has to change with the control arm length. There is a decent amount of adjustment in the tie rods, but if you go too long on the control arms you'll be forced to make new tie rods. Jon Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 17, 2004 Share Posted March 17, 2004 OK. The figure in Jeromio's link clearly shows it is the relationship of the control arm with the tie rod that causes the toe in and out. I was just thinking of the arc the control arm swings. So my believe that the control arm should be level may be close to the right answer, but based on an erroneous belief on my part. The ride height does vary, but the goal of having the toe change by as little as possible when the tire bounces up and down about an average height would be my goal. It would be interesting to do the laser pointer thing and see how much the toe changes over the entire range of strut motion, both before and after the JTR mod. It would be interesting to see if the toe could be held constant over the entire range, or if you can only minimize the change in toe about the average ride height only. Pete Paraska and a couple of others did the dial indicator method, but a laser pointer would take advantage of geometry to amplify even small angle differences. Plus I would think that moving just the strut would be prefferable to moving the entire car. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 17, 2004 Share Posted March 17, 2004 Your laser pointer idea has merit but would be a big hassle in the end, I think. You'd have to have one on the front and back, and have a reference point to check against. The track width changes A LOT when you start changing the angle of the control arm, so your pointer would make a wide arc regardless of how you changed the control arm angle. So you would need a way to compare the arc vs the centerline of the tire, unless I'm missing something. Using dial indicators is easy, because you can zero them and as you compress the suspension you can see how the front and rear indicators move in relation to each other. So you jack the suspension up an inch, and watch the dial indicators. If the front goes around once and the rear goes around twice (just an example) then you've still got quite a bit of bumpsteer. Move the pivot until the dial indicators move very close to the same amount over a few inch range where the car will spend most of it's time (meaning measure the bumpsteer at the ride height you'll be driving at, not at full droop or full bump). You can buy 2 magnetic base dial indicators from Harbor Freight for like $40, then just stick them to a big piece of metal and start measuring. Jon Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 17, 2004 Share Posted March 17, 2004 I see what you are saying. I kept thinking that as long as the two lines stayed the same distance apart, then the wheel hubs would be parallel. Not true if the tires get closer together. Maybe the way around that problem with the laser pointers is to attach one to the control arm and the other to the hub. Now the change in distance between the two dots is truly the change in toe. But don't know if beams will clear the wheel wells. You would need a good distance between the lasers and the wall to get much angular accuracy. I need to think about it some more. I have two of those laser chalk line things. Maybe I could work out something where the two lines (one on the control arm, one on the hub) stay parallel on the floor. But with dial indicators, I assume you had them sitting on the floor, hence the need for two? The whole wheel may move in and out as the car is raised and lowered, but the difference in the two readings indicates toe? And since the dial indicators are a fixed height off the floor, you need to move the car and not the wheel? If I am correct in my understanding, then my issue with the dial indicators is you first have to makes sure the car goes straight up and down without yawing, and second a 1/4 degree change in toe on a 12 inch rotor will only result in a about 50 thousands of an inch difference in dial indicator readings. I guess that should be measurable. But most importantly I already have a couple of laser pointers (zero is cheaper than $40) I need to go back though the old posts. I thought Pete did it with a single dial indicator somehow attached to the car. BTW, do you happen to know how much you moved the pivot point? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 17, 2004 Share Posted March 17, 2004 Maybe the way around that problem with the laser pointers is to attach one to the control arm and the other to the hub. Now the change in distance between the two dots is truly the change in toe. But don't know if beams will clear the wheel wells. Since the control arm and the hub move independently, any change here might not be a toe change. You'd still need a front and rear marker to tell the difference in the movement of the front vs the rear of the hub. So now you would need 4 laser pointers and a reference point. But with dial indicators, I assume you had them sitting on the floor, hence the need for two? The whole wheel may move in and out as the car is raised and lowered, but the difference in the two readings indicates toe? And since the dial indicators are a fixed height off the floor, you need to move the car and not the wheel? That's why if you buy a bump steer gauge it comes with a flat plate that can be bolted to the hub. I've also heard of people strapping a metal plate to the wheel itself. The plate gives you a lot more room to jack the control arm up and down without the dial indicators falling off the surface. Yes to your other statement. The difference in the movement between the front and rear dial indicators indicates the toe change. I don't know how much I moved the pivot point. Enough to fix the problem. I get kinda frustrated reading about people arguing about 3/4 or 15/16 or whatever. What should be done is it should be moved until the problem is solved. Anything else is (bad) guesswork. If anyone is going to go through the trouble to fix it, it may as well be fixed right. You're right about the potential for the car to yaw I suppose, although my rear wheels were on the ground at the time. I should have taken the time to make a plate to attach to the hub so that I could raise the control arm, but I can tell you the difference was night and day on the track. I did the test in a spot where my jack would not roll, and I had repeatable results. I didn't just check it once when I got to where I thought it was fixed, I made absolutely sure before I locked the bolts down, then checked it again to make sure nothing had moved. Jon Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 18, 2004 Share Posted March 18, 2004 Yeah, it gets complicated since they move independently. But those laser chalk lines project essentially planes of light, so if I think through the geometry involved I might be able to convince myself that it will work. The key is the control arm movement should stay nearly perpendicular to the car front to back centerline. I haven't given up on the idea yet. It make take a 6 pack to figure out. Good to hear about the difference it made. Probably helps answer the original question. As far as one size fits all, as long as the car's front geometry is near spec, then it seems reasonable that the same pivot point would work for all Z cars. If we can get a few people to actually measure their fix we could see how much variance there is. Quote Link to comment Share on other sites More sharing options...
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