Pagoda SL Group
W113 Pagoda SL Group => Drive train, fuel, suspension, steering & brakes => Topic started by: Benz Dr. on September 23, 2016, 21:42:19
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So, I have 230SL in that drives well but it will bump steer when ever I hit a bump or undulation on the road. We removed the rear axle and set it up to factory specs and checked everything carefully. The cross strut was bent so I fixed that and changed a few things to get it into alignment.
Before we started the rear axle was running about 3 - 4 degrees positive camber and now it's right around zero. I checked the measurement on all four corners of the car and it's within 2 mm so I don't think that's a problem. I also made a gauge to check the rear axle alignment and it looks to be spot on.
After doing two different alignments and finding nothing wrong I'm at a loss.
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Hi Dan - to be sure, what is ´bump steer´? The steering wheel turns a bit in your hands when the car hits a (slight) bump?
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Sort of. The car steers in either direction when you hit a bump but the steering wheel doesn't move. This is often due to some loose part in the suspension or steering gear although that's not the case here.
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Ah, ok. Would the front suspension not be a likely suspect? Loose balljoint, king pin,steering knuckle, whatever we call whatever keeps it all together (sorry, have never tackled the front end of my Pagoda and don't know all the technical terms yet as a result). Rather than the alignment/geometry.
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I have created bump steer in a previously owned track car (1972 BMW 2002) when I had very stiff springs and shocks, I wonder if the spring rates or shocks in your car are mismatched? Did I just offer a suggestion to Dan? Seriously.
g
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That's the problem when someone was there before me and I have no idea what they did. In this case, all of the rear springs were replaced and the spacers were too thick causing excessive rear wheel camber. We removed the rear axle and straightened out the bent cross strut. All of the geometry for the central mounting was out so I carefully reset all of that and I made a gauge to check the rear axle which appears to be right on.
After I changed spring pads and anything that would get the rear wheel down to zero I thought it would drive normally. The alignment specs are very close, front end has all new parts, I had it on the bench twice with everything set up correctly, and yet it still does the same thing, only not as badly now. When I hit a decent sized bump or a rough section of roadway, the rear of the car jumps out in either direction. It's usually just one blip and then it settles back into place.
According to the alignment bench, everything is straight and correct, yet it still bump steers. The guy who did the ( four wheel ) alignment has a lot more time on things like this than I do and he couldn't see anything wrong either.
I know I must be missing something here but I can't imagine what it could be. We fixed a number of things and it is better, but it's not there yet.
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Double check the steering damper. Just because its new doesnt make it good.
Also adding negative caster will help. If the ride height has changed with the new springs etc.., bump steer will be exacerbated. Get your alignment done with you in the car. The added weight makes more of a difference than you would think.
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(especially in Dan´s case, I believe he is 6´4´´)
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Thanks for the ideas. I'll try them.
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Are the rear swing arm bushes fitted correctly? I'm sure you know that they are installed in the opposite orientation to how they look like they should be fitted.
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Can't say for sure Stick. Which way is right?
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So, I turned the rubber mounts around at the stud mount and...............nothing. Same squirrely behavour. What now, I ask myself.
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How about the tire pressures? I've had a few issues before with incorrect pressures on left and right sides.
Making them all even is best. Try going a slightly softer(less psi) and test. If still the same, you can rule out the tires as the cause. Put tires back to proper specs again.
What about the subframe straps in front near the horns? Fastened properly? Bushings ok?
Walter
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Are your wheels running true? Or may be a bent wheel shaft? Or springs rated differently left and right? Or shocks adjusted differently left and right in case you have adjustable ones?
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Maybe I can throw something in, along the lines of the previous post. Please I am not trying to offend anybody's intelligence, but where knpowledge and experience do not give immediate answers, maybe lack of knowledge and experience from other area (off-road) may help.
What you described as symptom, sounds a bit like the flying rear. Quite common in off-road, in fact. Like driving empty pick-up truck. What I remember from my off-road days, the reason was always one: something was preventing suspension to give in on bump like it should. And from what I remember, again, what we were looking at were:
Springs:
1. Springs too hard - out of specs, too stiff.
2. Springs too short (e.g. worn) and suspension hitting the rubber buffers.
3. Springs too tight. I do not know how to express it in English - the spring "thread advance" is to small, the spring has no room to be pressed as the "layers" of rod the spring is made off are too close to each other.
4. Two different springs on each side.
Shock absorbers
1. Shock absorbers out of specs (too hard)
2. Shock absorber piston stuck
3. Shock absorber rod bent and stuck
4. Two different shock absorbers on each side
Suspension elements
1. One or more of the pivots/movement points stuck - movement is restricted
In Pagode we have the right axis movement - something may be stuck there, something prevents the spring being compressed, maybe something preventing change of length of the right axis.
I remember the simple test was to try to depress and start swinging each of the rear corners of the car. If one is noticeably easier to depress than the other - then the hard to depress one is to be looked at. That is how I figured I had upper outer pivot stuck in the front of my Pagoda - the side of the car in the front was harder to depress and to start swinging than the other.
Tyre pressure can contribute, certainly, as one of the other colleagues wrote, if it is too high.
This is what I rememeber on these topics from my offroad days....
Pawel
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Thanks for the ideas.
The rear axle was changed from drum to disc on this car but those dimensions should be the same. At the time, new rear springs were installed along with a new compensating spring. These are all MB parts although the rear springs were slightly different from originals in that they fitted about a 1/4 turn higher. We compensated for this with thinner spring pads and a used compensating spring. The rear axle was about 3 degrees positive before we did all of this work and now it's right around zero. I made up a gauge to check axle position and it's right on.
I had the axle out of the car and found that the cross strut was bent so that was fixed. I set the central pin to 90 degrees and 158mm from the front flange. All of the rubber has been changed except the bushings on the trailing arms where they fit on to the rear axle tubes. Everything is tight there and I can't see that being the problem.
All of the front end was rebuilt and the car has new shocks all the way around. Car is fitted with alloy rims - I doubt that should have any effect on how it drives.
I changed air pressure to 30 PSI front and 32 rear. I can set them all the same but I'm not expecting any real change. Car shoots left or right depending on which side hits a bump. It also has a slight tendency to veer from side to side while driving down the road - I'd call it more like slightly twitchy than veering.
I've had it to my alignment shop twice and it looks to be perfect after our repairs. It should drive perfectly but it doesn't.
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The car shoots left or right when the rear wheel hits the bump, is this correct?
And when you depress the corners and swing them up and down - does this go like you know from other cars or harder?
And agian from off-road days, just to make sure: did you double check the steering dumper (I know you wrote you had a new one), but on my G-class I had to replace the new one after a couple of weeks - it very quickly got worn, but you have to check for this wear in the middle of it, on a very short distance - it got loose on 0,5cm in the middle of its travel, where the central position of the wheels is.
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Know what, the only thing that stands out, actually, from your description is the non-standard springs... Maybe height was not the only parameter that was different...
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Part number on the springs were standard MB issue. Maybe different but not enough to make that much of a change. We were able to adjust the spring height to normal parameters so that should have removed most of that as an influence.
The thing that gets me is I've seen old worn out axles with beat up bushings in them and the car drove straight as an arrow. This thing is twitchy.
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Dan,
search for my previous posts on springs.
Mercedes' own springs do not match their own tech specs. The ones that come from SLS do.
The MB variation from spec can be as much as 16%.
I would get the springs on a rate tester and measure them.
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I had the car out for a long drive today of more than an hour and it ran well. The only time I noticed any real bump steer was when I hit a fairly large bump so I have it much closer than before. Not perfect but at least OK under normal conditions. The last time I had it out before today it was rather windy outside and it was throwing the car around quite a bit which I think led to me thinking the car was worse than it really was.
My next step is to put the car on a frame machine to make sure it's not bent or screwed up somehow. I also plan to take a different car and get the alignment specs so I can compare the two.