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Rear End Steer
I have a ford 9” rear end. It is a tri bar design, lowers rods are straight to frame; uppers angle in from outside towards the brakes to the frame. The uppers are 9.400 inches long from center of bolt to bolt and the lowers are 12.700 long. I have hime joints at both ends on all radius rods. Everything is new no used parts.
My problem is when I get on the gas the car drives to the right and when you let up on the gas the cars goes to the left. Is my rear end moving side to side or do I have an alignment problem? Should I install bushed rod ends at one end of each radius rod? |
Got any good pictures?
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Are you spinning the tires when this occurs? One tire spinning more than the other can cause this, but, I think you have an axle movement problem for some reason. like John asked, do you have any good pictures of the rear axle and suspension?
Byron |
Sounds like something is loose. What is locating the rear end side to side?
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sounds like mine. I have a triangulated 4 link but the same issue. If you jack up one rear corner i think it demonstrates what is happening: as you jack the one side, the wheel on that side actually moves forward, effectively steering the rear.
I think under acceleration the passenger side compresses, pulling passenger wheel forward steering you to the right. Assuming your links are level at rest. My links are higher at the front (I think that give sit more anti-squat), so I steer to the left under acceleration. I'm going to put sway bars on, I don't have any, so that should reduce (mask) the problem. |
sounds like mine. I have a triangulated 4 link but the same issue. If you jack up one rear corner i think it demonstrates what is happening: as you jack the one side, the wheel on that side actually moves forward, effectively steering the rear.
I think under acceleration the passenger side compresses, pulling passenger wheel forward steering you to the right. Assuming your links are level at rest. My links are higher at the front (I think that give sit more anti-squat), so I steer to the left under acceleration. I'm going to put sway bars on, I don't have any, so that should reduce (mask) the problem. |
Just for the heck of it. You may want to check if your housing is bent.
Ralphy |
Are you running an "Open" Differential, Limited slip or?
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Put some kind of rubber bump stops to limit the movement to within acceptable behaviour. Sway bars will help hugely. Also if the tire pressures are off it will pull too.
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i would guess alignment issues, what is the thrust angle?
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As was explained above the geometry is causing the axle to "steer" the car, so by limiting the movement so that the control arms movements remain close to parralel to the chassis.
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How much HP are you running? I think if the other items listed previously check out I would replace the bushings with poly or solid bushings. I'm guessing they are crushing under power.
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Is there adjustability in the angle of the control arms? The control arms are short so slight changes in the angles result in exaggerated axle movement under travel. As you jump on the throttle the torque wants to roll the car to the right, compressing the right rear suspension and lifting the left side. This is causing your rear end housing to change angles. In your case it sounds like the right rear is being extended while the left rear is being pulled. If you can adjust the angle of the control arms you can adjust some of this out.
Of course, I would double check the alignment and for loose parts as well. But, you sound like the pull is dramatic so I think it's the control arm angles, and length. The suspension design you are describing is a Satchel Link. This is used in a lot of circle track cars because it can be adjusted for "rear steer" and can be designed to help the car turn. Be careful! I'm not an expert, just my thoughts. |
when you do an alignment you can adjust the thrust angle to the right no more than 6 degrees, that will compensate for torque steer.
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My car did this prior to 4 wheel alignment and setting up the coil overs on scales.
The rear wheels toe in slightly now. oops my bad. Should be an easy fix with the solid axle |
Its likely not a bushing thing. I have all solid bushings and it still does it. I think John explained it better than I did, it's a geometry issue caused by short links. My links definitely angle downwards from front to back, so I have exaggerated issues. It scared the CRAP out of me the first time when i backed off the throttle quickly The upside is it hooks up really good!!
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Treblig, My Shell Valley came with a narrowed 9" and we felt it had movement also. After looking at the 4 link we could not locate a problem . We start to think it was a problem with the locker and we started to remove the axles to have alook. We could not remove the axles ??? they would not come out . After hours of pulling we drove the axles out we found that the flanges at the ends where welded at an angle and could be seen with the naked eye. The bearing where beat into the opening and where cracked in two places . Yes Shell Valley did the welding . I removed the rear end and took it to a specialists and he could believe it . You could not put the axle aliment tool in to the mouth of the housing end it was so far off. It was working on the axle like a lathe and it torn up the bearing and the housing . I purchased another 9" and had it cut down to fit . I hope you do not find this but another Shell Valley owner in Washington State did.
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Shell Valley Junk
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Thanks for all the input.
The ride height is set correct. My lower radius rods are parallel to the ground and my pinion angle is down 3 degrees. However there was a good point made on my rear end housing being bent. When I had the bearing races welded on, the shop said the housing was bent but he had it straighten. Need to double check that. On alignment, the only thing I can say, I measure from a hard point on the frame to the center point on the axels the frame to axels is with in .030. Question here: I was thinking on adjusting the rear axels to have a slight angle, let’s say offset the alignment .060 to .100’s maybe .200 and take it for a spin. If this corrects the problem or improves then it’s an alignment issue, if not need to keep digging??? When you punch it the rear end does not lift the car, it sits or stays level. Not spinning the tire. On the fit for all the joints / bolts, the rear end is tight. All bolts are a slip fit |
looking at your frame layout I don't believe it has adaquet torsional stiffness and you will get severe torque steer with a car this light and a lot of hp & tq. You might be able to counter act some of it with some anti roll bars. Also , your drive shaft looks more like it might have 12-15 degrees at the u-joint.
You might also go to a much higher rate rear spring to stop some of the torque roll.and it seems as though maybe your trans crossmember is the most rearward cross member?????????? |
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