Calling all suspension gurus
 

I'm rebuilding an old Cobra based on a Toyota Crown chassis. I'm having problems with the front suspension. I set it up with the desired camber and toe-in, however after I drive it once around the block, the camber goes from about -2 deg. to +1.5 deg on both wheels. This then impacts on the toe-in.

I had previously replaced all the bushings, bearings and ball joints. All the suspension bolts are tight. I'm at a loss as to why the camber is changing. I suspect it has to do with the angles of the top and bottom ball joints, however I've not altered the design of any of the front suspension components, so I don't know why this is happening.

Any ideas/help would be appreciated.....

From what I can see the Toyota Crown uses an upper A-arm and a lower single arm with a diagonal ball-joint brace arm. That brace arm can cause problems if worn.
Since the steering rack is front of the suspension, if the Brace is worn the hubs will shift aft when the brakes are applied causing Toe-in. Driving with a lot of Toe-in will cause the suspension to climb inwards and add Positive camber.

That or you already have a lot of toe-in and the climbing effect is the result.

https://www.nengun.com/oem/toyota/48660-51020

There are quite a few "IFs" in my response as I don't know your year model donor suspension info.

Its a 1985 Crown MS85. This is more like what the suspension looks like:

There is a diagonal brace on the lower control arm (which is not shown on the diagram). However, it seems to be in good condition. I assume its there to adjust the caster?
This model had a steering box which I have replaced with a rear steering rack.

Same principles apply on the earlier setup you have with the manual gearbox-pitman arm tie rods.

https://partsouq.com/en/catalog/toyo...-KR&group=4505

If the lower suspension arm deflects backwards during braking the tie-rods will increase your toe-in.

Hi gcarm, it sounds like you've had several goes at resetting this. What adjustment do you make each time to put back right again?
Have you replaced the rubbers at the front of the radius rods?
The lower control arm will shift back and forth if these are worn affecting toe, but shouldn't affect camber much

Hi Slipry,
The only thing I didn't replace was the rubbers on the radius arms, as I couldn't find a replacement. However, they seem to be in good condition, so I don't think this is the problem.
I think I may have discovered the problem - the lower control arms I thought were the same for left and right side. But after checking the diagram closely, they are slightly different as they have a small 'v' shaped bracket welded on one side. According to the diagram, this bracket should be pointing towards the front of the car. It seems I have swapped them when I reassembled the suspension.
I also discovered when I removed the lower control arms, the bottom ball joint is slightly different between the left & right sides. Although the pin is the same, the base is a slightly different shape. Not sure if this is an issue, but I will order a new ball joint so they both match exactly.
So hopefully, when I reassemble it, the problem will go away. Here's hoping, will let you know the outcome.
Cheers,
Greg

Whils the rubbers may appear good, all the forces on them are compression. There is a metal sleeve that determins how fare the nut tightens.
Still might pay to check the rubbers haven't compressed a bit allowing some fore/aft movement.
I couldn't source them either when i did mine. I adapted some others which I don't remember what they were off. This was to be temporary - 22 years ago.
I've been wanting to graft in a rack. Im courious how you did yours.

I had a lot of trouble finding a rack to fit as the track width at the front of the Crown is quite narrow. The rack I used is from a Daewoo Lanos.

The Lanos rack is a centre take off type. I made up a plate from 5mm steel which is bolted to the movable part of the rack. I then mounted a solid AL tube to this which is cut to a length which will minimise bump steer (well, that's the theory - yet to prove it in reality). The tie rods are screwed into the ends of the tube. Finally I used a linear bearing to support the plate so it will stay in place during loading.

I'm going to pull it out so I can repaint it once I get the suspension issues sorted. I'll post some pics then so you get a better idea.

Cheers,

PS> radius rods - I assume you mean the diagonal rods that go from the lower control arm to the front of the chassis?

Yes Greg, correct

It sounds like you have a bump steer problem from the info you have given. The problem spdbrake said sounds correct. If you have a bump steer problem. As the wheels toe in it forces the body up causing it to toe in more. I would sit the car on the ground at standard ride height. Measure the distance from the centre of the wheel to the mudguard. Write it down you will need it later. Jack up the front of the car and place on chassis stands. Jack up the rear, make the car level and place on stands. Remove the front wheels, disconnect shocks and remove front springs. Reconnect shock. refit front wheel. Place jack under lower control arm and jack up wheel. Jack up until the distance from the centre of the wheel to the mudguard is the same as standard ride height. Now place a piece of wood or angle iron on a crate or drum at the same height as the centre of the wheel. Place a weight on the wood/angle iron to hold it steady. Now measure from the wood/angle to the rim edge and adjust until it is square to the face of the rim. Now jack up control arm until the wheel has moved up 20mm. Now check the measurements from the wood/angle to the rim edges. The difference in the measurements is the bump steer toe in. The difference should be less than 2mm for a road car. Hope you can understand all that and it helps diagnose your problem.

Greg, sorry I've got to ask this and no insult intended, are you setting your alignment with the front jacked up and the wheels off the ground? When you drop it back down the alignment will change.

If it is a bump steer issue as Bender suggests, has this only appeared since the installation of the steering rack?

So I've swapped the lower control arms and replaced the ball joint so I now have two of the same.
Re-tightened all of the nuts and it seems the camber and toe is not changing as per before.
The only problem left is with the camber. Its positive on both front wheels. I tried inserting some shims in the top control arms which has given me around -1.4 deg on the left wheel, but the right wheel is still slightly positive.
I would have assumed its better to have the same amount of negative camber on both wheels?
Or is this ok, as I've read some geometries allow for less camber on the right side due to the crown of the roads.

Its best to have at least 1/4 to 1/2 degree negative camber. This is so that the tyre is not rolling on the outside edge in a straight line. It is not unusual to have 1/2 degree more negative on the lhs. Normally you keep the difference between the 2 sides at a max of 1/2 degree for road cars. Race cars can have a bigger difference due to the fact race circuits tend to have more corners in one direction. Hope this helps.

You want to set it up the same side to side for a road car as you can never tell which way the corners you will travel around will go. Don't worry about crown in the road, you want predictable handling left and right.

OK, so I added some more shims to my drivers side front top control arm to get some more negative camber in that wheel. I'm now getting around -0.9 on both wheels. I think this should be about right for a road car.
What I failed to take into account is the level of the ground I am measuring upon. Although my driveway looks level, there is a definite crown on it. I'm now doing the measurements in the garage which is more level.