BMW Rallying Forum

i have just swapped the wheels over on my compact ready for a rally on saturday and noticed that the inside of both the rear tyres was really worn. i had this problem before and replaced the rear beam thinking that something was bent. now that it is happening again i checked the rear toe and found it to be toeing 2 degrees out. i have had a look on the net and all i can find is 0 toe is ideal.
I spoke to a friend with an e30 rally car and his tyres wear on the inside also.
i am running gaz coilovers about standard ride height with a m42 4pot. the tarmac tyres are hankooks and the dirt tyres are michelins. i am quite a fan of the handbrake and do like a slide so the tyres are going to suffer but I'm surprised how uneven the wear is

does anyone know what the idea rear toe should be?
i jacked the car up about 100 mm from the diff and checked the toe and it was still toeing 2 degrees out.
nothing looks bent and both the rear tyres are wearing the same. i am extremely economical with my tyre buying and these ones were bought second hand about 4 years ago and have had a hard life

While I cannot speak about specific numbers, changes to toe and camber as the wheel moves through its travel are both inherent to semi-trailing arm rear ends.

You will probably find that the rear toes in and gains significant negative camber under acceleration, and this is probably where the excessive wear comes from.

My suggestion is to remove one rear spring, support the body at ride height, and then jack the hub up and down. Measure the toe and camber at various points. Then look carefully at the geometry changes - you may find that a change in ride height or possibly limiting the travel will have you working in a better range that avoids significant tyre wear.
For example, when I built the struts for my old gravel RX-7, I gave them heaps of travel (about 280mm, IIRC). This seemed like a good idea, except that at full droop the geometry went to pooh - lots of positive camber and lots of toe in. Landing from jumps was simply terrifying until I used travel limiting straps to avoid the last 40-50mm of droop travel, and kept the geometry under control.

My guess is your rear arms will be bent we usually need to change ours every 3rd rally as the rear wear gets to excessive

i measured my daily road compact and the rear toe was 0.1 degree toe in. I'm not sure if that is where it should be but seems a lot better than 2 degrees toe out. i have extra arms so will change everything on the rally car but will measure the old arms to try and see what has bent. i will have to keep it as it is for the rally on saturday and change it afterwards as i don't have any time.

i put this together last night, it has z3 arms e30 325i diff and e36 328i shafts (only tried one to see if it fits ok, the lh shaft is the z3 shaft that is putting up a fight to get out of the hub) it works out 65mm wider per side which should work out well with the arches i have and to balance up the extra width of the e46 arms on the front.

This is with standard arms, 25mm spacers and e46 wheels
I will get the z3 armed rear beam cleaned up and fit new bushes and should be able to straight edge the disks before i fit it to check that it is roughly where it should be.
The rally i am doing on saturday has huge pot holes on the corners which the car hits pretty hard and this might be the cause of the bending.
i won't worry about the arms on saturday and will measure them afterwards, with any luck i might bend them straight!

i finished the rally yesterday with the toe out and shredded my rear tyres after 4 short stages
the outsides were ok but they were coming apart on the inside
apparently this could be caused by a bent rear beam so i will swap the arms into another beam and see how they line up.
this is stage 4 and the noise at the end is the tyres coming apart.
https://www.youtube.com/watch?v=sIdfXMrudwY
i don't take it too seriously and the car is pretty low with very stiff springs but i really enjoy the event and would rather compete in the wrong car than not at all. Lots of fun and smiles

I went to the uk, bought 2 z3 rear beams, stripped them in halfords car park, brought them home cleaned them up, fitted new bearings, bushes and welded strengthening arms before i gave them a coat of satin black i then fitted adjustable hangers on the rear beam, fitted it all together and the front of the nearside wheel was about 25mm further in the arch than the rear, the toe was so far out that i couldn't drive it. I've had to pull them off and strip them down, buy another 2 wheel bearings and rebuild the second (first pair i tried) going to press the hub in tonight and build it back up and hopefully i can get some decent geometry with them. i tried adjusting the camber with the rear wheel jacked up but it was so different to the camber when the car was resting on the ground, I'm going to have to try and adjust everything with the car on the floor. i am aiming for slight toe in (about 1 degree) and about the same in negative camber. i don't know if that is any good but it is somewhere to start