Rear Subframe & Suspension
Posted: Fri Sep 21, 2012 1:27 am
I was going to reply to another thread, but thought I'd drop my two penneth into a fresh post that might help people in future.
I've had endless fun and games with my saloon suspension over the years, as the members of the 406 Coupé Club will testify. Firstly I ought to point out that all the variants of the 406 have the same basic suspension setup - the only major differences are dampers and springs - so my experiences could apply to all owners. Almost all rods/linkages/bushes are universal across saloon/estate/coupé of the same year, although some were changed during the production-run (front drop-links changing from Z-shape to L-shape being a good example).
Anyway, back to the main point. My car always had a habit of pulling left, and I found it would scrub the left sides of the front tyres. After all the fun and games I had, lots of money spent on tracking and tyres, I discovered my rear subframe was wrongly aligned with the chassis. I can only presume this alignment problem was created in the factory, and at the time it appeared a few of the coupé owners had similar issues with their cars pulling left. I should clarify that this mis-alignment at the rear of the car scrubbed the left hand side of the FRONT tyres - not the rears.
Evenly spread wear on rear tyres could well be down to badly set tracking (assuming it's badly set both sides), but anything that stops the rear wheel alignment being equal at both sides will scrub the fronts instead as you steer to compensate. The rear tyres should have a small amount of toe-in at kerb height, but there's enough spare in the adjusters that they could be set to toe-out. The adjusters are a weakness too, and they eventually get hammered by speedbumps and so on, so the wheels end up toeing-out regardless. I have to do do my own tracking several times a year because of the damage done by the local speedbumps
I've found many tracking places simply set the car to a universal setting for FWD cars (parallel rears, fronts toed-out slightly - like the 306/405 torsion bar rear setup would be) and this is very wrong. The problem with this is the rears begin to toe-out under compression (the rear-wheel steering effect) so driving a car like this with a full boot will scrub the inner edges of the rears, as will enthusiastic driving. It is WELL WORTH getting all four wheels correctly aligned. It'll save you a fortune in tyres, a fortune in fuel, and better still your car will handle FAR better than you ever realised.
In the case of my subframe problem I'd fabricated my own laser rig (initially to save the cost of constantly paying to get the tracking done only for it to never be right) and through frustration decided to measure the rear wheel alignment along the sills, rather than at the front wheels. When I did this I found the rear subframe was a whopping 10mm too far left. A little bit of wiggling it about got it perfectly centred, I re-tracked my rears and the difference in handling was unbelievable...
The problem with the subframe problem is that whenever you get four wheel tracking done, they align the rear wheels to the front wheels - but if the rear wheels are mounted offset to the left in the first place, they will be aligned to point right - thus steering the car left. The overall amount of toe-in is correct, so the rears won't scrub, but the rear is steering you left so you correct with the front and you can wave goodbye to those lovely new Michelins pretty damn quickly...
Bah, I've gone and rambled like a fool. Sorry chaps. The simple point is that the 406 is a sweet-handling car when the suspension is setup properly. And it really is worth getting it done right. And don't assume the factory got it right either - they got mine VERY wrong
I've had endless fun and games with my saloon suspension over the years, as the members of the 406 Coupé Club will testify. Firstly I ought to point out that all the variants of the 406 have the same basic suspension setup - the only major differences are dampers and springs - so my experiences could apply to all owners. Almost all rods/linkages/bushes are universal across saloon/estate/coupé of the same year, although some were changed during the production-run (front drop-links changing from Z-shape to L-shape being a good example).
Anyway, back to the main point. My car always had a habit of pulling left, and I found it would scrub the left sides of the front tyres. After all the fun and games I had, lots of money spent on tracking and tyres, I discovered my rear subframe was wrongly aligned with the chassis. I can only presume this alignment problem was created in the factory, and at the time it appeared a few of the coupé owners had similar issues with their cars pulling left. I should clarify that this mis-alignment at the rear of the car scrubbed the left hand side of the FRONT tyres - not the rears.
Evenly spread wear on rear tyres could well be down to badly set tracking (assuming it's badly set both sides), but anything that stops the rear wheel alignment being equal at both sides will scrub the fronts instead as you steer to compensate. The rear tyres should have a small amount of toe-in at kerb height, but there's enough spare in the adjusters that they could be set to toe-out. The adjusters are a weakness too, and they eventually get hammered by speedbumps and so on, so the wheels end up toeing-out regardless. I have to do do my own tracking several times a year because of the damage done by the local speedbumps
I've found many tracking places simply set the car to a universal setting for FWD cars (parallel rears, fronts toed-out slightly - like the 306/405 torsion bar rear setup would be) and this is very wrong. The problem with this is the rears begin to toe-out under compression (the rear-wheel steering effect) so driving a car like this with a full boot will scrub the inner edges of the rears, as will enthusiastic driving. It is WELL WORTH getting all four wheels correctly aligned. It'll save you a fortune in tyres, a fortune in fuel, and better still your car will handle FAR better than you ever realised.
In the case of my subframe problem I'd fabricated my own laser rig (initially to save the cost of constantly paying to get the tracking done only for it to never be right) and through frustration decided to measure the rear wheel alignment along the sills, rather than at the front wheels. When I did this I found the rear subframe was a whopping 10mm too far left. A little bit of wiggling it about got it perfectly centred, I re-tracked my rears and the difference in handling was unbelievable...
The problem with the subframe problem is that whenever you get four wheel tracking done, they align the rear wheels to the front wheels - but if the rear wheels are mounted offset to the left in the first place, they will be aligned to point right - thus steering the car left. The overall amount of toe-in is correct, so the rears won't scrub, but the rear is steering you left so you correct with the front and you can wave goodbye to those lovely new Michelins pretty damn quickly...
Bah, I've gone and rambled like a fool. Sorry chaps. The simple point is that the 406 is a sweet-handling car when the suspension is setup properly. And it really is worth getting it done right. And don't assume the factory got it right either - they got mine VERY wrong