Best place for alignment work.

[Jaguarnut]

I missed a question I think which basically is what the hell is "Ackerman". It is a difficult concept to explain but it is a theoretical ideal which in practice is seldom acheived fully. There are a number of angles, theories etc., all interrelated that are built into steering geometry. This one concerns the steering angles of the front wheels. No doubt we have all looked at wheel tracks in left mud or on dry surfaces after driving through water. If you can imagine these tracks after the vehicle has been turning say to the right on full lock. You get four independant tracks all following an arc from some imaginary centre point. The inner nearest this point will be the RH rear wheel, the next furthest out will be the RH front, next LH rear and the last LH front. Clearly the back wheels are fixed and therfore follow two seperate but parrallel arcs revolving around this imaginary centre point, the difference of the radius being the track width of the rear wheels. This arc radius (and the position of the imaginary centre point) is determined by the amount of steering lock applied. The more lock, the tighter the turning circle, the less arc radius. Now because the position of the back wheels relative to each other is fixed, the front wheel angles have to be different to maintain a turning circle otherwise significant tyre scrub will be seen! The difference in the angle between these two wheels at the point when projected they meet is the ackerman angle I belive, not fixed amount as it varies from zero to a maximum figure as more lock is applied. Fine in theory, but in practice, the various requirements of steering and suspension inevitable means a compromise, hence the degree of front tyre scrub usually witnessed on full lock. Back to the main point, at the straight ahead position, (wheels, rack bar and steering wheel), this angle is at zero. If anyone is avoided insomnia through reading this, does it make sense???

[leorest]

It makes sense and I do understand what Ackerman is but I couldn't see how taking the steering wheel off and putting it back on made any difference. Once the toe angles have been set with the newly defined "centre" then this makes sense but not before. I think this boils down to "It's difficult to explain a complex model in a few words"

[Mr Whippy]

If the rack is central AND the suspension geometry pickup points are symmetrical about the rack centre point, then I guess we can assume ackerman effects will be symmetric one way or the other.

My concern is if the rack is say 1cm off to one side, so the TRE's are adjusted to keep it straight, then the ackerman effect will be in force at dead ahead. You may well take a left hand turn and the ackerman zero's it's effect **before** starting to turn the outside wheel more, leaving you with screwed geometry.

Is that effect negligible or not I wonder? Do race teams even worry about the slight offset?

Surely there must be a decent alignment shop out there

Dave

[combemarshal]

If your steering wheel is not straight when going dead ahead the answer isn't just to take it off and centre it yourself, there must be another underlying problem that put it out.

[Mr Whippy]

combemarshal said:

If your steering wheel is not straight when going dead ahead the answer isn't just to take it off and centre it yourself, there must be another underlying problem that put it out.

Yep, it's had a new TRE on the nearside and the wheel isn't straight. Thats worrying because either the steering rack is out by say 5 deg at the steering wheel, and the TRE's are adjusted to be straight, or the wheel was removed and re-fitted badly, and the TRE/rack is spot on, or a combination of both!

Hence the quandary

Dave

Edited by Mr Whippy on Thursday 6th July 20:20

[leorest]

Was it straight before the TRE wore out?

[Mr Whippy]

leorest said:

Was it straight before the TRE wore out?

Dunno, previous owner

Copper slip and new "looking" TRE were the give away on a W plate car with 72k on.

Dave

[mototec]

What a load of old boocks

leorest said:

Unless you get a cast iron good personal recommendation you would probably be better off doing it yourself.
Flat bit of ground, fishing line, axle stands, el cheapo laser spirit level, steel rule, tape measure, plumb line, metal plates, grease, and some schoolboy mathematics. With all this and a bit of research on the web you should be able to do full four wheel alignment with high accuracy, just not as quickly as with the professional kit.

[Mr Whippy]

Well I took the steering wheel off recently to fit a leather one (mmmm), so I re-set it to the right a bit but then it was more right than it was left

So I'm guessing that where it was is right (looked like it hadn't been off when I took it off), so must be down to the TRE adjustments.

Since tyre wear is pretty sweet across the patches at the front I think I'll wind a few turns off one side and add a few to the other side and see how it evens out

That said will be fitting new front dampers sooner than later so may just get down to that Sunside place in Halifax after to get it checked 100%

Dave