setting rear IRS camber

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Roger King
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Re: setting rear IRS camber

Post by Roger King »

StewbieC wrote: Tue Jun 23, 2020 2:51 pm 1/2 degree of negative camber is ca 3mm difference between the top of the rim and bottom of the rim measured against vertical (taken from a 15" rim that measures ca 380mm dia).
I used a simple Bosch laser level that provides a vertical plumb line, you can use a plumb line suspended somewhere close to the wheel and measure to the string
camber = tan -1 (difference in measurement to the plumb line (in mm) / diameter of rim (in mm))
so tan-1(-3mm/380mm) = -0.45 degrees
I remember my dad telling me I'd need A-level maths one day....
Well, it was no use whatsoever throughout university, post grad and a career in dentistry (dentists don't measure anything - with a very few exceptions) but 48 years later I had to setup my Mustang's front end, so I dug out my book of log tables, reacquainted myself with SOHCAHTOA and Bob's yer uncle!*


*although in my case he was my Dad.
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StewbieC
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Re: setting rear IRS camber

Post by StewbieC »

I went back and did my degree 10 years ago, couldn't afford to go to uni when I left school. I ended up having to help half the class with algebra and calculus as they hadn't learnt anything in school. I did old school O level maths taught by teachers who wore black gowns and the head master had a mortar board to boot. I quite enjoyed going back and doing engineering related maths, calculating Spring damper responses is a bit tougher using second order differential equations than working out a camber angle using a bit of Trig :? Shall we do it with complex J numbers rather than polar geometry?? I'll get my coat.....
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Hawk 289, 66 Mustang Fastback with a 289 maximum smiles per mile..
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simonjrwinter
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Re: setting rear IRS camber

Post by simonjrwinter »

Did anybody else duck as that went over their heads.....? 😜
TR6 (V8) Hawk 289 FIA (V8) Doing my bit for global warming.
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Roger King
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Re: setting rear IRS camber

Post by Roger King »

When I left school, a university education was still means-tested and therefore free for the son of a schoolteacher.

This is supposed to be progress.
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StewbieC
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Re: setting rear IRS camber

Post by StewbieC »

simonjrwinter wrote: Fri Jun 26, 2020 5:21 pm Did anybody else duck as that went over their heads.....? 😜
Didn't mean it that way Simon, tan-1 or arc tan is the same and will be a button on any scientific calculator even most smart phones nowerdays tan-1 on a Casio or arc tan on a Texas calculator
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Hawk 289, 66 Mustang Fastback with a 289 maximum smiles per mile..
CobStang
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Re: setting rear IRS camber

Post by CobStang »

I use a section of 20 x 40 box section steel with a couple of studs screwed in each end set at the wheel diameter. The studs need to be long enough to allow the box to clear the wheel spinner, and need to be set at the same distance out from the box, ie 80mm - 100mm.
By holding the box vertically with the 'pins' resting on the wheel rim it is simply a case of holding a digital spirit level on the box. You get super accurate repeatable data and with the tyre pressures equalised it is simply a matter of adjust, take reading, adjust until you have the desired camber.

Works for front and rear obviously, and is fast.
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Roger King
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Re: setting rear IRS camber

Post by Roger King »

About fifteen years ago I bought a full set of Dunlop gauges on ebay from a garage clearout. The tracking gauges get a lot of use - the old mirror kind - and the camber gauge gets used now and then, but obviously camber tends not to need adjustment so often. They've paid for themselves several times over.
I bought them because I'd had an expensive four-wheel laser alignment done on my Mustang (£90 seemed a lot then...) which produced rather vague steering (well, vague for an old Mustang) and very rapid front tyre wear. I did a very thorough four-axle-stands-and-string-'box' alignment and found the rear axle was very slightly out of alignment with the front axle. I don't know if the fancy laser kit was trying to work around this, or what - but the result, checked empirically, was terrible.
I measured very carefully, cut the axle perches off the Currie 9" live axle, and rewelded in the correct position for my car. At the same time I reduced the pinion preload angle as this was Currie's 'performance' axle, with a real drag racer's negative pinion angle to cope with drag launches, not relevant for my usage and allowing a much more favourable set of driveline angles to reduce vibration.
Handling on the car, and tyre wear, were transformed and it's much nicer to drive. Every year or so, depending on usage, I check camber and toe-in (both easily adjustable on a '68) with the Dunlop kit. I think the benefits of modern four-wheel alignment systems is questionable on older cars with live rear axles.
My Cobra is spot-on to the AC manual numbers, and only toe-in is adjustable, so not much to bother about there.
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Re: setting rear IRS camber

Post by CobStang »

+1 for Dunlop gauges.
And we string line every new DBR at work.
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John H
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Re: setting rear IRS camber

Post by John H »

Well done Simon. I never checked mine, Gerry said use the same shims from the IRS I used off an XJS. Once I get my hubs back I will check and see what it is, always felt planted though so can’t be far off. Stu I am assuming you numbers are how they should be?
Built St Piran Hawk289FIA in Scotland!
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