Tracking a BX with a laser

Frequently asked technical questions and common modifications/improvements
tom
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Tracking a BX with a laser

Post by tom » Mon Feb 05, 2007 9:30 pm

Using a commonly available laser level, it is quite easy to measure and if necessary adjust the tracking on a BX. The 400mm type with a spirit level is ideal.

This example is for steel wheels and an estate car.

glue a small magnet to a ruler. place this in the bottom of the rear wheel, and measure the distance to the edge of the rim (45mm in this case)
A=45mm
The front track is 1420mm. (1434 GTI, 1440 16V)
The rear track is 1364 for estates, (early saloons 1354, late ones 1360. 1378 GTI and 1384 16V.)
Halve the difference (56mm/2) in this case.
B=28
find the distance between the base of the laser and the beam. It is often marked on the level (20mm)
C=20

A+B+C=93mm. This is the offset. If the wheels were pointing straight ahead, this is where the laser beam would be seen on the ruler.

Divide wheelbase (2655) by wheel diameter (355) =7.4
Multiply by half the allowed toe out which is 3mm
This gives 11 as nearly as you can measure and this is the maximum toe out.

Mark the ruler with tape at 93 mm and 82mm (93=11).

On level ground, centre the car's steering, start the engine and wait for normal ride height.
Place the laser with the line generator lens (which must be set at right angles to the laser to give a vertical line) on the wheel and use the spirit level to check it is level. If the wheels are both pointing straight ahead, the beam will touch the ruler at 93mm If they are fully toed out by 1.5mm each side, the beam will touch at 82. Somewhere in between is ok. If they are out more on one side than the other, the steering is askew but with care, the steering wheel can be centred and the toe out adjusted to perfection.
The ruler should be placed in the bottom of the wheel because the rear wheels toe in. Setting the ruler at hub level at front and rear of the wheel will show how much. I may return to the topic of rear wheel toe at a later date, for it is seldom discussed and worth knowing about.
Last edited by tom on Wed Feb 07, 2007 8:33 pm, edited 3 times in total.
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Jaba
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Post by Jaba » Mon Feb 05, 2007 9:59 pm

Brilliant idea Tom.
How much does a laser gadget cost ? This might be another item on my wish list for a club tool sharing and loan scheme.
I paid 32EUR last month to have mine checked and was told it was bon and no adjustment was needed.
The Joy of BX with just one Citroën to my name now. Will I sing Bye Bye to my GTI or will it be Till death us do part.

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cauchoiskev
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Post by cauchoiskev » Tue Feb 06, 2007 12:33 pm

Yes, a brilliant idea indeed.

Where did you find the figures for the front and rear tracking, Tom ? Can't find them in Haynes.

Also, as I remember, the rear wheels not only have a toe-in, but also about 1° of negative camber, i.e. they're a bit knock-kneed. Would this not affect the measure if it's taken at the bottom of the rim ?

tom
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Post by tom » Tue Feb 06, 2007 8:14 pm

Intelligent questions! The laser cost me £4 (in Super U, IIRC). You won't need to spend more than £8.

THe rear wheels shouldn't have more than one degree of negative camber but inevitably will have if the back end is worn. As the back affects the handling more than the front (!) I'd assume you'd have it in good order. The bottom of the wheel is taken to be correct as far as the vehicle's quoted track is concerned because track is the distance between the centres of the tyres at ground level

Regarding the figures, I used Russek's manual. The figures also appear in the owner's handbook.
The basic formula is much as stated:
(Front track - rear track)/2 to get any offset and determine straight ahead position,
then (wheelbase / wheel diameter) x (total toe/2), adding for toe in and subtracting for toe out (as is the case for the BX).

I'll edit the sticky posting to include the figures shortly bacause they will be of use. I could do with knowing, for the sake of completeness the inset of an alloy rim on which the ruler needs to be fixed with a sticky pad or blutak. I'd suggest a using a white ruler and twilight unless you have a garage or a very fierce laser!
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cauchoiskev
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Post by cauchoiskev » Tue Feb 13, 2007 9:34 pm

If it doesn't provoke the anger of the wizard too much, could I add the following details for the benefit of numpties like myself ?

1) Don't try to adjust the toe-out from the engine bay (just about possible with a 14, impossible with any other model). Put the car in high position, and turn the steering to the opposite lock from the side you are doing. you can then get to the relevant nuts from the wheel arch. The lock-nut is 19mm and the adjusting nut 13mm.

2) One clockwise (as you look from the wheel) turn of the adjusting nut will increase toe-in by roughly 15mm at the ruler (i.e. the laser line will move away from the car).

3) I vote for Tom's post as "Post of the Year" (yeah, I know it's only February). You can check your geometry in 5 minutes flat, and adjust it in half an hour (if the lock nut isn't jammed solid....). I had just changed my tyres, and the old ones had uneven wear, so this has probably saved me a few thousand miles of rubber. Thanks again, Tom !

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Post by citronut » Fri Nov 30, 2007 10:44 pm

i thought you are ment to check tracking at normal ride hight, as this is when the track rods are level or near level
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DavidRutherford
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Post by DavidRutherford » Fri Nov 30, 2007 10:48 pm

citronut wrote:i thought you are ment to check tracking at normal ride hight
You are, which is why
tom wrote:centre the car's steering, start the engine and wait for normal ride height.
.. and I don't think anyone has said otherwise have they?
this might be a signature