In my previous post I was talking about setting the tracking on our 500 and mentioned the laser tracking gauges I made some years ago. I spent a dozen or more years of my early working life at an independent garage that specialised in, amongst other things, suspension geometry alignment and adjustment. We used an amazing piece of kit manufactured by HP Anderson, I believe they now trade as HPA. Having been out of the trade for so long now I don’t have access to such equipment any more but it is an absolute must have item in my opinion.
I spent a while watching auctions on eBay and waiting for something to fall in my lap but it never did. And the problem was compounded by another matter of a rather delicate nature. Lean in close, I’ll say this quietly because it will piss a lot of people off! There are quite a few vendors selling laser alignment tools that hang on the wheel and shine across the car to a flag on the opposite unit. Some profess to measure toe—in/out in mm and have units marked on the flag to represent these. Many of these devices simply DO NOT WORK. Fast fit establishments love them, because they are quick and pretty much guarantee you’ll be back for another set of tyres!
How can you tell? Well it’s pretty simple really. When you measure toe in degrees you can’t go wrong. Degrees is degrees, is degrees. That’s why the venerable Dunlop optical gauges are still a fantastic tool. They work very well, although they are a bit of a fiddle because they are free standing and don’t move with your adjustments (They are also nearly £500 a set and really hold their value!). When you measure toe in mm it gets a bit more complicated. The measurement is taken from the edge of the wheel rim. So if you think about simple trigonometry, 1mm of toe-in on a 10” Mini wheel is a much greater angle than 1mm toe-in on a set of 19” Boom-Shakka Drug Dealer alloys. It’s approximately double, in fact. So if the gauges profess to measure ONLY in mm’s and have no means to specify wheel size then they can only possibly be accurate for one size of rim. There is also the less significant issue of the width of the vehicle, which also affects the reading.
So I had little choice but to make my own gauges. I bought 4 cheap laser air pistol sights and mounted them on adjustable fittings on a 1” square box T frame that I welded together, taking great care to keep them straight and maintain accurate measurements. The units hang from the tyre and rest against the rim on three matched length allen bolts. The flags have a simple scale in centimetres. I calibrate these to read right angles. I used to use the back axle of my MKII Escort but now that’s gone and the Spider is in bits I’ve made a calibration rig, using some accurately measured cables.
The next part is the clever bit. Dunlop gauges have a protractor attached that you enter the wheel size into and it uses slide rule hocus-pocus to convert degree to mm for you. That old HP Anderson rig had a micro processor in it to do the same, some pretty funky mojo for the mid 80’s. I’ve got a look up table in an Excel spread sheet. I understand the trig but I’m not that good with Excel. I’m not a mathematical genius either. Fortunately I know a man who is! My old friend and colleague Steffan taught Excel and has a bucket full of Math smarts….and a bucket full of OCD too! I rather cruelly planted the seed of what I was trying to do in his mind one Friday night. He left with my detailed design plans (an old envelope with some triangles drawn on it) and at 4.45 AM on the Sunday he mailed me a working spread sheet. It even corrects for the width of the vehicle….Bless him!
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