Setting suspension geometry using string.

It’s well known that suspension alignment is crucial for handling and safety, but how much can be checked at home? The main one that we can usually adjust is the front toe angle, but even though there might not be a simple adjustment for rear toe, camber, castor etc. it is worth checking them soon after buying a vehicle, just to see if it is straight.

What’s needed is some sort of datum point from which to take measurements to the wheels. The simplest way of doing this is to use two pieces of string, the thinner the better, one each side of the car at hub height and parallel to each other. With the car sat centrally between them we can measure from the string to the front and rear edges of each wheel and find out at what angle they are pointing in or out.

This method takes practice and initially may take time to set up and get consistent results, especially when you mess thing s up by tripping over the string. But with perseverance it becomes a very quick and surprisingly accurate operation.

ACCURACY VS PRECISION

It’s very easy to spend ages taking detailed readings, but beware. It’s easy to have accidentally moved the string, thus highlighting the difference between accuracy and precision. For example if you measure a block of metal that is in fact 50mm wide with a cheap digital vernier that reads 47.9836mm, it is very precise but not very accurate. By contrast if you use an old tape measure it may read 5cm, not very precise but completely accurate.

With the string method we are looking for accuracy, and consistent results.

Park the car on flat and even ground, this is very important; you can’t do it on grass.

Attach a piece of string to a suitable stand that won’t move when you pull the string tight, such as a breezeblock, at each end. The string must be at the same height, give or take a couple of millimetres, as the wheel hub centres and extend beyond each end of the car by a foot or so. The string should be as close to the car as possible without snagging on any bodywork etc. Ideally we would get this parallel to the centre line of the car, but as that can be difficult to define, just make it parallel to the sills to start with. It can be adjusted in a moment. Next put the other string on the other side in the same manner.

Now measure the distance between the strings at the front and the rear of the car to check they are parallel and adjust the string stands evenly until the same measurement is achieved at both ends.

Bear in mind that most cars have different track widths front and rear, so don’t expect the same hub to string distance front and rear.

When taking a measurement hold the ruler just underneath the string and take a reading from the edge of the string closest the car. With practice, and fine string, it is quite possible to get precision to 0.5mm, more than enough for our purposes.

You should now have two parallel lines at hub height, so to measure toe angle you can measure at a right angle from the string to the front and rear edges of the wheel or tyre, but beware, there are pitfalls here. Toe angle is very rarely quoted as an angle, that would be far too simple, so instead we work with a measurement which is the difference in the distances between the front and rear edges of the wheel/tyre. Now, to make it more complicated there are three systems in use, usual European method is to measure to the edge of the wheel rim (which is often bent and suffering kerb damage making the measurement difficult), the Japanese method is to use the step before the rim (more sensible as less prone to damage but depends on wheel design as how easy it is to get ruler on), the Americans go for the mid point of the tyre side wall (how you judge where the middle is can be tricky). So it is important to check the manufacturers’ specification before measuring your car.

Now, as we won’t be measuring between the wheels, but instead from the string to the wheel, we are effectively looking at half the car. So what we need to do is combine the reading from both sides.

For example on the car pictured here, the measurements from the string to the front left wheel are 47mm at the front of the wheel rim and 46mm at the rear edge. So the front edge is 1mm further inboard than the rear.

Now onto the right front wheel and I have 51mm at the front edge and 49mm at the back. So it is 2mm inboard at the front.

That means that I have a total 3mm toe in. And the car is slightly closer to the string on the left and I also haven’t got the steering set quite as straight as I thought, but the latter doesn’t affect the results.

Measurements at the rear are 47mm on the left and 50mm on the right. In both cases it is the same at the front and rear edges. So the rear wheels are completely parallel.

All this makes the assumption that the wheels are not bent or dented, so it is useful to check the run out first.

Jack and properly support the axle up until the tyre is just off the ground, then place a suitable datum, a breeze block perhaps, next to the wheel and clamp a ruler onto it so that its edge is just touching the wheel rim. Next, spin the wheel and see if the gap opens up or the ruler gets pushed back, anything more than a fraction of a mm is less than ideal, although its amazing how much wobbly wheel factor some people live with. If the wheel is bent then it is still possible to check the geometry, but the wheel has to be set so that the front and rear edges where we take measurements from have the same offset, but its obviously best to get it fixed first.

Another potential problem is that as the car moves forward or backwards, the suspension moves on the bushes. This means that you get a different toe angle reading depending on whether you reversed the car or went in forwards before measuring it. The best bet is to take a set of measurements then drive the car in the other direction for a few feet and take another set. This usefully highlights any potential bush or bearing troubles too.

Normally it’s the readings taken when the car has moved forward that are more important, after all that’s the direction you normally drive in, but the driven axle has to cope with being pushed forward when accelerating and being pushed backwards when braking. So both readings must be within specification.

Next up we can check to see if the front and rear axles are centred properly. The first step is to check the wheelbase on both sides and compare this to the manufacturers’ specification, bearing in mind that some cars, such as the Renault 4, have a different wheelbase on each side.

Having set the strings up to be parallel measure to the hub centres, and also to suitable datum points on the body/chassis such as the seam on the sill. If the car has been badly repaired after a shunt then one axle could be offset causing the car to crab. Should the results show the car is out of alignment, it might be a case of rebuilding the suspension or possibly the body may need straightening. However, before embarking on any drastic action based on the string and bricks method its worth double-checking at a professional alignment facility.

The next step is to use another piece of string and a bolt to test camber, most cars have the tyre inboard of the wheel arch, or at least they should. So by attaching a simple plumb bob to the top centre of the wheel arch with some sticky tape we can measure from the string to the top and bottom edges of the wheel and see how far it leans in or out. Now, this is absolutely dependent on the car being on flat ground, so check the ground with a long sprit level first. Also the tyres need to be inflated correctly and be matched so the wheels on each side are the same height off the ground, check this by measuring from the ground to the wheel.

Now, camber measurements are usually quoted as angles, so we need to convert our mm measurements in to degrees. This involves a small amount of school trigonometry, what we have is a triangle; if the top measurement (O) was 31mm and the bottom one was 21mm then there is 10mm difference, if the top and bottom measuring points on the wheel are 300mm apart then the triangle has a long side (H) of 300mm and a short side of 1mm, so the camber angle (a) is the inverse sine of 10/300 = 1.9 degrees.

Checking your car’s steering and suspension geometry can be quite simple, once you get the hang of it, and although the accuracy isn’t as fine as with laser alignment kit it does give a very useful idea of which way your wheels are pointing, can help highlight suspension faults and it’s a technique that is used by some of the top motor racing teams to this day.

Practice this method until you become proficient and you’ll have a simple and easy to set up system ready for use almost anywhere.



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