Car Setup

In the run-up to Formula Student UK 2014, Head Design Judge Neill Anderson will reveal the major issues teams need to address if they want to succeed in the competition's Design Event. The tenth article in the series sees Neill looking at the all important process of car setup.

As usual with these articles this is aimed at first year teams and tries to steer you from the usual beginner mistakes and also tries to stop you panicking about things which are really not that difficult, time consuming or important. We can be business-like and call it “prioritising” if you wish.

By now you might have actually assembled your beast and hopefully ironed out the basic sub systems functionality. Time is very tight, maybe even non-existent and there is probably little left in the bank either. But before you can do even a shakedown run of your creation you do need to check again that it is safe and whilst doing that (spanner checking assemblies, wiring and plumbing checks etc.) you may as well take a little bit extra time to square it up so the wheels (and therefore tyres) are at the “correct” angles.

Despite popular myth you don’t need expensive equipment, a flat floor or lots of time to do a good enough job (for this sort of car in this sort of event). I would estimate if you have not done this before you need a space big enough to walk/kneel around the car, 4 axle stands, some string, a spirit level and two straight edges (or lengths of tube that by eye are straight), one that is approx. the tyre diameter in length and one that is about equal to the wider track width of the car. And a reel of cotton thread. Two lengths (2m each) of 15mm copper plumbing pipe is helpful.

You will also need something to weight the car with and to level the floor area locally under the tyres: vinyl floor tiles are great for this or even glossy magazines which can also serve as “turntables” under the front wheels. As your creations are going to be quite light then for about £30 you should be able to but 4 bathroom scales each of which will accommodate about 150kg.

I am assuming that you don’t have lots of time and so I suggest the following is order of priority: all measurements and adjustments need to be finalised for the vehicle complete with “average” mass driver and healthy complement of fluids on board. If time really runs out use the same order but do all measurements by eye: it’s amazing how good the human eye is for seeing lack of symmetry especially. The tyre supplier will provide some guidance on preferred angles and pressures and you should of course have understood those recommendations at the design stage…

It makes sense to quickly scheme up a “Set Up Sheet”, examples abound on the Internet but should be graphically obvious as to which is the front of the car. When undertaking his first set of adjustments you may have time to note the adjustment sensitivity, i.e. one full turn of the tie rod changes the toe by x mins or mm. Note this on the Set Up Sheet.

How to do these is explained after the list:

1. When you assembled the car you should have set all the various adjustable parts equal side to side, and to your design specification.
2. Level the floor under the tyre contact patches
3. Ensure tyres are fitted correctly and symmetrically to the right ends of the car!
4. Pump the tyres to something close to a hot (warm?) running pressure (note that this will likely be a few psi higher than the recommended cold starting pressures).
5. Check each wheel/tyre for run out: rarely are wheels truly straight. Mark two diametrically opposite point in the rim of equal runout: these will be the positions you measure from.
6. Ballast car or fit average mass driver
7. Get ride height somewhere near and ensure that you have legal suspension travel in bump and rebound and are unlikely to bottom out severely anywhere when running. It should be equal left to right and usually the rear will be higher than the front for a variety of reasons (more complaint rear for traction, greater rear mass bias, aero, need to have a “pointy” front feel on turn in etc.)
8. Set the front camber angles and then the rears. There is a massive difference in requirements between radials and cross plys. Many camber adjustment designs also influence toe, so do camber first.
9. Set the castor angle at the front: if you don’t have much time at least get it equal side to side.
10. Set the toe for each wheel, i.e. angle in plan view.
11. Check camber again.
12. Set corner weights accepting that if they are a long way off and using soft springs then you may have changed the ride height a lot and thus need to go round the car again from Step 8.
13. Bolt anything loose back up tight. Every time, otherwise inevitably you forget to go back and torque it up.

How to do it

Almost everything you adjust will have associated influence: usually as you adjust castor you will also affect bumpsteer etc. Bear this in mind if you make significant changes.

1. It is probable that to adjust some things you will need to jack the car up. For repeatable consistency you need the car to settle back to the same position each time. So you need to set all dampers full soft, and disconnect one anti roll bar link at each end to allow the car to be as supple and independent as possible.
2. Roll the car into the working area, draw or mark around the tyre locations and move the car away. Using the level, shim the marked areas until the floor is level side to side. It is useful to mark on the floor the thickness of shims required to level the floor.
   1. If you prefer you can add the shims on top of the scales and do all the adjustments on the scales. You gain some clearance but lose the ability to roll the car back and forwards.
3. Note tyre details
4. Note tyre pressures
5. You will do camber first so set the two marks to be vertical
6. Add the driver or bags of sand!
7. Measuring to the floor, or when really uneven, to a stretched piece of string stretched between the tyre/shim (ground) interfaces, set the ride height to your design specification. A ruler is adequate for this.
   1. If you don’t know the ride height then it makes sense to aim for about 30mm, as the Rules require an inch of travel and no bottoming out
   2. In almost all cases this will be done by adjusting the spring platform height to compress the spring.
   3. If you have some progressive connection (push or pull rods) then you really need to set the pushrod length initially to get the rocker in the design position and then adjust the spring collars to suit and then go round the loop again to keep the rocker in the right position at the correct ride height.
   4. Be aware that springs are notoriously poor quality and that that the end coils are “dead”, i.e. contribute little to the overall support. They are also quite variable despite what is marked on them, variances of over 5% are quite normal even in “good” springs.
8. Camber next, and whilst a nice digital gauge is convenient you can make do with a spirit level or even a plumb line. And whilst it improves accuracy to measure over the greatest distance the tyre will bulge at its base and so it is usually easier to measure to the rim, where we earlier marked the two equal runout points.
   1. Calculate a simple angular camber to be a lateral measurement from true vertical across the rim diameter, e.g. 1 deg on a 13” rim is about 6 mm. hold the level against the straight edge touching the base rim point and when the level is vertical measure the gap to the upper rim point. Or vice versa if positive camber exists.
   2. Cross ply tyres work best almost vertical, radials need a lot more initial static negative camber generally, more so again at the front. 3 deg negative is not uncommon.
9. Castor is hard to measure directly but you have the advantage that you probably know the relative locations of a feature on the front uprights, e.g. caliper mounting bolts. You can therefore measure directly the angles of these features in side view and relate back to the kingpin axis. Simpler still, and if in a hurry is to lay a straight edge across these bolts, one each side and simply adjust until they appear equal when viewed together across the car! A simple eyeball from above of the upper and lower balljoints will tell if you have any castor at all.
   1. If you have proper castor gauges they work by measuring the camber angle for a given steer angle at the rim. So you need some way to measure steer angle which can be lines chalked on the floor using a protractor! Glossy magazines under the front tyres will allow them to steer without moving the car.
10. Now for the time consuming part. If you don’t have the fancy laser gear this is where it takes you a bit longer. What you are going to do is to create a perfectly square rectangle around the car from which you can measure in to the rims (the marked points) so roll the car ¼ of a wheel turn to get the marks horizontal. The simplicity of this method is that the string rectangle is referenced to the wheels and not to the chassis and thus allows for any asymmetry in link lengths etc. In other words it squares up the tyres’ direction relative to the other tyres, which is after all the important point.
    1. The reference rectangle will be constructed from stretched string, cotton thread in a contrasting colour to the workshop floor is best and it needs to be fairly non stretch. This needs to be at rim centre height.
    2. I find it easiest to get two 2m lengths of 15mm copper plumber’s pipe, and about 25mm in from each end and exactly the same distance apart (WFront and WRear in the sketch) on each pipe, I create a groove using the cutting tool designed to cut such pipe, the ones with the little roller knife wheel. Make a slight groove on the pipes without cutting through the tube wall, enough to locate our thread.
    3. Position an axle stand about two foot forwards and one foot outboard of each wheel front wheel and the same in reverse at the rear. We can then rest these pipes across the two rear stands and across the two front stands. They serve to keep the two threads equidistant from each other which saves a lot of time.
    4. Stretch the thread front to rear each side and ensure they are taught.
    5. Now measure from the thread each side to a fixed point on the wheel hub area, at centre height, e.g. to the flat face at the centre bore, to a brake disc face or similar. Note the measurement to about half a millimetre (this is quite possible with a steel rule and stretched thread).
    6. Get this dimension equal left to right at the front. LF and RF in the sketch.
    7. Don’t let anyone trip over the thread or kick the axle stands
    8. Do the same at the rear, the rear distance to the hub will most likely not be the same as at the front. LR and RR in the sketch.
    9. Go round in afew iterations until the car is central in the string box to within ½ millimetre measured to the wheel centres
    10. Don’t let anyone trip over the thread or kick the axle stands
    11. Now simply measure the distance between the thread and the forward rim (marked point) and the rearward mark for each wheel. Write these down as you go for each wheel, toe out is when the forward reading is smaller than the rearward reading for a wheel.
    12. As a staring point perhaps aim for about 15 mins (15 mins is about 1.5 mm on 13” rim, quite convenient to remember) toe out per wheel for a pointy car. Bear in mind that any compliance that causes a wheel to wobble between some to in and some toe out will feel awful so where you have flexibility have enough static toe to avoid it going past centre etc.
    13. Adjusting the toe will sometimes cause the car to ”re-align” within the box, especially if you need to jack it up. Simply reset the box as per steps (e) to (j) again and remeasure. Note the sensitivity of the adjusters.
11. If you made significant adjustments you need to check camber again.
    1. This may then entail another round of setting the toe!
12. Corner weighting is again not mysterious or hard, just time consuming and easy to get frustrated by! Essentially you are looking to balance the car’s mass across the 4 contact patches, the basic accuracy of which is dictated by the car’s design mass distribution. However, at the expense of “perfect” ride height, you can adjust the individual corner weights by raising or lowering a corner: raise the ride height at a corner to increase the load carried by that tyre.
    1. With pushrod/pullrod linkages you want to do this on the link to keep the rocker at the desired geometrical position (most have significant rising rate in their geometry). For outboard suspension raise or lower the spring collar to change the spring preload.
    2. You should be able to do this whilst the car is on its wheels: if you need to jack it up then sometime sit is hard to be consistent and repeatable because of the dampers and any friction in suspension pivots.
    3. From a driver confidence perspective it is usual to prioritise minimising the difference between the two fronts but if you have limited rear droop travel and a weak LSD then it may be beneficial, performance wise, to prioritise the rears. Depending on the basic mass distribution you probably won’t get much better than 5kg difference across an axle.
    4. If you changed the ride height much, and depending on your camber gain characteristics you may need to go round again from Step 8.
13. Reconnect the anti-roll bar links, ensuring they are adjusted to fit without any preload: they may well now be different left to right.
    1. Check everything is tight.
    2. Reset the dampers to where they were/should be. If in doubt err on the soft side as it is easier to feel excessive softness than the other way round.
    3. Check travel to the bumpstops: there should be some! Cheap bumpstops tend to be pretty solid and with minimal progression, proper ones can be used effectively as they are very progressive (rising rate). Very simply the more conical the bumpstop the better. Note the internal shape of the spring cap also influences the rate of the bumpstop.
    4. Measure the fitted length of springs, pushrods, adjuster links etc. and record them along with the car and driver mass and distributions. These will come in very handy should you take the car apart at any time.

If you really want to check things thoroughly then you can do a bumpsteer check, i.e. measure change in toe at each wheel at every ride height increment. You can do this inside the string box but you will need to remove the springs and have a jack or blocks of wood etc. under the car to support it.

It’s worthwhile doing if you have time but only if you have some method/adjustment available to correct it: usually this means shimming the vertical position of the steering rack or toe link pivot.

Wishing you all success at Silverstone in 2014 and I will see you there.

Any feedback is genuinely welcomed.