According to the Atom manual “More Castor increases the amount of steering and aggression, less Castor gives less steering but also makes the car easier to drive and can prevent traction rolling.”

So it seems that setting the right Castor angle is very important, but how easy is that?  Well in my opinion not very easy at all! In fact with the standard plastic front hubs, you had to dismantle the front of the car, put washers under the front beam pivot to get the ride height you want for a given tyre diameter and then adjust a grub screw to set the angle of rake on the beam to get the Castor angle where you want it.  However, the process of setting the beam angle raised or lowered the axle relative to the chassis, which changed the ride height!  So, fiddle and adjust some more, till you get what you want, maybe having to dismantle the front beam pivot again to add or remove washers. And how is the actual Castor angle set?  Adjust the length of a grub screw, which according to the manual is measured from where to where exactly?  Is it the total length protruding out of the holder or maybe minus a small bit on the end which has bitten into the chassis, or is it the distance between beam and chassis which can’t be measured with a ruler and even with a digital Vernier caliper, take 3 readings and they are all different due to the slope. I showed the picture in the manual to a few people who all said something different about what to measure.

All in all, not exactly a simple or even precise procedure and when racing at YeovilRC impossible to achieve between heats when you’ve only got 10 minutes to marshal a race and then deal with battery, tyre and additive changes and maybe then check and replace anything that has fell off, come loose or been broken in the last heat!  So, with these constraints I found myself getting an increased Castor angle as the evening went on and using the nice grub screw to regain my ride height as the front tyres wore down as I couldn’t be bothered with the tedium and time involved of adjusting the Castor angle properly. The car itself therefore drove more aggressively as time passed which with my now ‘limited’ abilities from 40 years ago, means what I need is something more sedate and predictable and not make the shortage of rear grip even more noticeable when the front bites. I always had the option to shout something like “I’m not ready yet” while reassembling the front beam, but the thought of the boos, hisses and other abuse aimed in my direction while they’re all stood on the rostrum waiting to start a race didn’t really appeal, so aggressive steering it is 🙂

So, how do you accurately measure the Castor angle you’ve set from the manual’s chart of 5° to 8° and in my case aiming for something just less than the kit setting of 7°  Simple, use the expensive setup system you bought that can measure Camber, Toe & Castor, well no, as the axle isn’t in line with the kingpin so that won’t work! Use a protractor, you know the plastic thing that’s in your kid’s pencil case except it isn’t exactly particularly accurate for this purpose, as the diameter of the kingpin makes it anywhere around 6ish, not my idea of precise!

Simple answer, go get a digital protractor! Certainly accurate on a flat and level surface and even on a pit table good enough to know it hasn’t moved much since the last heat if the ride height needed changing. Easy!

When I started this little project, I didn’t have a digital protractor and only got one to confirm my findings having used what I already had.  So my thinking was that what I wanted to calculate was an angle in a triangle, i.e. the triangle formed from the chassis on one side, the slope of the beam on another other side and the vertical side being the chassis to beam pivot.  Oh look! one of these angles is already a right-angle and I remembered that someone at school had told me something about right-angled triangles, didn’t they?  So having dug out the schoolboy trigonometry text book, which strangely has renamed itself to Google over the years, I remembered searched how to make the calculation to find out the angles.

In order to find the angles, I needed to know at least 2 of the lengths of the sides of my triangle, so I started measuring and established after much fiddling that the distance from bottom of the chassis to the top of the beam where it pivots is 10.4mm plus any rake spacer or washers. The distance along the chassis from the centre of the pivot to the centre of the kingpin was 26.3mm.  I was of course using my Vernier to get this level of accuracy and armed with this info plus my Sine/Tangent tables I could now create my equation to find the Castor angle.  Well it seems that my old maths exercise book was also absent, so I had to get a new one, which was called Excel 😉

By deducting the height of the beam off the ground from the total height of the pivot I now had the true height of the vertical side of my triangle. Measuring the height of the beam isn’t easy so decided to measure from the ground to the top of the kingpin and then realised that if I also included the thickness of the camber strap and the button head screw, nothing would have to be dismantled to take this measurement except removing the wheels so that the chassis was flat on the ground. (for ground, read setup board). This total height is therefore the measurement you enter into the calculation as well as the amount of rake spacing added.  The teacher then said “the square of the hypotenuse is equal to the sum of the squares of the other two sides”, so now I know the distance along the slope of the beam too and the 3rd side of my triangle. So the rest was easy once I’d found the sine and tangent functions in Excel.

So how do you measure the total kingpin height?  Well you could do it this way, but again not exactly accurate.

Using the depth gauge on your Vernier calliper is a lot more accurate.

And there you have your reading!

This is all much easier with the new alloy front hubs as the ride height doesn’t affect the Castor angle so these can now be set independently, which makes life much easier and quicker. In my case I have a fixed 6.6° Castor angle and I can forget about it until I decide I want more or less aggressive steering.

Without a digital protractor it is possible to work out the Castor Angle and the 2nd part of my Excel sheet has a lookup table to shows the heights required to get the 4 set angles in the manual.  Just enter the amount of rake spacing you have under the front beam pivot and it will tell you what the total height of the kingpin, camber strap and button head should be for all the angles from 5° to 8°.

If you want a copy of the Excel sheet you can download it here: https://yeovilrc.org/wp-content/uploads/2018/01/Atom_Castor.xlsx

Assuming you’re not asleep by now, read my post on the new alloy hubs, coming soon!