In case you haven’t worked out from the lack of us being at races so far this year, 3galoo has not been progressing smoothly.
We’ve had numerous problems with the wheels and driveshafts at the rear of the car, and I thought it’d be a good idea to record what we’ve learnt so that other teams out there don’t repeat the same mistakes.
Without further ado:
We had the hubs and bearing spacers for the wheels machined by a local engineering shop. Having checked my drawings with one of the technical specialists at work, I had good confidence that the drawings were correct, and most importantly, correctly toleranced.
Sadly, the parts we received back were not up to scratch. The recesses within the hubs for the bearings were too small for us to fit the bearing, although not by an amount that we could measure with a set of calipers. It wasn’t until I sent the parts to my friendly technical specialist that we realised that they were less than 1/10th of a mm too small – this was enough to completely prevent us from fitting the bearing.
Foolishly, we wrecked one hub by trying to turn out the extra hundredths of a mm with our bench lathe which really was not up to the job – we went from not being able to fit the bearing, to it falling immediately back out.
This all put a massive damper on progress, and enthusiasm – as the first thing that we’ve not done ourselves, for it to go this poorly was a bit of an eye opener. We know for next time – make sure our tolerancing is documented properly, make sure the person doing the machining understands and agrees what is required of them, and if possible, get the bearings to them that you want to fit!
Our next problem was with the shaft that we bought to use as the rear axles. To our calipers, the shaft we bought was a constant 18mm diameter the whole way along, so we couldn’t understand why it would only fit part of the way through the holes that we needed it to, and why it slopped around at other points.
Again, a measurement with a higher resolution tool showed us that the shaft was definitely not a constant diameter along its entire length. We were advised to instead purchase Cold rolled bright mild steel, which has a much more consistent outer diameter, and has really reduced the amount of wobbliness at the back of the car.
From this we learnt how much difference an imperceptible change in diameter can make, and therefore the importance of using the right materials.
Tolerance Stack up
The design we came up with introduced a lot more ‘critical tolerances’ than it needed to, meaning that many dimensions on many parts needed to be machined perfectly for the design as a whole to work.
The redesign at the rear end used a shaft that we knew would fit well into the I/D of the bearing with no further work, reducing the number of ‘critical characteristics’ by two.
This approach of designing to minimise the ‘critical characteristics’ of any given part makes the entire system more robust, and easier to manufacture, which is a big win!
Hopefully this has been of some use to someone who’s looking at building their own car!