Costly mistakes at Cotswold…

Often it seems that stories of failure are as educational as stories of success…

The SPC (Super Production Cup) came to the Cotswold track yesterday. Last year, I had a great time, winning the Mini class with my Xevo Triumph, and I have been looking forward to this race meeting all summer.

Weather was a lot better than forecast, with only a couple of showers disrupting proceedings.

This year I was competing in Mini Formula, and my trusty M-05L has been setting good times at club meetings all season, so I was pretty confident of a good result.  That confidence turned out to be misplaced…

A brief shower before qualifying had dried from the track before my first race, but the grass and painted kerbs were still damp. I clipped the kerb through the fast kink at turn 4, and before I knew it I was facing the wrong way – the first of several spins and trips through the grass as I tried to keep the car under control while the rear tyres dried off. This left me almost a lap down on the leader, but I managed to find a rhythm and set the quickest laps of the race on my way to finishing second in the heat and third in the round.

Round 2 started better, and with the sun having moved over the track the shadows near race control were gone and the grip was much better. The back end of the car was a bit flighty, especially when changing direction, and a spin into the grass on my last lap cost me nine seconds. I was lucky to set the fastest time of the day so far, by just one second from Jamie Carpenter.

Round 3 followed a rain shower. The racing line was 80% dry, but the area around race control was still damp, and my car was very tricky indeed. The runners with Tamiya S-grips in their pitbox were a lot faster than my Sweep-slick-shod car, and I realised that if the finals were held in these conditions, I would be well off the pace. So, with the aim of increasing rear grip, I made the bold decision of rebuilding the car to “M” wheelbase with the HPI Civic shell to put more weight over the rear tyres. I also put some new Sweep 33s on the front, as the previous set were very worn on the shoulders and I wondered whether they might be biting too much on turn-in.

Round 4 was dry again. But the car was diabolical. The back end was extremely unstable, to the extent that it would do 360s on the apex for no apparent reason. I pulled off after a couple of laps. Fortunately for me, the track was a little slower than Round 2 (probably because of the earlier shower), and my TQ remained.

Before the first leg of the final, I decided to change back to the old front tyres – assuming that the new boots were the problem. I was wrong – the car remained undriveable, and after two laps I pulled off again.

I made an extra close examination of the car before the second final, and noticed that the pin in one of the front CVD driveshafts had come loose, and was only secured in one side of the axle. This is the kind of thing that can cause handling problems – so I repaired it. I also spotted a couple of small tears in the rear tyre sidewalls – so I re-glued them.

Confident that I must have solved the problem this time, I lined up on the grid for the second final. I got through the first two corners OK, but when I got to the tight hairpin at turn three, the rear end kicked out, and I couldn’t carry the speed through to the exit, losing the place to Jamie. By the start of the second lap, I was losing more places as the back end jumped around, and after a few trips into the grass I finished back in 8th place. The car was still basically undriveable. All my changes had been for nothing.

After the race meeting finished, I decided to have a few practice runs to see if I could get to the bottom of the problem.

First of all, I lowered the rear ride-height and disconnected the anti-roll bars. It made no difference.

Then the answer came to me. The 3Racing gear diff was the problem. I had a suspicion that it was getting looser over time, which would explain the oversteer problems as the inside wheel “diffed out” during cornering, causing the outer wheel to slow down, turn tighter and swing out the rear end. Changing to medium wheelbase would only make it worse – because although re-distributing the weight to the rear should improve rear grip, taking weight off the front would only make the diffing-out worse, because there is even less load on the inside tyre during cornering.

So, I rebuilt the car with a TA03 diff, which turned out to be quite a lot tighter than the 3Racing diff with 5K oil which hadn’t been re-filled for many, many months. The handling was much improved, although there was still some diffing-out, and the car had very poor traction out of the corners. A switch back to the “L” wheelbase and the Mito shell gave me back the Tamiya Mini I had become used to – great traction out of the corners, and a pleasure to drive. A change to Sweep 40s on the front end seemed to be another small improvement, and a newer set of Sweep 25s on the rear was a small improvement again. You still needed to be careful from a standing start before the rear tyres got cleaned off by the track, but after that I could run lap after lap at a good pace.

So what made the 3Racing diff loosen up? Well, I have yet to strip it down, but I did see some thick oil around one of the bearings when I took it out of the car, so it may have leaked a little. It’s also true that the standard o-rings used to seal the outdrives are very hard and a very tight fit. These have loosened up a lot with running, to the extent that the diff probably needs a thicker oil again to get the same feel that it had when first built.

Congrats to Jamie on the overall win. If the SPC is at Cotswold again next year, I will be back with a bag full off diff oil!

Dave

3 thoughts on “Costly mistakes at Cotswold…

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