Why did the MIRA Lindley Special crash

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MIRA Lindley Special

by Charles Armstrong-Wilson

Having seen the hill that the organizers of the Border Bogie intended to use for the event, I badly wanted to take part. But not having the time to build my own bogie, I called in a favour and rather cheekily turned up with MIRA’s Lindley Special. It was originally built for the Goodwood Festival of Speed’s gravity challenge where it competed successfully in 2001 and 2002. It was now sitting in a shed gathering dust at the company’s Warwickshire HQ and they were keen to see it used. Thankfully it was well received at Denholm and everyone, very sportingly, seemed happy to see it run.

The original cost of building the car caused some interest with the estimates overheard in the crowd ranging from £30,000 to £85,000. In fact, the total cost of parts was around £500. However, the cost of designing it would have been much higher if the trainees who built the car didn’t have the MIRA’s resources available for free.

Predictably the car was very competitive and after putting up the fastest time on the first run, I was keen to improve, particularly as I felt I had been a bit conservative, braking twice. Unfortunately, two corners from the finish on the second descent the car swapped ends, clobbered the bank on the left of the track and went backwards into the bales on the right. The looks on the faces of the girls sitting behind that particular straw bale were a picture.

But why did it crash? Did I just go in too hot and run out of grip, or did I do something very different or make a big error that pitched me into the scenery. Fortunately, prior to the event I had managed to borrow a motorsport data logging device from Race Technology which recorded a mass of data from my runs that can be analysed at leisure. The DL1, data logger 1, uses GPS and accelerometers to determine the position of itself over time from which it can work out direction, speed, and other useful information. I also supplemented this with brake and steering position sensors.

Data was saved from two of the three runs; the first run sadly proved unreadable, but the time from the third was virtually identical and comparing it with the ill-fated second run is very revealing. My maximum speed on both runs was very similar, 42.32mph on the second run and 41.18mph on the third, achieved just before braking to squeeze through the gap at the junior start line. However, I was trying to brake less on the second run and only rubbed off one mile an hour compared to four mph on the third one. I suppose you could call it the gravity equivalent of a confidence lift.

However, the problems came at the second braking point where the course approached the area with bales and spectators. On the second run I was arriving at 39.28mph but actually started braking eight metres earlier than on the third run. Thanks to the brake position sensor we can look at this more closely. Initially I pressed the pedal for less than a third of its travel, before coming almost completely off the pedal and immediately pressing it to its full travel. Just 11 metres later I was in the bank. The question is why? Was I just going too fast?

According to the data on run three I was doing 38.99mph at the same point so the speeds prior to braking were similar. But I braked more gently so was I carrying too much speed into the corner? Again the data tells us that, because I braked earlier on run two, from that point on, my speed on run two was lower than on run three.

Excessive speed doesn’t seem to have been the problem after all. Perhaps then I did something stupid. Maybe I turned in while still on the brakes. Fortunately the steering position sensor allows us to examine that possibility. It shows that there was virtually no left lock at that point prior to winding armfuls of right lock to try and catch the spin.

One possibility haunted me. The Lindley Special has one brake pedal on the right. Could it be that, being a seasoned car driver, I braked then, as I turned in, started to feed in the throttle, which of course is not a throttle but a brake? The brake data, showing two consecutive applications, suggests this could be a possibility. Alternatively was the second application just damage limitation once the car had gone past the point of no return. Sadly I can’t remember as it was all a bit of a blur, but maybe the data can help us again. To test this we need to know whether the car started to spin during the first or second braking event.

We can see from the speed trace that when the brakes are applied the first time the car starts to decelerate. All being well, as you come off the brakes you expect the car to decelerate less. In this case, though, the deceleration continues unbroken. This suggests that the brakes were, by now, doing little to slow the car and at this point it was the wheels travelling sideways, not their usual direction of rotation, that was slowing us down. By this time it was clearly beyond catching, hence the reapplication of the brakes to stop it going too far backwards.

But that leaves the question, why did it spin? The only theory here comes from the amount of pedal travel used on the second and third runs. The Lindley Special brake system has cables operating drums on all four wheels. However, there is no real attempt in the system to compensate the forces and balance is achieved through adjusting each cable individually.

Preparing the car for the event we had moved the brake pedal to make more legroom and had to take up the slack on the cables and readjust them accordingly. On reflection it seems that good firm pedal application pulls all the brakes on together. However, poor adjustment may have meant that light pedal pressure brought the rears on before the fronts with disastrous consequences.

So it was the mechanics fault and not the driver’s. Sadly as I was the one doing both jobs, so I don’t get off the hook. However, it shows the value of a simple data logging system to look at gravity runs in fantastic detail. Even if you do manage to keep the vehicle out of the scenery, it can be a big help to going faster.

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