Ophitoxaemia
06-22-2012, 08:50 PM
I've autocrossed my Mark I car for over 12 years now (1400 runs and counting), and made extensive modifications to the suspension during that time. Increasing the spring rates has been part of that plan. After retrofitting the car with IRS in 2001, spring rates have gradually climbed to their current 750lb front and 1000lb rear.
However I've had a growing problem with inside right rear wheelspin, which worsened to the point the car would lift the right rear tire off the ground. Removing the rear bar and a low rebound setting on the shock did not help. The suspension does not bind.
I think my frame was bending enough to lift the tire off the ground. I called FFR to ask if I could have worn out my frame, and they did not think this was possible.
So I embarked this year on retrofitting a backbone. My backbone needs to be removable, as in my tiny garage, my only choice is to get the transmission out is through the top. And given the speed with which I go through clutches, that is fairly often.
We were able (with the incredible help of David Borden, his shop, and Dan Pellow) to get the lower welding done, with both forward diagonals welded directly to the motor mounts.
The rest of the tubing is relatively normal, but the top K section pieces are meant to be removable. My FEA analysis of the frame 12 years ago indicated that 90% of the torsional stiffness is lost without the K pieces.
However, there is some increase in bending rigidity, and so at the last autocross event, even though still missing key pieces, the car no longer lifts a tire, puts power down better, and has crisper turn-in. The ride is better on the freeway as well.
Who else has retrofitted a backbone into their early car, and what were your impressions?
However I've had a growing problem with inside right rear wheelspin, which worsened to the point the car would lift the right rear tire off the ground. Removing the rear bar and a low rebound setting on the shock did not help. The suspension does not bind.
I think my frame was bending enough to lift the tire off the ground. I called FFR to ask if I could have worn out my frame, and they did not think this was possible.
So I embarked this year on retrofitting a backbone. My backbone needs to be removable, as in my tiny garage, my only choice is to get the transmission out is through the top. And given the speed with which I go through clutches, that is fairly often.
We were able (with the incredible help of David Borden, his shop, and Dan Pellow) to get the lower welding done, with both forward diagonals welded directly to the motor mounts.
The rest of the tubing is relatively normal, but the top K section pieces are meant to be removable. My FEA analysis of the frame 12 years ago indicated that 90% of the torsional stiffness is lost without the K pieces.
However, there is some increase in bending rigidity, and so at the last autocross event, even though still missing key pieces, the car no longer lifts a tire, puts power down better, and has crisper turn-in. The ride is better on the freeway as well.
Who else has retrofitted a backbone into their early car, and what were your impressions?