Clutchless shifting a dog gearbox. 2

[daveylynx]

We are currently thinking of incorporating a full throttle gear shift system to our racing transmission. The system detects when a shift is about to be made and cuts the spark to the engine during the shift. The idea is that the clutch isn't used during up-shifts and shift time can be reduced to less than 100ms, giving obvious advantages.

What I would like to know is this: how much extra stress is placed on the transmission when shifting without the clutch? If you shift with the clutch, the drive is obviously taken up slowly enough to minimise shock loadings as the dog faces meet. However, if the clutch isn't used, the gearbox has to mechanically force the engine to lose maybe 2000rpm to match the next gear - in an instant! This will obviously put shock loadings through the entire transmission, but are the forces involved likely to be more than the forces created by the engine torque?

I appreciate that if the rotating inertia of the engine is greater and the speed mis-match between gears is greater, the shock loading will be higher, but can anyone guestimate which might place the greatest forces on the transmission - the engine torque of 200ft/lbs or shock loads due to engine being forced to reduce speed as the dogs engage. Remember that as the dogs engage, the engine isn't producing power but is spinning perhaps 2000rpm too fast to match the next gear.

 

[evelrod]

Using the TR ratios at 3.0 and 5.0 FD and, using 9000 rpm as a shift point and 23" diameter tires (not an unreasonable combination)...

Speeds in 1st, 2nd, 3rd and, 4th would be---

With 3.0 FD...~121, ~158, ~188, ~205 mph and,
With 5.0 FD...~~72, ~95, ~113, ~123 mph respectively.

The change in rpm between shifts (drop in rpm on upshifts) would be the same with both FD's...2118 rpm from 1st to 2nd, 1454 rpm from 2nd to 3rd and, 743 rpm from 3rd to top.

The 'close ratio' JKD dog box I used had ratios of 1.75, 1.36, 1.19 and 1.0 to one ratios---my 'clubman' gearset is 2.573, 1.722, 1.255 and 1.0...my FD's are 3.55, 3.78, 3.9, 4.12 and 4.44 to one and I have used all of them from time to time, depending on the track. Never noticed ANY difference in the shifts in either setup simply by changing the FD. It's primarily the drop (change) in rpm that determines the shock loading at each shift.

Rod