Clutchless shifting a dog gearbox.
Clutchless shifting a dog gearbox.
(OP)
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.
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.





RE: Clutchless shifting a dog gearbox.
Well I don’t have the precise answer for you, but cluchless upshifts are why people use dog engagement transmissions in the first place. Almost all racing motorcycles, and a lot of street ones upshift without the clutch. As I understand it, a downshift produces more stress on a transmission than an upshift. In an upshift the amount of torque multiplication is reduced, in a downshift the torque multiplication is increased, thus the downshift is more stressful.
More to the point, when you cut the spark, the engine is effectively in a no load state. In such a state, engine rpm can change dramatically over a tenth of a second.
Bye for now
RE: Clutchless shifting a dog gearbox.
There is a redline gear oil called HeavyWt Shockproof. It has some funky qualities. Its really slippery, viscous as all get out, refuses to release from metal surfaces, has a suspension of solids to absorb shock, a beautiful strawberry color, and smells to high heaven!
I'm on my third mix with their (redline's) 75W90NS (keeps some synchro action when cold) and have no trouble. I was reccommended this by several performance rally teams.
I'm sure that someone who knows more about dog-boxes will be along presently. Keep the shiny side up.
Nick
I love materials science!
RE: Clutchless shifting a dog gearbox.
I had the chance to take my crew chiefs much modified WRX out for a few laps at Las Vegas last year...handling acceptable...impressive power...gearbox sucks.
Rod
RE: Clutchless shifting a dog gearbox.
(It is however quite manageable if you're patient, and kind.)
RE: Clutchless shifting a dog gearbox.
A dog clutch gearbox will only go into gear when Engine rpm and box rpm are syncronised! there cant be a 2000 rpm difference.
There is no slip of Syncro rings trying to match speeds to allow gear engagement.
Ken
RE: Clutchless shifting a dog gearbox.
Regards
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RE: Clutchless shifting a dog gearbox.
Just as an example, F1 gear changes take less than 20ms - during this extremely short period of time, the engine will not slow down by any measurable amount of its own accord - even F1 engines have significant inertia, especially when spinning close to 20,000rpm! The engine is literally forced to match the next gear when the dogs engage. Greater rotating inertia and greater speed mis-match meang greater shock loads - just how much greater is the question I asked in the original post.
RE: Clutchless shifting a dog gearbox.
If you can find the Hotrod Mag article from 59/60 or so from their archives...that's where I got the idea back then.
Rod
RE: Clutchless shifting a dog gearbox.
For racing, anything goes!
Rod, Impressive times for back then! I have heard of those tricks being used on syncros to speed shifts. Liberty cut syncros rings a bell.
Ken
RE: Clutchless shifting a dog gearbox.
http://www.f1technical.net/articles/66
I believe that the engine speed is changed by the ECU cutting the fuel.
RE: Clutchless shifting a dog gearbox.
Just apply simple kinematics. You need to look at how many degrees engine rotation you can get from the flex you have in all the parts from the clutch disk to the tires, and assume it all acts like a torsion spring. Estimate the engine inertia. Calculate the resultant torque (T = I x angular accel) required to decelerate the engine speed from one gear to the next over that many degress.
RE: Clutchless shifting a dog gearbox.
RE: Clutchless shifting a dog gearbox.
There is very little compliance in the drive train between the motor and the clutch as it is a number of relatively short shafts connected via steel bevel gears and steel (Hardy Spicer type) universal joints. There is a lot of compliance after the clutch as the propeller is a very low efficiency (at idle speed) hydraulic coupling.
Forward or reverse is selected at idle. The reduction in the drive ratio certainly helps to a greater degree than the extra mass and surface area of the propeller as the inertia is 1/2 mass times velocity squared.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Clutchless shifting a dog gearbox.
I doubt that your troublesome WRX gearbox is a dog-type; much more likely it uses synchro rings.
RE: Clutchless shifting a dog gearbox.
(And I dont have troubles with my WRX trans (knock wood) after 100k mi. but it isn't the best feeling I've ever driven.)
RE: Clutchless shifting a dog gearbox.
The wide ratio box was 2.91 - 2.21 - 1.43 - 1.0
The close ratio box was 1.70 - 1.30 - 1.09 - 1.0
And there were rubber pieces in the clutch to absorb some shock in both clutches. I suppose the springs in automotive clutches would absorb some of the shift shock.
Have a great day.
Pancholin
RE: Clutchless shifting a dog gearbox.
Final drive ratio can also be an important factor.
RE: Clutchless shifting a dog gearbox.
Rod
RE: Clutchless shifting a dog gearbox.
This is why I mentioned “can” as we still do not know what it is being used on. Final drive ratios multiply all other ratios and the differences between them. The higher the numerical FD ratio is, the greater or "wider", all transmission gear ratios become. If my math is right...
Using only the close ratio Triumph gear ranges listed above as an example
Close Ratio = 1.70 - 1.30 - 1.09 - 1.0
3.0 FD = overall ratio of 5.1, 3.9, 3.27, 3.0 for a difference (upshifting) of 1.2, .63, .27
5.0 FD = overall ratio of 8.5, 6.5, 5.45, 5.0 for a difference (upshifting) of 2.0, 1.05, .5
These are not way-out ratios, 5.0 provides very nearly double the RPM difference between each gear than 3.0.
RE: Clutchless shifting a dog gearbox.
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