Torque, Horse Power and Acceleration (2014)

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comnoz said:
Generally speaking short rods will increase the torque in the lower to middle RPM ranges if you have a small fast burning chamber like a Norton.

A longer rod increases the torque at higher RPM because the ideal rod to crankshaft angle comes a few degrees later so the fuel has more time to burn and raise the cylinder pressure.

The differences are very small. Jim
Click to expand...

+1

As stated earlier, for any given engine configuration (rod to stroke ratio) peak torque is at peak volumetric efficiency - the rule applies across the board.
 
Dances with Shrapnel said:
comnoz said:
Generally speaking short rods will increase the torque in the lower to middle RPM ranges if you have a small fast burning chamber like a Norton.

A longer rod increases the torque at higher RPM because the ideal rod to crankshaft angle comes a few degrees later so the fuel has more time to burn and raise the cylinder pressure.

The differences are very small. Jim
Click to expand...

+1

As stated earlier, for any given engine configuration (rod to stroke ratio) peak torque is at peak volumetric efficiency - the rule applies across the board.
Click to expand...
+2
 
So, what I am gathering here is that the rod that puts the least pressure on the thrust side of the piston would be the best. The JS longer rods would do that while fattening up the torque in mid to upper range.
 
If ya just look at the mechanical and friction aspects of r/s ratio might miss its most significant performance feature on how it aids engine breathing per rpm and piston speed. Long rods delay decent acceleration from TDC more than shorter rod, so guess which glups it in sooner. Yet... Longer rod delay of piston jerk down eases loads some so helps raise rpm threshold to some degree over shorter rod. Longer rods are less tolerant of octane detonation onset too. So what's this do to simple rule of thumbs?

Torque, Horse Power and Acceleration (2014)
 
motorson said:
So, what I am gathering here is that the rod that puts the least pressure on the thrust side of the piston would be the best. The JS longer rods would do that while fattening up the torque in mid to upper range.
Click to expand...

If what you are looking for is torque in the high rpm ranges -then yes -a long rod does help.

Unfortunately there is a tradeoff in durability when you use a longer rod without raising the deck height -the thrust will be moved from the piston skirt to the ring land area. Jim
 
rod stroke and rotation demo for clarity of physics involved.
[video]http://www.youtube.com/watch?v=qYyVRU5Z0so[/video]
 
Just a mind tesaser similar to 'optimal' r/s ratio which usually gets dictated by the space the engine must fit in a mass produced craft and the rpm it must tolerate, then fudge intake and fuel and timing to make the best with that ratio. As comnoz and most references and dyno testing show r/s ratio is one of the more minor things to alter for performance reasons. Factory Norton's are in the generally considered good ratio zone, 1.8-ish, give or take a point, or even 2.

Here's a few evenings worth of generalized reasons behind r/s ratio selection
https://www.google.com/#q=optimal%20bor ... ke%20ratio
 
I suggest there is a compromise involved. If the rods are longer so is the rock-over period around TDC, also the angularity suits higher revs better. The strength of the commando engine and controlling its potential to explode involves keeping the revs below a certain level and concentrating on improving pulling power. I still think Jim Schmidt's long rods and light pistons are great.
I've watched Kenny Cummins' videos which show his Seeley with the top end motor racing - most impressive, but expensive. I suppose it is still the same old story 'how fast can you afford to go ?', however I'm amazed how much I've got out of my 850 motor with virtually nil modification. When I first built the bike back in 1978, I looked inside the motor and thought it was rubbish. I think differently about it now.
 
Yep r/s ratio is a compromise mostly dictated by manufacture fitting reasons. I've had 2 Nortons that required dragster planting to snap throttle off idle or would run out from under, which was horrifying to me at first until reflexes/grip developed to tame/tolerate it and then power just got stronger into red zone so only pilot control prevented over rev crank jump rope let down, like from Peel's stuck throttle event. I'm likely the only one to experience the 2nd level of torque onset of over size hogged out ports once past the red zone, ugh. There is a different skill set of cycle behavior involved when too much power most the time than tire can take and reason for moderns to have traction and wheelie 'puter controls. Thank goodness P!! and Peel both had exquisite, electric rheostat like one to one throttle to tire spin rpm setting, ie: crack throttle exactly so much above present power acceleration tire grip for an instant or 3 of tire spin and get set for the predicable hook back up jerk forward straight ahead or snap back from the out of line steering change the short skips allow for true point and aim out of there thrills.

Some day will put Peel's some what ruined 11,000 rpm Combat engine back together with the stock items and few mods of stock items and send to comnoz or Big D to dyno then figure out what to do with it that's is not Commando related, long silly chopper or trike or even a boat. Going by Peel's pure performance against my SV650 and inline 4's from 2000-2004 era implies over 70 hp and almost as much torque. Peel had lighened flywheel and small port head and was only so so Combat until the 2-1-megaphone dialed in with 12 ga slug, that woke her up to shocking levels to me and some surprised angery Sneering strangers, just like many here do me on my claims. Can't imagine if I knew what was doing and degree'd cam in to match.
 
While we are here, some mention was made earlier of slipper clutches.
Which was not really correctly applied.

A slipper clutch in this context simply dis-engages the drive to the back wheel - to prevent the back wheel locking up going into corners when going back down through the gears.

http://en.wikipedia.org/wiki/Slipper_clutch

It would not be of any benefit to torquey, or gutless, engines under acceleration.
Its not like a centrifugal clutch, at all...
 
Rohan said:
While we are here, some mention was made earlier of slipper clutches.
Which was not really correctly applied.

A slipper clutch in this context simply dis-engages the drive to the back wheel - to prevent the back wheel locking up going into corners when going back down through the gears.

http://en.wikipedia.org/wiki/Slipper_clutch

It would not be of any benefit to torquey, or gutless, engines under acceleration.
Its not like a centrifugal clutch, at all...
Click to expand...

In your Wiki reference, are we sure the John Gregory/TC Christianson Hogslayer slipper clutch is the same as the modern slipper clutches you are referring to. I always thought they were different.
 
A good question - there are zero references, so we can't go and have a look. !
Can anyone ask TC - or know of some details somewhere.

Drag racers these days do have somewhat adjustable clutches, so it seems there may be 2 different concepts here ?
Can't say this is common motorcycle technology though...
 
Dances with Shrapnel said:
In your Wiki reference, are we sure the John Gregory/TC Christianson Hogslayer slipper clutch is the same as the modern slipper clutches you are referring to. I always thought they were different.
Click to expand...

A drag race slipper (bob weight) and modern road (torque activated cam set or ball ramp pressure plate lifter) race type slipper unit have next to nothing in common.

Edit.
Road or track.
Torque, Horse Power and Acceleration (2014)


Drag race.
Adjustable for lock up point.

Torque, Horse Power and Acceleration (2014)
 
Rohan said:
While we are here, some mention was made earlier of slipper clutches.
Which was not really correctly applied.

A slipper clutch in this context simply dis-engages the drive to the back wheel - to prevent the back wheel locking up going into corners when going back down through the gears.

http://en.wikipedia.org/wiki/Slipper_clutch

It would not be of any benefit to torquey, or gutless, engines under acceleration.
Its not like a centrifugal clutch, at all...
Click to expand...

As I understand it, the slipper clutch is intended to allow the engine to brake without locking the back wheel - so that you can change down without matching the revs. - Not much to do with acceleration. I've sometimes seen kids at ride days braking for corners and changing down two gears with the clutch pulled in. In the old days it would have meant instant crash as the rear tyre lost contact with the road. It is certainly relevant to a torquey commando engine with a standard gearbox. If you don't match the revs the motor opposes revving up when the clutch engages, this means the rear wheel brakes. If you are cranked over ..... ? If the slipper clutch releases on back-off or down-change - you don't get the sudden retardation of the back wheel. You don't usually need them on two strokes because the flywheel effect is much less, as is the comp. ratio. The other clutch on this page is a locker - the device used on early Ford sidevalve V8s, to stop slip under power. You should not need a locking clutch with a CR gearbox on a commando. If you do, you probably bend mainshafts.
 
The slipper clutch mentioned back here though is the centrifugal version that the drag racers use.
This makes them like postie bikes - no clutch lever - twist the throttle and go.
Was hoping to get some comments from someone that has fitted and used one.
And if they are useable on the street.
Or are they full throttle short life use ONLY ?
 
Slipper clutch helps when a cycle and pilot is not good enough to be on power all the way *into*, *through* and *out of* a turn so they let off throttle so much the engine drag causes rear tire drag to hop and slip. Its more a problem with hi rpm engines that are geared back down to get enough torque to the ground speed, so like trying to push start in lowest gear the engine drag has too much advantage over tire traction, ESPECIALLY if trail braking which lightens the rear planting as well as the front. Be careful and wise testing your power band on leans.
 
@Time Warp - pretty much my understanding of slipper clutches. Pretty sure the HogSlayer Norton drag bike was designed to meter out the torque on the throttle, not off the throttle.

Another case of terminology and context.
 
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