I have a related question that has been bugging me all weekend with regards to tuning engagement for peak torque coming out of corners. Hopefully, this isn’t a dumb question.
Every forum post on the subject of clutch engagement tuning seems to recommend tuning for just before peak torque. In theory, this makes sense: transfer peak torque to the sprocket/driver to accelerate the kart. But why does tuning for peak torque really matter coming out of a corner if the higher rpm engagement is going to result in slippage between the clutch and drum? Slippage is loss of torque: the clutch drum is connected to the axle via the chain/driver, so the engine can’t possibly instantly accelerate the kart; i.e. the clutch drum and clutch shoes will turn at different rates, resulting in slippage.
Since slippage means loss of transferred torque, then couldn’t it be possible that the actual torque transferred, after accounting for the temporary slippage, is similar to the torque the engine puts out at a lower rpm without slippage? So, why not just have the clutch engagement set to the lowest rpm corner? This would mean less wear on the clutch, would it not?
My question really only applies to whether tuning matters for corners. With regards to starting from a stand-still, I get why tuning would probably be important: the engine can likely accelerate the kart from a stand-still faster, even with the slippage, starting at peak torque (e.g. 3400rpm) vs idle (e.g. 1600rpm). But all the races my daughter competes in use rolling starts.
Am I thinking about this in the wrong way or missing something? Does tuning clutch engagement for corner exitreally matter?
Clutch tuning only matters if the corner speed is low enough to disengage the clutch. If your clutch never becomes disengaged then clutch tuning is not important. Side note: We could have a debate on amount of clutch slip while locked up, but thats for another day…
You want the drivetrain to transfer the most torque from the engine to the rear axle during acceleration. Yes, some energy is lost to heat while the clutch is slipping, but the engine is also putting out more torque. The amount of gain by locking up at peak torque is much higher than the amount lost through the extra slippage. In other words, you want the clutch engagement to transfer maximum torque to the axle. The easiest way to do that is to make sure the engine is at peak torque.
If Al Nunley was still with us, he would tell you to test it. Put a scale between the nose of your kart and a garage wall. Try different clutch settings and see which one produces the highest number on the scale. That will be the setting that propels you forward the fastest.
Conventional wisdom/experience will save you some trouble and tell you that peak torque of the engine is that point.
So, I guess my question then is really whether a slipping clutch can still transfer the expected peak torque. For example, my kid’s engine (yellow slide) makes 10lb-ft @ 3300rpm. If the output side (drum/sprocket) is rotating at 2800rpm (e.g. slow corner), when the clutch engages at 3300rpm, there will be major slippage, at least initially. But even with the clutch shoes slipping, should I still expect approximately 10lb-ft to be transferred to the sprocket? i.e. can a slipping clutch at 3300rpm still transmit more torque than a fully locked clutch at 2800rpm? This would suggest the torque lost through slipping should be relatively minimal.
I wonder if this could be tested on a dyno by accelerating a known mass (flywheel) from 2800rpm to 3300rpm, once using a fully locked clutch and once using a clutch set to engage at 3300pm. I think there would be too many variables to do this on track.
Edit: dug through some physics forums and the consensus is that yes, a slipping clutch can continue to transfer more torque (to a point), which answers my question. It therefore does seem to make sense to tune the clutch to engage at peak torque even though it will slip quite a bit.
It can be tested with a bathroom scales, see Derek’s post above. Try different engagement RPM (while being mindful of heat of course) and see what force it exerts on the scales.
I’m gonna make up numbers, but hopefully prove my point.
Lets assume an engine produces 10 ft lbs at peak torque and 8 ft lbs at 1000 rpms less than that.
At peak torque, the clutch is going to be slipping more and five up 5% torque loss. Therefore, we are only transferring 9.5 ft-lbs to the rear axle. When compared to the 8 ft lbs, we are still accelerating faster.
Now, obviously, I skewed these numbers to prove my point, but losses in the clutch USUALLY do not outweigh the gain in torque rise.
Can it be tested on a dyno? Absolutely! Did we test it (probably wasting way too much time) to find our clutch settings at Ghost Racing? You better believe it.
Now, a few things that were mentioned on the other karting forums that are important here, as well:
The 206 has a very flat curve, so all gains and losses are going to be very small.
The long slide engines (green, red, etc) have to be tuned to the stop watch due to their unique short curve, no top end power, and light weight allowing faster acceleration.
As Mr Carlson said, sometimes there is benefit in setting your clutch to never disengage once you are on the track. This removes the “dip” that happens during clutch engagement.
Ultimately, the stop watch is your ultimate decider. Also, dont be afraid to change/test track to track. This is often lost in todays racing world, but trying things like that and coming up with your own conclusions is part of the fun to me.
Overall, not really.
There’s a dispute between many of whether outboard adds extra strain on the crank due to the pulling force being on the end rather the middle of the crank. Personally I think that force is pretty negligible, still there, but negligible as long as you take care of your equipment. @fatboy1dh probably knows more.
Overall, I’d just slap the clutch on either way you need, make sure the shoes are orientated to your preferred direction and drive your heart out.
Right now I have it inboard but after two races I DNF due to a thrown chain I am going to get a sprocket guard. I have not got the guard yet but the sprocket carrier may be to close the the frame. I might have to flip the clutch because I like engine mounted where it is.
I’ve played around with my Hilliard Flame, I’ve tested the following:
2 Black/ 2 White
4 Black (Just cause I was curious)
2 Black/ 2 Green
2 Black/ 2 Green with clutch weights
Nothing I did seem to make a huge/ noticable different honestly.
Currently I’m 2 black/ 2 Green with no weights, thats what I’m testing the next time I race.
What I’m really courious about is how it would feel running the scrocket inboard. As my chassis is 2 stroke with the 3rd axle cassette, so unless I want to go ape shit with a hammer to the seat post, I have no option but to run it outboard which I have always done.
Might also make sure if you have an engine bump stop to use it to stop the engine from getting pushed back from bumps and scrapes and also tightening that engine mount down should help as long as you make sure the chain tension is right.
Dean, let us know how the testing goes . I am running 2 black/2 white with no weights in my clutch right now. I am thinking about trying 2 black/ 2 yellow with no weights.
