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Torque converter/CVT tuning/adjustment

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massmanute Alan Rockwood
Riverton, USA   USA
I am not a Kart owner, but I am interested in learning more about Karts. In particular I want to learn about torque converter/CVT tuning/adjustment. Let me run through a scenario with a question or twoat the end.

Suppose in a recreational application I have an engine that is nominally 13 horsepower at 3600 rpm, and the governor is set to limit the engine to 3600 rpm. Let's not get into engine enhancements right now. Initially I am running on a long level stretch, with the throttle set at full, but the governor is limiting the engine to 3600 rpm. Obviously, the gearing is not set for top speed on the straightaway in this case. Let's just accept that this is intentional.

Next, the Kart starts climbing a hill with moderate slope, and the governor starts opening up the engine to maintain 3600 rpm. So far nothing has happened with the torque converter, i.e. it is running in “high gear”. Let's assume high gear corresponds to a 1:1 ratio.

Eventually the hill gets steep enough so that the governor opens the engine up to full power at 3600 rpm. At this point the engine torque is 19 foot pounds, and since the gearing is 1:1 the load on the driven pulley is also 19 foot pounds. Here we are assuming no power loss in the torque converter, i.e. it is 100% efficient. Let's call the condition of the system at this point “Condition S”. Under condition S the engine is delivering maximum power.

I would like the torque converter to start downshifting once the system reaches condition S, i.e. at a 19 foot pound load on the driven pulley with the engine running at 3600 rpm. If the hill gets steeper the torque converter with downshift even more. I would like the torque converter to be adjusted so that the engine continues to run at maximum power (i.e. at 3600 rpm and with the throttle fully open) as the hill gets steeper. This means the engine must continually downshift to maintain the engine at full power. The downshifting of course would ultimately reach a limit once the hill gets too steep, perhaps at a ratio of 2:1 or so.

Question: can the torque converter be adjusted to meet these requirements? If so, how would one go about setting up the adjustment?

As a followup question, suppose the engine is not actually 13 horsepower but due to mufflers and other issues, including torque converter efficiency that is less than 100%, it is actually running at some lower (and unknown) horsepower figure. How does one adjust the torque converter in that case?

Thanks.

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MichaelR Avatar
MichaelR Michael Richard
Albany, GA., USA   USA
As I understand it you are wanting a variable torque converting torque converter. As I recall the CVT can only engage at the rpm it is set up to start applying power and once there it adjusts itself as rpm is applied causing an uptick in gearing. So you have a fixed engagement rpm, followed by a gearing variable that increases as rpm is increased. I don't think the CVT in any way adjusts according to the torque values fed to it, only by rpm. The only way I see getting to where you want to go is to have a way of varying the gearing of the torque converter manually which might could be done but at the cost of a whole lot of machining. Essentially you're asking a torque converter to act as a transmission that can be geared up or down as needed. The only gearing changes on a cyclekart are done manually by replacing the final drive sprocket with one giving a different gear ratio.
Not sure what you're working on, but most Karts on a fixed formula so I don't think this has been bandied about much. Interesting problem, but I see no economic way of accomplishing it. Imagination and knowledge accomplish great things though so press on. Just remember "Reality often astounds theory". Then agin I could be totally wrong and someone with more hands on head scratching will chime in. Nice topic though and well explained.

massmanute Alan Rockwood
Riverton, USA   USA
In reply to # 36458 by MichaelR As I understand it you are wanting a variable torque converting torque converter. As I recall the CVT can only engage at the rpm it is set up to start applying power and once there it adjusts itself as rpm is applied causing an uptick in gearing. So you have a fixed engagement rpm, followed by a gearing variable that increases as rpm is increased. I don't think the CVT in any way adjusts according to the torque values fed to it, only by rpm. The only way I see getting to where you want to go is to have a way of varying the gearing of the torque converter manually which might could be done but at the cost of a whole lot of machining. Essentially you're asking a torque converter to act as a transmission that can be geared up or down as needed. The only gearing changes on a cyclekart are done manually by replacing the final drive sprocket with one giving a different gear ratio.
Not sure what you're working on, but most Karts on a fixed formula so I don't think this has been bandied about much. Interesting problem, but I see no economic way of accomplishing it. Imagination and knowledge accomplish great things though so press on. Just remember "Reality often astounds theory". Then agin I could be totally wrong and someone with more hands on head scratching will chime in. Nice topic though and well explained.

"Reality often astounds theory", kind of reminds me of a quote from America's most original philosopher, Yogi Berra: "In theory there's no difference between theory and practice. In practice there is." Or what my dad used to say "Sure it works in practice, but the theory's no good."

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carChips Avatar
carChips Victor Harnish
Kelowna, BC, Canada   CAN
1933 MG Magnette
1973 Triumph Spitfire 1500 "Chip"
1989 GMC Sierra 1500 "Bush Truck"
Your typical Comet style TAV works on engine rpm, which will engage at 2200 rpm. The only down shifting you're going to get is when the engine slows down.



S'all for now!

JTremain Avatar
JTremain Justin Tremain
Oak HArbor, WA, USA   USA
1927 Nash Legion Special "Legion Special"
Vic, thank you for explaining that in a way simple mind can handle! I was lost there! Hahaha

In reply to # 36464 by carChips Your typical Comet style TAV works on engine rpm, which will engage at 2200 rpm. The only down shifting you're going to get is when the engine slows down.



"Magic Man"

1927 Nash Modified: Legion Special
"Build" Thread - https://www.cyclekartclub.com/phorum/read.php?2,26661

massmanute Alan Rockwood
Riverton, USA   USA
Tanks for the replies. If shifting only depends no RPM I wonder why the driven pulley is sometimes called a "torque sensitive pulley".

CmdBentaxle Avatar
CmdBentaxle Dave D
Federal Way, WA, USA   USA
1950 CycleKart Italian "1950 Ferrari 166 F2"
The CVT is completely reactive to the engines drop in rpm, it's indication of being loaded.
As with any automatic transmission, it's going to be just a bit behind, as far as adjusting for peak torque efficiency goes. Only the driver can anticipate a load not yet encoutered, and select a ratio slightly early to advantage.

But once again, other than for hypothetical rumination, why bother with it on something as knucklehead simple as a cyclekart ?

andrewroudny Avatar
andrewroudny andrew roudny
peterborough, ON, Canada   CAN
1939 CycleKart German "The Vulture"
I thought this would be the place to post a question that has been fermenting for a while:

Has anyone ever replaced the upper shaft of a Comet TAV (the one with the chain sprocket on it) for a longer shaft which could be used as a direct shaft drive? Anybody ever see that done?



"I never lose. I either win or learn.”
-Nelson Mandela

Antioch Bob M
Langley, BC, Canada   CAN
1929 CycleKart French "LR2 Special"
For the Comet Tav2’s you can adjust the springs (which are colour coded) which determine at what rpm the engagement happens... below is a listing of what the colours & the part numbers at least used to be.

For larger HP motors you would change to a differnt (in this case comet) cvt.... on some of those you can change both the springs & the roller weights.

I would recommend you chat with a company that handles the range of comet cvt’s to get the one that would work best for you...and the hp you intend to use....which in turn let’s you know what the available parts are...or are not.

Now after comet went through several other owners (hoffco, certifued parts, etc) parts availability (& possibly colour coding & numbers) may have changed as well.

Obviously for other cvt companies same process should apply.

011184A Orange 1500 rpm
011186A Pink 1800 rpm
011188A Std Blue 2200 rpm
011109A White 3100 rpm
216111A Flur. Yellow 3300 rpm
216115A Blue/Silver 3600 rpm



Rosengart LR2 Special

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