Seeking ideas for buggy ground speed governor

geekoftheweek

Joined Oct 6, 2013
1,201
One more quick drawing... I don't know what it is about this project, but it has my brain thinking.

pedal.jpg

Top to bottom...
Throttle cable from pedal pulls a lever that pivots in middle.
Lever pulls spring that pulls second lever (that spring could be pretty heavy to keep it from stretching unless needed)
Spring pulls second lever.
In an over speed situation the actuator would push the second lever back.
Second lever then pivots in the middle and connects to your governor spring or throttle linkage on engine (however you see fit to do it).
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
One more quick drawing... I don't know what it is about this project, but it has my brain thinking.

View attachment 211805

Top to bottom...
Throttle cable from pedal pulls a lever that pivots in middle.
Lever pulls spring that pulls second lever (that spring could be pretty heavy to keep it from stretching unless needed)
Spring pulls second lever.
In an over speed situation the actuator would push the second lever back.
Second lever then pivots in the middle and connects to your governor spring or throttle linkage on engine (however you see fit to do it).
I think I see what you are showing me, and maybe I see a simpler way to achieve it. If there is going to be a linear actuator anyway, Instead of having these pivots and linkages, the sheath of the throttle cable could be mounted to the linear actuator. After all, the pivots and linkages are only effecting the range of motion of the throttle lever given a movement of the cable core relative to the sheath. And we can effect that relative movement directly. Or am I mistaken in my interpretation?
 

geekoftheweek

Joined Oct 6, 2013
1,201
After all, the pivots and linkages are only effecting the range of motion of the throttle lever given a movement of the cable core relative to the sheath. And we can effect that relative movement directly.
That actually sounds incredibly simple. In my head it works, but would have to mock something up to prove it to myself. I think you have the answer.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
That actually sounds incredibly simple. In my head it works, but would have to mock something up to prove it to myself. I think you have the answer.
Yeah, I think so too. I can't see any reason not to try it. I'll report results; soon, or not, I don't know.
 

geekoftheweek

Joined Oct 6, 2013
1,201
If set up so home position is the normal mounting for the cable then extend to simulate letting off the pedal it would be safe in the sense of not running away. Beyond that a way to cut spark as a backup if say the speed doesn't change after a few seconds would be a thought. Permanent disable until reset by password.

The throttle stops sound good in theory, but could cause issues starting off on an incline or incline in general since you will need more torque to climb which would equate to pressing the gas a little. Still a good idea on flat ground.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
My concern is, can you make that fail-safe?
I suppose there is a gradient of "safe" failure conditions, with max-throttle runaway condition being at the far end. I do not see the possibility of that, with this setup. Pretty much as described here:
If set up so home position is the normal mounting for the cable then extend to simulate letting off the pedal it would be safe in the sense of not running away. Beyond that a way to cut spark as a backup if say the speed doesn't change after a few seconds would be a thought. Permanent disable until reset by password.

The throttle stops sound good in theory, but could cause issues starting off on an incline or incline in general since you will need more torque to climb which would equate to pressing the gas a little. Still a good idea on flat ground.
Thanks for the suggestion of permanent lockout. Like it.

That is the why of my previous solenoid suggestion
Sorry I just realized I never addressed your suggestion. The reason I don't like the idea of throttle stops is that engine RPM isn't directly tied to speed, due to the CVT. Imagine if you had a car with a block under the accelerator which limits you to 1000RPM. If trying to launch uphill, you might not be able to move at all. If driving down a long hill, you might be able to get all the way to 4th gear and 60mph.

As I said, I don't fully understand the CVT. It seems a simple enough device and to watch it operate, it seems very intuitive, but there is actually a lot of physics going on behind the scenes. I am surprised to find that just to reach 10-15mph running across the yard, the engine RPM is quite high. In researching ways to modify the CVT for better speed range, I was baffled by the web of effects that changing just one of many components of the device can have.

I would like to leave my options open to engine and CVT modifications in the future, without having to go back to the drawing board on my speed control each time. So it seems to me that treating the engine and drivetrain as a black box with an input and an output is best. Accelerator pedal > black box > ground speed. Solution can be implemented before or after the black box.
 

crutschow

Joined Mar 14, 2008
34,281
I am surprised to find that just to reach 10-15mph running across the yard, the engine RPM is quite high. In researching ways to modify the CVT for better speed range,
This may help you understand its operation.
Perhaps a reduction in spring tension would allow it to reach a higher ground speed at a lower engine speed.
 
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Thread Starter

strantor

Joined Oct 3, 2010
6,782
This may help you understand its operation.
Perhaps a reduction in spring tension would allow it to reach a higher ground speed at a lower engine speed.
Thank you. As are many things, these CVTs are like an onion; many layers. The article is top layer, maybe 2nd layer.

Yes, a lighter spring would do as you say. As would increasing drive pulley weights. The weights and the spring tension are a balancing act (balancing on what, I don't know) to get the desired RPM:reduction ratio. And the spring tension of the driven pulley affects that ratio as well, as does belt width and belt starting/static tension. This is what I mean; many connected parameters, each effecting the other in one way or another. I plan to play with one parameter at a time until I fully grasp the situation. These things are not typically talked about or published by manufacturers. Reasonable people go to the CVT manufacturer and let their applications Engineers spec a CVT package with all the appropriate pulleys, springs, weights, sheave travel, dimensions, belts, etc. which meets the desired performance criteria. I am not one of those people.

I have already modified the drive pulley which resulted in a very appreciable gain. The drive pulley still had ~1/4" before the plates touched when the internal stop bottomed out. I machined the internal stop back until the plates could come all the way together and touch. This small change resulted in a 60% gain in top speed.

Before:
20200628_154616.jpg20200628_154742.jpg
Modification:
20200628_162949.jpg20200628_164052.jpg
After:
20200628_165023.jpg20200628_165556.jpg

I suspect the driven pulley hard stop is preventing it from fully opening in a similar way. I intend to study that pulley and determine whether a similar modification is possible/feasible.

I have also purchased a set of heavier weights for the drive pulley. I have not tested their effects yet. I suspect top speed will not be effected, but that I won't need the engine fully wound up any more to get across the yard at 15mph.
 

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shortbus

Joined Sep 30, 2009
10,045
Comet Clutches used to have some good information on how these worked and how to modify and troubleshoot them. Many of the snowmobile makers and racers used Comet clutches. But I couldn't find the information now, Comet has changed hands a few times since I was interested in the use of them.

I have also purchased a set of heavier weights for the drive pulley.
If I remember correctly, heavier wheights will make the engagement RPM level higher. Lighter springs let it engage at lower RPM. Some of the old information had charts on how to fine tune engagement and disengagement RPM's and the combinations that worked.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Comet Clutches used to have some good information on how these worked and how to modify and troubleshoot them. Many of the snowmobile makers and racers used Comet clutches. But I couldn't find the information now, Comet has changed hands a few times since I was interested in the use of them.



If I remember correctly, heavier wheights will make the engagement RPM level higher. Lighter springs let it engage at lower RPM. Some of the old information had charts on how to fine tune engagement and disengagement RPM's and the combinations that worked.
Thanks for the advice about Comet. I will look into it. I did find an incredibly detailed resource on these CVTs, and I'm pleased to have confirmation that I wasn't being a big baby in insisting that these aren't as simple as they seem. 155 pages of detailed explanation there. Most of it is over my head and I would like to find that comet material which might be more on my level.

I put the heavier weights in today. No improvement in top speed (as expected) but I think the whole rpm:ratio curve shifted downward. It now engages just barely above idle, which is a very welcome improvement. No more "lurch" on takeoff, feels pretty much like driving an automatic transmission car.

Also did some unsanctioned machining modifications to the driven pulley. The pulley faces had "steps" worn into them so I resurfaced them to the proper 12deg angle. No improvement. Moved the hard stop back about 1/4" as I had done on the drive pulley, so now it opens 1/4 wider than it used to. Still no improvement. Seems the belt circumference is the limiting factor, so I machined a 1/8" "washer" to separate the pulley halves so the belt rests deeper in the pulley at rest, adding to static belt circumference. Then moved the engine forward to take up new slack. 22% improvement in top speed from yesterday.

Per the manufacturer (CVTech) this CVT has a low-range ratio of 3.0:1 and a high range ratio of 0.43:1. When I got this, the ratio was nowhere near that. Much worse. Now with the modifications it has a low range of 3.49:1 and a high range of .408:1
 

geekoftheweek

Joined Oct 6, 2013
1,201
I never considered multiple forward speeds... kind of throws the tracking the pulley position out the window.

The clutch concept sounds simple enough and looking at them you wouldn't think there is a whole lot going on, but there are a lot of variables other than RPMs to account for. Even simpler looking yet was the John Deere setup I mentioned earlier with the sliding sheave. I can't remember anymore how exactly it was done, but I spent several hours adjusting it to try to make it not gear down as much under a heavy load. Just getting in to tall grass would make it slow a bit which actually helped a lot when mowing with it. I would start somewhat slow and gain speed as you moved along in fourth gear and on flat ground it would stay the same., but a hill would cause it to gear down a little. Third gear wasn't as bad... it would pretty much immediately settle into place in first and second gears. It was a snow plowing beast because of it in the long run. Instead of spinning out when it got rough it would gear down, bite in, and keep pushing.

Edited... unlike the newer stuff out there where the pulley ratios are the only speed setting and a simple forward / reverse gearbox the old John Deere was a separate unit before the actual transmission so there was four forward gears there and along with the vari-speed it kind of allowed for infinite adjustment to some extent.
 
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geekoftheweek

Joined Oct 6, 2013
1,201
Actually were you talking the pulley ratios or is there a high and low speed in the gearbox also? Just realized I may have misunderstood. I thought some of those vehicles have gearboxes with multiple forward gears and with the pulleys figured in it allows for a wide range of posibilities.
 

geekoftheweek

Joined Oct 6, 2013
1,201
Out in the garage tinkering and thought maybe I should add I'm not trying to say the John Deere setup was superior in any way to the one gear setup with variable ratio pulleys. It all depends on the application. Snowmobiles, golf carts, and such have used only the variable ratio pulley concept for decades because it just simply works.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
No multiple speeds; not yet anyway. By low range i meant drive pulley fully open, driven pulley fully closed, and so on.
 

geekoftheweek

Joined Oct 6, 2013
1,201
I had a feeling after I wrote all that I was wrong thinking that way. Pretty wide spread for the ratios... comparable to an automotive 3 speed in the end.
 

shortbus

Joined Sep 30, 2009
10,045
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