speed vs acceleration... not satisfied

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Tony01

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Nov 28, 2012
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So in my quest to get the highest speed with the simplest transmission, I've found the maximum speeds and capabilities of different types of transmissions for my 3.5hp Briggs powered Schwinn. But none of them gives me both 50mph and good acceleration.

CVT gives under 40 with awesome acceleration, MT at 8.02 gives a reliable 40+ cruise with embarrassing but bearable acceleration.


Setup: Final reduction: 44t sprocket with 12t or 15t drive.

MaxTorque 3.2" to 7" pulley half inch A-section v-belt

(12t) 8.02:1. 47mph @ 4900rpm. slow acceleration under 100% hookup at 20mph.
(15t) 6.42:1. 51mph @ 4200rpm, after half a mile. Acceleration almost non-existent, ran many red lights.

CVT comet knockoff .Listed ratios are w/o CVT. CVT goes 2.7:1 to .9:1. Then 10t to 23t, then to final reduction (triple overall reduction)

CVT will not let my engine rev past ~900rpm above spring rating, no matter what I do:

(12t, 8.43:1, 2200rpm springs) 32mph @ 3150rpm, .9:1
(12t, 8.43:1, 3300rpm springs) 37mph @ 4100rpm, 1:1
(15t, 6.75:1, 3300rpm springs) 39mph @ 4100rpm, 1.2:1 (testing incomplete, will continue in the morning)

..............................


The MT when hooked up 100% above 22mph, gives the best top speed and all-around performance. CVT pulls hard to 30+ then levels off. The Briggs power band is 3000 to 5100 from my testing. Is there any way to double the CVT rpm range? Make the MT hookup better?

I'm strongly considering selling the CVT and using the funds to construct a 2nd jackshaft and two-speed autoshift transmission. I'm happy with the MT from 25 to 45mph and could use a lower, freewheeling gear to get that 'kick'

Any easy suggestions on what to do next?
 

racie35

Active Member
Nov 17, 2012
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When you say cvt I think you're referring to a TAV...if so ,on the driven unit the spring can be moved to another hole to either delay the opening of it(to top end) or speed it up.
That and careful sprocket choices should get you closer to what you wanna do.
Your hp may be limiting you at 3.5, the tav uses up some of it. The cvt on scooters is smaller and robs less on those peanut engines. Still not really wonders though.
 

Tony01

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Nov 28, 2012
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Yea, tav.

Hmm.. I will check out the scooter cvts. Good suggestion. Looks like they use a cent clutch to hookup to a cvt which is always engaged. Hmm...

Switching to lowest driven tension from middle netted a 3or 4 mph gain from 30mph. This told me that middle position never went into overdrive. It just shifts too fast....hmmm

LIGHTBULB! Changing the driver springs is about engagement and tuning the cvt for the beginning of the ideal powerband... Could the driven be used to increase useable powerband?

Then there is cvt slip. Still strongly considering fabricating a 2-speed though. Those MT clutches when hooked up have very close to zero slip or none at all.

There is a thread on OMB about 2-speed vs torqueconverter, post 4 details a drag race between the two and post 6 details slippage. here


So I guess I'm basically stuck huh... If I had a bigger motor we wouldn't be having this conversation... But the challenge is exciting.
 

Davezilla

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Mar 15, 2014
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Sounds like you got it right with your idea.. Use the drive side to set the engagement rpm, then use the driven side to control the shift rate. You can also get different rate springs to even further fine tune it.

I had a Comet converter on a go kart when I was a lot younger and it worked great until I started messing with the engine to make it faster and have more power, then it wasn't so great because it didn't match the engine's power curve. After complaining about it to the right person I was shown how these work and that other springs were available if it still wasn't right. After I got the right spring for the driven side and in the right position the acceleration was very nice and so was top speed.

I do think that the moped/scooter type variators are a better design for the drive side tho since the weights are more readily available and there are even more options for the rear spring on the driven side either by being adjustable instead of just having 3 positions to set it at, and there are more srping rates available if a spring change is needed.

Then once everything goes the way it should, then you can change the rear sprocket either to a larger one for better acceleration or a smaller one for more top speed, and if going with a smaller or larger sprocket changes the acceleration too much, it's always possible to slow down or speed up the cvt's shift rate to compensate as long as your final ratio is reasonable.
 

Tony01

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Nov 28, 2012
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Thanks for your post Dave. I'm going to keep messing with it but not for long.. Took it out today with 6.75:1 and it was not any more spectacular than at 8.43:1, though the acceleration starting from 3300rpm is still taking me getting used to. It does take off the line real quick.

Numbers:
(15t, 6.75:1, 3300rpm springs) 41mph @ 4100rpm, 1:1

It appeared to be at least direct drive, if not overdrive.

My gear ratio calculator shows I should be going 46mph at this rpm with direct CVT drive, so if this really was direct drive, then I have 13% slip. If this was overdrive, then 21% slip. Either way, the slippage it killing it for me!

I'm going to take your advice and put the driven on the heavier side, to try to increase my CVT's use of the powerband. If it doesn't work.. I will get sell it and use the funds to build up a 2-speed auto.. Out on this ride today I found myself missing the (above 20mph) performance of the single speed.
 

Tony01

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Nov 28, 2012
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@Trey: was unaware of that. Guess I must be way outside of the 6-sigma range here :d

MB is also my DD and only transportation other than bus.

40mph is about the upper limit for 3.5hp and CVT. Tried heavier preloads, it gives higher rpm but doesn't shift into high gear.
 

Davezilla

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Mar 15, 2014
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Main thing to keep in consideration with only 3.5hp is that you can have decent speed or decent acceleration, but not both... the CVT will help some but won't be a complete cure all