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Old 08-18-2008, 10:29 AM   #1
FiveFlat
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Default how much HP does the rearend rob?

Just curious if anyone has dyno'd their carts. I don't have a dyno anywhere locally that I could take mine to. Not sure I want to go through the trouble either. But i was just curious. if you have a 24HP v-twin, what have you found at the rear wheels?
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Old 01-11-2016, 06:32 PM   #2
southseaisland
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Default Re: how much HP does the rearend rob?

No dyno info, but I have a 23 hp V-Twin and it runs fast enough that I don't care about dyno results. I just hold on to keep from falling out. 36 mph before the switch from 8" to 12" and high speed gears. I'll be over 40 mph and in a golf cart, that's fast enough for me
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Old 01-15-2016, 03:44 PM   #3
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Default Re: how much HP does the rearend rob?

CVT systems suck a lot of power and are difficult to get accurate dyno results from because of the movable pulleys creating different gear ratios. This is likely why you don't see people getting their carts dyno'd.

If I had to guess, I'd think that you will have about a 25%-30% hp/torque loss from the drivetrain. So if you are in fact at 23hp, your actual to the wheels would be in the 16-17 hp range.

You can have your engine dyno'd. You'll want to look for a small engine builder in your area and contact them. They may have a dyno, or might be able to refer you to one. It's probably not cheap. And it's not going to be convenient to pull your engine out and take it somewhere.

But here's the reality. Why do you need to know? Bragging rights? Unless you are doing some type of racing where 1-2 hp matters....then you really don't need to know. Yeah it's cool to think about, and yes if I had a dyno near me I might think about it, but I probably wouldn't do it.
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Old 01-15-2016, 05:14 PM   #4
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Default Re: how much HP does the rearend rob?

Andy the FYS place on 18 by the jvs just got a dyno for street bikes and atvs so I think he could do a cart
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Old 01-15-2016, 05:54 PM   #5
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Default Re: how much HP does the rearend rob?

Quote:
Originally Posted by slonomo View Post
If I had to guess, I'd think that you will have about a 25%-30% hp/torque loss from the drivetrain. So if you are in fact at 23hp, your actual to the wheels would be in the 16-17 hp range.
There is no loss of HP through a ratio change, except the power it takes to turn the reducer.

Loss/gain of torque can be calculated by knowing the ratio, then a slight loss from the friction of the reducer.

Example...
100ft/lb torque at the motor(input of reducer) equals...
1:1 ratio = 100ft/lb torque at output of reducer, minus friction loss
2:1 ratio = 200ft/lb torque at output of reducer, minus friction loss
1:2 ratio = 50ft/lb torque at output of reducer, minus friction loss
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Old 01-15-2016, 07:58 PM   #6
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Default Re: how much HP does the rearend rob?

Mostly you get friction loss with the belt in a cvt, small amount from the gears/axles, some from tires maybe more on a dyno where they are tied down. There is a dyno run video posted here of a HF v twin doing 18hp iirc on a gokart. Vegas carts has posted dyno info on his facebook page I think it is located. CVT show up poorly on a dyno, they have the most loss under the highest loads, and they can't show the gains they make from not having to shift/clutch like other trans do between gears. They can also apply peak power all the time if your clutches are tuned for that.

As a rule gears and bearings have pretty low losses compared to belts, but belts can serve more functions such as engagement and in a cvt they can change ratios ('gears') and they can be a 'fuse' in the drivetrain; it is very seldom something breaks with the belt drive because the belt will slip first, there is no shock load with a belt.
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Old 01-15-2016, 08:52 PM   #7
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Default Re: how much HP does the rearend rob?

Quote:
Originally Posted by 1989Marathon View Post
There is no loss of HP through a ratio change, except the power it takes to turn the reducer.

Loss/gain of torque can be calculated by knowing the ratio, then a slight loss from the friction of the reducer.

Example...
100ft/lb torque at the motor(input of reducer) equals...
1:1 ratio = 100ft/lb torque at output of reducer, minus friction loss
2:1 ratio = 200ft/lb torque at output of reducer, minus friction loss
1:2 ratio = 50ft/lb torque at output of reducer, minus friction loss
Yeah I guess I was trying to say the "friction from the drivetrain" will cause some power loss. My bad. I'm not an engineer ya know? LOL
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Old 01-15-2016, 08:52 PM   #8
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Default Re: how much HP does the rearend rob?

Quote:
Originally Posted by chevydmax6 View Post
Andy the FYS place on 18 by the jvs just got a dyno for street bikes and atvs so I think he could do a cart
Dyno's aren't going to be accurate with a CVT though, at least that's what I've read. Mostly because you can't hold it in gear. When they apply the load the CVT will try and downshift and compensate.
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Old 01-15-2016, 09:41 PM   #9
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Default Re: how much HP does the rearend rob?

Belts actually have the highest power efficiency, as high as 98%, which is higher than chains, gears, or shafts, but it sounds like the cvt system with the springs and weights added in must suck more out. One could appy a cvt gear locker like posted in another thread or perhaps just a temporary roller to limit the maxium gear to get a 1:1 or a constant ratio for dynoing.
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Old 01-15-2016, 09:46 PM   #10
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Default Re: how much HP does the rearend rob?

These gokart guys tried to dyno a 670cc twin with CVT, it did not yield any real results.

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