cvt

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lenny
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cvt

Post by lenny » Tue Sep 10, 2019 4:19 am

Trying to get my 67 year old brain around this, anyone out there to put me right? I understand the general operation of cvt, but have a question.
When moving on flat road, constant 3000 rpm and constant speed, 1/4 throttle, drive and driven pulleys in equilibrium.
Now increase to 1/2 throttle. My question is, at that point, what happens to the drive, is it the equivalent of a chain drive with a direct connection of engine to rear wheel or do the pulleys suddenly change in some way when the increased power is applied?

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Has been maxi tours
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Re: cvt

Post by Has been maxi tours » Tue Sep 10, 2019 5:02 am

The CVT is always trying to get the belt to top of the pulley friont pulley to get best ratio ie higher gear. What holds it back is the spring in rear pulley .More throttle more power will over come the spring belt get to top of pulley you go faster .Once its as far as it can go revs continue to rise to go faster .On you tube there's videos shows how belt moves in front and rear pulleys .

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Re: cvt

Post by MrGrumpy » Tue Sep 10, 2019 5:32 am

In general, a engine with a CVT likes to sit at a fairly constant rpm - say 5000-5500 rpm. At those sort of revs opening the throttle might increase revs a bit, but the main result will be from the CVT pulleys giving you a higher ratio, at least once acceleration has been achieved and the forces and loads through the CVT have balanced out. For low revs like 3000, twisting the throttle will probably cause revs to rise as it wants to get to its sweetspot, but the CVT ratio will also change. When speed has increased so that the CVT is in its highest ratio, then there will be a more direct connection between revs and speed. The CVT is always balancing load / drag from the road and power from the engine - if you meet a gradient on the road without the altering the throttle, the CVT will lower its ratio.

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Re: cvt

Post by lenny » Tue Sep 10, 2019 5:58 am

MrGrumpy said
'at least once acceleration has been achieved'
This is the specific part of cvt operation I'm trying to get straight in my head.

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Re: cvt

Post by Has been maxi tours » Tue Sep 10, 2019 7:01 am

Take belt case off put it on centre stand rev it up you'll see it working .I see your on a Silver wing ecu does a cross check between revs and speed at about 60 mph belt light will come on if it doesn't match up .So nothing to worry about really .

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Re: cvt

Post by MrGrumpy » Tue Sep 10, 2019 8:03 am

lenny wrote:
Tue Sep 10, 2019 5:58 am
MrGrumpy said
'at least once acceleration has been achieved'
This is the specific part of cvt operation I'm trying to get straight in my head.
What normally happens when you twist the throttle is that the revs rise a bit, but then stay constant whilst the CVT does its work.

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Re: cvt

Post by StephenC » Tue Sep 10, 2019 2:04 pm

Which is why, at least how it seems to me, that traditional motorcycle high revving engines are wrong for scooters with a CVT. What we need is peak torque low down so that the box can move up through the "ratios" without eating fuel. Instead what we have now are engines pulling 5k revs the moment you open the throttle.

I'd like to see smaller capacity versions of Honda's 750 in the Integra, etc. Or at least engines with that design concept anyway.

Is that possible?
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Re: cvt

Post by SH125Paul » Tue Sep 10, 2019 2:23 pm

With the increse in revs at the crank, the variator weights are thrown further up the ramps... Centrifugal (dont start on the centrapetal...) force...
The squeezes the front pully pushing the belt up... Which by virtue the belt is a fixed length, must open the rear pully against the conti spring... Dropping the belt down...
Think push bike - front chain wheel size v's rear sprocket size...
Search you tube, also take a look at what Dr Pulley rollers do...

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Re: cvt

Post by StephenC » Tue Sep 10, 2019 2:44 pm

XMax400Paul wrote:
Tue Sep 10, 2019 2:23 pm
With the increse in revs at the crank, the variator weights are thrown further up the ramps... Centrifugal (dont start on the centrapetal...) force...
The squeezes the front pully pushing the belt up... Which by virtue the belt is a fixed length, must open the rear pully against the conti spring... Dropping the belt down...
Think push bike - front chain wheel size v's rear sprocket size...
Search you tube, also take a look at what Dr Pulley rollers do...
Yeah, what he said. Meant to write that myself but got sidetracked by executing some timey-wimey stuff instead
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roadster
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Re: cvt

Post by roadster » Wed Sep 11, 2019 4:18 am

Here is the direct answer to your question on the assumption that your scenario is actually achievable for your particular vehicle. But note that in many cases it would not be possible to achieve an equilibrium state under the conditions you stipulate.
When you increase the throttle the engine develops more torque this torque creates more tension in the drive belt and how this affects the pulleys depends on the weights on the front pulley which are trying to force it to a larger diameter versus the tension in the belt which is doing the opposite and the counter spring in the back pulley which, in combination with the wormdrive which grips the belt more firmly if there is any tendency for the belt to slip, is also trying to achieve a bigger effective diameter itself.
So you will see that the answer is not simple but it can be summarized by saying that the design parameters for any given scooter are set to match the performance capability of the engine. In general the front pulley effective diameter will become smaller thus allowing the engine to achieve higher rpm provided road speed is well below the scooters practical maximum speed but if the scooter is already travelling at somewhere in the top 25% of its speed range ( unlikely at 3000 rpm ) then the variator is going to stay at maximum effective diameter.
Realistically most scooters in standard form will go up to an almost fixed rpm on any wide throttle opening and then when the front pulley reaches maximum effective diameter the rpm will begin to increase in proportion to road speed. When the throttle is closed rpm will reduce in proportion to road speed and the variator won't start to change ratio until a relatively lower road speed if at all.
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