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Jaggs
06-29-2010, 07:12 AM
Hey guys
We are going to use CVT for the first time for Baja SAE India competition. Last yr we used a manual tranny. We are going to use a CVT from CVTech. The engine that is used here is Lombardini Kohler
Rated Power: 8 Kw @4400rpm
Max. Torque @3000 rpm
At what value of rpm we should be aiming at engaging the CVT? And how to go about selecting the gear ratios for a 2-speed reduction gearbox?:confused:

Buggy Builder
06-29-2010, 12:34 PM
C'mon guys are you engineering students or not?

Draw a picture of your system from engine shaft to to rear wheels.

Derive an equation that relates engine rpm to car velocity

Make an educated guess about you maximum car speed. Base this on performance of last years car.

Use equation to calculate unknown values for gear box.

Jaggs
07-01-2010, 08:03 AM
ok i got these calculations( I derived the gear ratios) but still I need to find out what my engagement range will be? For d specs of the engine mentioned above will 2000-2200 rpm range be appropriate??

Buggy Builder
07-01-2010, 11:16 AM
Too low. You want it to engage above peak torque rpm.

minibajaman
07-01-2010, 11:11 PM
ok i got these calculations( I derived the gear ratios) but still I need to find out what my engagement range will be? For d specs of the engine mentioned above will 2000-2200 rpm range be appropriate??
Yes, assuming the peak torque is still made around 2600 rpm.


Too low. You want it to engage above peak torque rpm.
What? Please explain. Below but near peak torque RPM sure, but not above.

Buggy Builder
07-04-2010, 11:33 AM
The motive is performance. MiniBajas are soooo horsepower limited, you don't want to give any away.
If you need to start on a hill, you want the clutch to engage at an rpm that provides as much hp as possible, ie at a high rpm. These engines are designed to run on a governor at the rpm for max hp. The clutch is designed to keep the engine running at the rpm of max. hp. while varying the speed of the car.

Why engage the clutch at an rpm anything much below the rpm of peak hp?

Jaggs
07-05-2010, 11:36 PM
Max torque @3000 rpm
Max power@ 4400 rpm
Governor setting restricted to 3600 rpm

If I engage the CVT in a range higher than 2000-2200 rpm,then running the vehicle for a duration of around 4 hours for endurance wont lead to overheating of the engine?????

If I engage in this range then I am not too close to the max torque range..

minibajaman
07-06-2010, 08:45 AM
The motive is performance. MiniBajas are soooo horsepower limited, you don't want to give any away.
If you need to start on a hill, you want the clutch to engage at an rpm that provides as much hp as possible, ie at a high rpm. These engines are designed to run on a governor at the rpm for max hp. The clutch is designed to keep the engine running at the rpm of max. hp. while varying the speed of the car.

Why engage the clutch at an rpm anything much below the rpm of peak hp?
Were you on a Baja SAE team? I just wanted to know if this is a result of direct experience. For reference to others, the US competition series 10hp Briggs has limited RPM of 3800, makes peak HP at 3800 RPM, and peak torque at 2600 RPM.
When I was in Baja SAE, we experimented with various clutch settings, although never one where it was engaging at 3000+rpm. Still, we had good results engaging the clutch just below the max torque rpm. Not all applications require max power, sometimes max torque is needed (like tow pull and sometimes hill climb). So if you are engaging at 3000+rpm, then the engine is already past it's max torque. I guess if what you are saying is true, then why wouldn't every CVT drive vehicle engage the clutches at peak HP as well? Other than the obvious drive-ability issue.


If I engage the CVT in a range higher than 2000-2200 rpm,then running the vehicle for a duration of around 4 hours for endurance wont lead to overheating of the engine?????
Not relevant. The engagement RPM is only where the clutch starts to cause the vehicle to move, but the engine will continue to rev to it's max RPM. While driving, the engine will spend most of it's time maxed out at the governed RPM. The engines are designed for this kind of use and won't overheat.

Buggy Builder
07-06-2010, 08:49 AM
With a governor setting of 3600 rpm, the peak hp spec at 4400 rpm is irrelevent to your needs. You will want to figure out how much hp the engine makes at 3600.

These are industrial engines, they are designed to make rated hp all day long while the engine is stationary. You will have no overheating problems.

Engagement rpm will have zero effect on overheating.

Engagement rpm will determine how much acceleration you get "coming out of the hole" It will determine how quickly your car will accelerate if you need to start on a hill.