Diameter of front and rear drive pulleys

[sxonwls]

Hello everyone,

I am trying to improve the acceleration on my 2015 RT-S. I am attempting to do this be changing out the rear pulley on my RT and replacing it with an F3 rear pulley. The F3 rear pulley has 10 more teeth than the RT. The front pulley is the same as the RT.

What I need your help with is a diameter measurement of the front and rear pulley on an F3. If someone knows this measurement or happens to have the tupperware off their F3 I would greatly appreciate your help. This measurement will allow me to calculate the change in rpm at a designated speed.

Thank you in advance for your help!

Daniel

[spacetiger]

Daniel,

In case no one responds back, you might do it by the teeth count (as a close enough approximation); all you need is the teeth count from your bike since you know the difference in the rear is 10 teeth.

Post back what you work out - I'm curious.

Jerry

[Chupaca]

these are strange belts and not easy to find. You need to be sure you will be able to fit it on and have room for adjustments...from the parts list the F3 has 28 teeth front and 79 rear STD and an optional 89 tooth for the rear..belts vary with US and europe. Parts list from cheapcycleparts.com...:thumbup:

[billybovine]

I agree with a previous post to calculate the difference in ratio just use the number of teeth on each sprocket. When I was researching putting the bigger sprocket on my ST I learned. You should be able to swap out the rear sprocket and belt from a NA F3 and it should fit 2014 or 2015 RT. Since you have 2015 RT, it likely has the 6 spoke rear rim and you should have no trouble accessing the tire valve. On 3 spoke rims the tire valve will be blocked by the 6 spoke sprocket.

Let us know how it works out.

[PMK]

Daniel,

In case no one responds back, you might do it by the teeth count (as a close enough approximation); all you need is the teeth count from your bike since you know the difference in the rear is 10 teeth.

Post back what you work out - I'm curious.

Jerry

This is the proper method, by tooth count converted into a ratio.

Since this is final drive, you should be able to calculate the percentage difference between the pulley sets, and this should be the increase in percentage of rpm or mph decrease.

PK

[Bob Denman]

Quick and simple:
89/79=1.1265822
Multiply your current rpm at a given speed by that amount... :thumbup:

Since the rear sprocket is the only variable: the difference from one sprocket to the other, will be the only factor that determines thedifference in rpm levels...

[pauly1]

F3
Front Rear Belt #
US 28 89 705502127
OUS 28 79 705501304

RT-S
28 79 705501304

The diameters of the pulleys are only part of the swap: the distance between them is the other. The F3 models made for world distribution have a 28/79 sprocket ratio and drive belt the same as the RT-S models. If the sprockets are the same and the belt is the same, then the center to center distance must be the same between the F3 and the RT-S. So installing the 89-tooth sprocket would use the F3 belt for the North American market. Regardless of the sprocket tooth spacing/size, assuming both pitches are the same (which they are), the increased tooth count = larger diameter = ~12-13% increase in leverage at the rear wheel. That also is the increase in RPM because of the gear ratio F/R difference.

The rear wheels have the same part number, comparing the metallic silver, between the F3 standard and RT-S: the sprockets should bolt interchangeably on the wheels.

(all information sourced from the BRP website under the Parts tab)

[billybovine]

F3
Front Rear Belt #
US 28 89 705502127
OUS 28 79 705501304

RT-S
28 79 705501304

The diameters of the pulleys are only part of the swap: the distance between them is the other. The F3 models made for world distribution have a 28/79 sprocket ratio and drive belt the same as the RT-S models. If the sprockets are the same and the belt is the same, then the center to center distance must be the same between the F3 and the RT-S. So installing the 89-tooth sprocket would use the F3 belt for the North American market. Regardless of the sprocket tooth spacing/size, assuming both pitches are the same (which they are), the increased tooth count = larger diameter = ~12-13% increase in leverage at the rear wheel. That also is the increase in RPM because of the gear ratio F/R difference.

The rear wheels have the same part number, comparing the metallic silver, between the F3 standard and RT-S: the sprockets should bolt interchangeably on the wheels.

(all information sourced from the BRP website under the Parts tab)

Yes both sprockets will fit all rear wheel types, but the 89 tooth sprocket is a 6 spoke and will block the air valve on a three spoke rim. Not a big deal but has to dealt with.

[Bob Denman]

F3
Front Rear Belt #
US 28 89 705502127
OUS 28 79 705501304

RT-S
28 79 705501304
If the sprockets are the same and the belt is the same, then the center to center distance must be the same between the F3 and the RT-S. So installing the 89-tooth sprocket would use the F3 belt for the North American market. Regardless of the sprocket tooth spacing/size, assuming both pitches are the same (which they are), the increased tooth count = larger diameter = ~12-13% increase in leverage at the rear wheel. That also is the increase in RPM because of the gear ratio F/R difference.
)

Is 12.65822% close enough for you? :D

[PMK]

So, if the math is correct, 12.6% more engine rpm when compared to a stock RT.

And should be about 12.6% more torque to the ground, same HP unless you consider increased rpm.

12.6% reduction in max speed.

PK

[Bob Denman]

It most likely won't hurt the top-end at all.
These bikes are aero-limited; not rpm-limited.

[larryd]

I too would like to do this change...My dealer said he would work with me on the parts...My dealer also check with a BRP factory tech guy and here is a copy of his reply...

Sorry, I cannot copy and paste this...If you will send me your e-mail address I will send it to you...Maybe you can paste it???

My e-mail...lbdemay@verizon.net


http://www.spyderlovers.com/forums/{A7B2DE27-F0AE-4F83-8CEC-0793FC2C7ED0}/photo.JPG

[jerpinoy]

Deng if the bennis worth the trouble I'm all for it. Good luck and let us all know.

[billybovine]

larryd sent me a blurry picture of the email so I typed out the text.

Quote from email
"This is not possible ..... well physically it is but limp mode home is inevitable.

The final drive ratio will be changed and this will trigger codes with the TCM. Engine speed (RPM) and wheel speed are closely monitored to detect clutch slippage, changing the rear sprocket will put these readings out of range and put the unit into limp home mode immediately. "
End of quote.

Basically the same answer Lamont got when I brought out the idea of doing this to my ST.

[pauly1]

Yes both sprockets will fit all rear wheel types, but the 89 tooth sprcket is a 6 spoke and will block the air valve on a three spoke rim. Not a big deal but has to dealt with.

In my quote from BRP parts catalogs both sprockets bolted on the same rim based on part numbers.

The nanny is a problem.

[Bob Denman]

The nanny is a problem.

Why? :dontknow:
If she's not sensing wheelspin; do you think that she keeps an eye on the speed/rpm readings, and their relation to one another? :dontknow:
Unless the Gear Position Sensor is also tied into the equation; she'd just think that you're running in 5th gear...

[billybovine]

I looked around the demo version of BUDS and I cannot find any parameter changes that can be done for the final drive ratio.

Can someone with experience using BUDS confirm that to be true?

[pauly1]

The traction control system should monitor relative speed differences between the wheels, along with lateral acceleration, not the speed in which gear at RPM. Perhaps one way to tell would be the settings comparing the EU version to the North American version.

[pauly1]

I was pondering doing just the opposite switch after talking with OJ UK at SpyderFest a few weeks ago. He commented he didn't experience the belt dynamics on his F3.

[Bob Denman]

The traction control system should monitor relative speed differences between the wheels, along with lateral acceleration, not the speed in which gear at RPM. Perhaps one way to tell would be the settings comparing the EU version to the North American version.

You're not changing the wheels; just the sprocket; nothing would change that she would notice.

[Bob Denman]

I was pondering doing just the opposite switch after talking with OJ UK at SpyderFest a few weeks ago. He commented he didn't experience the belt dynamics on his F3.
That's because the European version is running the RT's gearing and belt; this setup must be just a bit more resistant to the harmonics. :thumbup:
Just be aware that you'll lose a little bit of that bottom-end pull that the F-3 owners rave about.