Author Topic: sport 90 gearing changes  (Read 3299 times)

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buddy rawls

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sport 90 gearing changes
« on: April 24, 2011, 01:16:43 AM »
Everyone that has messed around with these bikes, I need some info on the motor response to higher gearing.

I really need to obtain a 55mph cruise, as opposed to the 45mph the bike is currently happy with.  According to the gear/rpm charts this is right around 5000 (peak torque and highest VE)

If I increase the gearing 20-23% it is going to put me right where I want to be.

This is about the same as going up 1 tooth on the front and down 4 in the rear.  Or up 2 in the front and down 1-2 in the back.

This is going to take some off-idle response out of the bike.  For the ones that have played with the gearing, what is about the limit the bike could take and still be a worthy rider. 

Offline OldSwartout

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Re: sport 90 gearing changes
« Reply #1 on: April 24, 2011, 10:12:45 AM »
Either combination will work and there's not much reason to prefer one over the other, just see that you have room for the larger diameter on the front sprocket if you decide to go up 2.  The choice may depend on availability of the sprockets. 

I have not played with gearing that tall on a 90, so this is just an opinion, not experience. I would be concerned about the engines ability to pull that gearing; 20% is a lot of change. It may depend on your weight and wind resistance, if you're skinny, it may work reasonably well.  Only trying it can really answer, unless someone on here has already done it.
Karl Swartout
Mooresville, IN
BS175 Roadracer. BS200RS, BS350 GTR

buddy rawls

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Re: sport 90 gearing changes
« Reply #2 on: April 24, 2011, 10:56:00 AM »
...Only trying it can really answer, unless someone on here has already done it...

I was hoping for the easier answer, like 20-25% is too much, but 10-15% seems to work without much penalty, etc etc.  Or maybe 20% works but is right on the edge.

I have been mapping out the power curve versus used rpm and speed, and the 20-23% falls right about at the mid point between 1-2.  Which means the bike will be starting out right between 1-2 (as it is currently geared).  I am going to ride it some today starting out in second and monitor carefully.

Regardless, a cruise speed in the 55 mph region with max speeds in the 70-75 would be a great asset.  Or maybe I will do the experimenting and let everyone know about where the max gearing (for the stock motor) can go to.

subrew

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Re: sport 90 gearing changes
« Reply #3 on: April 25, 2011, 12:14:34 AM »
What size rear tire do you have on the bike?  They make plenty of slightly taller tires in 2.75 and 3x17 that will lengthen gearing too.  You may find a taller rear tire and 1 tooth up on the front will give you what you need.

buddy rawls

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Re: sport 90 gearing changes
« Reply #4 on: April 25, 2011, 09:48:24 AM »
I am running the Michelin Gazelles on it, 2.50's on both.

I played a lot with 2nd gear starts and 1st gear starts this weekend, really watching the speed and motor response as it begins coming into a usable rpm band, then back calculating into rpm ranges.  I do not think the stock sport 90 (or mine) can pull a 20-25% taller gear, which was my goal.  the true fix is another gear in the transmission, which isn't going to happen.  More rpm would work too, but delaying the intake closing will pull cylinder pressure out of the motor (torque decreased).  I do custom camshafts and valve event calcs for a side business, and I have been really looking into the rotary valve mods.  I am very hesitant with the age of the motor though.

so,  I am going to begin with a 1 tooth larger front (~7%), then either tune with the back, or see if it will take a 2 tooth larger front (~14%).  I am thinking 10-15% may be about the limit. 

For the ones that have stock sport 90's, is 45mph about the best cruising rpm you can obtain too.

 


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