Jump to content

pmk

OSSA MAR Countershaft Sprockets

Recommended Posts

9 minutes ago, feetupfun said:

Are you referring to my OSSA (the green and white one as shown in the photos) or pmk's OSSA?

I too was not sure which specific machine was being referenced.

My OSSA runs super clean at all rpms.  Carb is not worn, as it has an almost new Smart Carb on it.  Carb tuning is accurate, and simple, since all tuning is accomplished externally.  My riding buddy has ridden and compared my OSSA to his, and I should do the same.  He stated my OSSA has a smoother lower idle than his with the OKO conversion carb.  He also noted my engine pulls cleaner and stronger right off idle into higher power.  Maximum power seems the same.  I replaced both crank seals with new seals as the resto mod upgrades were accomplished.  The rubber intake manifold, airbox to carb rubber boots are both new.  The rear silencer was rebuilt and freshly packed, plus is a Krizman style (no screen) spark arrester so it is not clogged.  The midpipe was opened and repacked, plus the exhaust header remains tight with no leaks.

I did not pull the top end, but the motor is quiet with no rattles and great compression.  It starts and runs very well.

I keep refrencing back to the purpose built TL125.  That bike simply thumps along slightly above idle, even in softer terrain.  At 12/46 my OSSA would not do that, with 11/46 it does, guessing 10/46 will do the same, with even easier control, provided I do not simply tip over.

  • Like 2

Share this post


Link to post
Share on other sites

To keep this slightly updated, it was decided to try the 10/46 ratio initially.  This makes for a lower cost simpler change to see if the gearing change is worthwhile.

The 10 tooth countershaft sprocket was ordered along with the stepped nut.

I held of on ordering the flat sprocket adapter, knowing I can easily get that later, and it will be required if I go to a larger rear sprocket.

The numbers regarding ratio changes indicate the swap from to 11 to 10 will be a larger percentage change than the swap from 12 to 11.  Possibly the 10/46 will be very good.  If not, I am leaning towards 10/51 next, which brings second gear to about what was first gear when using the 12/46.

Slow speed control with minimal clutch use, in an effort to simply gain a smoother effort while riding sections is the goal.

While waiting on the parts, time to go back inside those rear shocks and soften tne damping even more.

  • Like 1

Share this post


Link to post
Share on other sites

Hi Guys, I think with all bikes, if you lower the ratio, it makes using 3rd gear an option for some sections.   Last year we had three organisers that give us a  hill-climb type section, where a powerful 3rd gear was just right.    Admittedly one organiser put a left / right squiggle in his graded hill climb, but as long as you went round that bit quick enough, the engine would not stall. (tried changing down on the corner on one lap, but that didn't work!).

.

  • Like 1

Share this post


Link to post
Share on other sites

10 tooth sprocket arrived today.  Actually, 2 sprockets and 2 stepped nuts, designed to allow clearance for the chain. 

So, regarding the parts, the Talon sprockets look great.  These have a forged in step on the backside, to allow the sprocket to be automatically spaced away from the cases / seal, and it still remains a very close fit.

Regarding the nuts, two identically ordered parts, however, dimensionally they differ.  One nut is thinner overall than the other.  Also, the thinner nut has a shallower step prior to the hex.

Regardless, even the nut with the longer step will pinch the chain and totally bind on the master link. The longer step sprocket can clear the chain, provided I install a 2mm shim between the nut and the sprocket.  The shallow stepped nut will require more thickness to the spacer.

With all this, it seems the easiest solution to allow using the stepped nut will be to modify the nut further, to allow clearance at the chain.

So I ask, has anyone run the 10 tooth Talon front sprocket and the stepped nut?  If so, what was your solution to allow it to work without contacting the chain?

Edited by pmk

Share this post


Link to post
Share on other sites
5 hours ago, pmk said:

10 tooth sprocket arrived today.  Actually, 2 sprockets and 2 stepped nuts, designed to allow clearance for the chain. 

First, I want to say I asked the IN Motion guys about this install a couple months back.  They advised having sold many 10 tooth sprockets and step nuts for the OSSA MAR.

So, regarding the parts, the Talon sprockets look great.  These have a forged in step on the backside, to allow the sprocket to be automatically spaced away from the cases / seal, and it still remains a very close fit.

Regarding the nuts, two identically ordered parts, however, dimensionally they differ.  One nut is thinner overall than the other.  Also, the thinner nut has a shallower step prior to the hex.

Regardless, even the nut with the longer step will pinch the chain and totally bind on the master link. The longer step sprocket can clear the chain, provided I install a 2mm shim between the nut and the sprocket.  The shallow stepped nut will require more thickness to the spacer.

With all this, it seems the easiest solution to allow using the stepped nut will be to modify the nut further, to allow clearance at the chain.

So I ask, has anyone run the 10 tooth Talon front sprocket and the stepped nut?  If so, what was your solution to allow it to work without contacting the chain?

Pretty sure I machined the nut, leaving just enough hex there to tighten it with

  • Like 1

Share this post


Link to post
Share on other sites

I resolved the concern of the chain not having clearance to the nut.  Using the lathe, I machined a chamfered edge on the step side of the nut, providing clearance.

This could be done with a file if needed.  

Certainly something to watch for.  Possibly not every situation has this clearance issue, but worth checking to prevent a possible problem.

Share this post


Link to post
Share on other sites

Wanted to add some details about this countershaft sprocket change.  Initially when built and during my rebuild, the 73 OSSA MAR came with stock gearing of 12/46.

After some use, rides, I made a change to 11/46.  This was a very noticable change.  The downside being tight sections or some climbing required use of the clutch.

Most recently, I installed a 10 tooth countershaft sprocket.  Initially, I rode the 10/46 setup in my yard, prior to yesterdays event.  The change from 11 to 10 was also very noticable.  With the 10 tooth, the low speed control is improved.  The need for using the clutch is almost never.

The good of the 10 tooth was low speed control.  Steep climbs were not a problem, and my riding buddy commented there was no tire spin at all.  A secondary benefit it how improved the rear braking has become.

The downside of the 10 is how quickly the engine revs out.  Riding between sections, this is a non issue.  The main drawback so far has been when using the throttle to lift the front wheel.  With the 11 or 12 tooth, the engine had a longer pull and could keep the front wheel lofted longer.  Understanding the lower gearing not being able to hold the front end up as long, timing becomes a bit more important.

At the event yesterday it was a learning curve, for various reasons and my mistakes were my own.  The bike certainly did not struggle.  For now, I plan to run the 10 but if I deem needed, will go back to the 11.

  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×