Jackshaft Primary Ratios

twostroketoowoke

twostroketoowoke

Member
There’s an old post from years ago that talked about belt and clutch life and how it directly correlates to primary ratios that I can’t seem to find. We all know that coveted 2.71 primary ratio (21 engine, 57 pulley) is pretty much the standard for two cycles with a minimum of a 21 driver on the dry clutch. The post I can no longer find talked about how that primary ratio getting bigger eats up belts and is hard on clutches because you actually lower the engagement significantly to match up to the much higher jackshaft speed you’ve created by downsizing the engine driver, say an 18 or 19 Instead of a 21. Can anyone school me a little on this subject and tell me why I should or shouldn’t put an 18 on my engine so I can keep my 21 driver on my clutch and not have to run a 70 on the rear gear for the very small indoor tracks where I need a 9.5-10.0 final ratio? OR what’s stopping me from leaving said 21/57 and just putting on an 18 or 19T on my clutch instead? Isn’t the whole point of a jackshaft to begin with to keep heat off the crank and rear gears out of the dirt no matter what final ratio you need?
 
I run a 50/20 primary with my Birky clutch on a Bully jackshaft (2.5 primary). I have run a 50/21 before too. I run what Birky told me it is better for my engine and clutch and he sets the clutch accordingly. It works good for me. The clutches that have lasted longer for me without rebuilds have been low stall dry engine clutches on JICA 100cc port engines. Years without touching them with top performance on asphalt sprint track, like the one that came with my P51. That engine is so fast that outrun some of my ICA direct drive reeds.
 
I'm becoming a fan of the theory; smallest gears possible club. But you can put any combination you want in this spreadsheet. It's part of nine sheets, my free collection of karting utilities.
jack shaft 10 1 20.jpg
I
 
So I guess the question is... on a disk clutch jackshaft setup, is lowest possible engagement a good thing? And I’m not talking about engaging at a lower engine RPM. I mean this by raising the primary ratio, and lowering the clutch engagement to match up to the same target RPM. Here’s an example:

21/57 = 2.71 primary
Desired engine RPM engagement = 10,800
RPM divided by primary ratio = 3,985 engagement on the clutch.

Now raise the primary ratio by going smaller on the engine: 18/57 = 3.16 primary
Same 10.8K desired RPM engagement with that bigger primary LOWERS the clutch engagement to 3,400 RPM’s. That is a significant drop. Is it better? Worse? The lower the clutch engages, the less heat correct?
 
From my perspective, you are on the right path. I would check with Buller on what his limits are for the Jackshaft clutches. He may have spring and weight packages that will get you dialed in without too much trouble.
 
The heat would be developed by the slipping , more then the engagement rpm .
More disc's better ability to transfer heat and power .
Where it engages in relation to the average rpm would be a factor .
If your rpm drop below engagement rpm during a race that would create excess heat .
The other thing your also changing the torque. Applied to the clutch .
 
I wonder, your lowering the engagement RPM but your increasing the torque. Where's the trade-off? Time for the old infrared heat gun, whichever one is generating the most heat, is the one that's getting less power to the axle. An interesting test.
 
Why such a high stall? I believe that in itself will creat unwanted heat and could also lead to heat sink in the engine alone. The average in clutch lockup I’ve ever truly seen has been from 9.2 to 10.2 from axel, dry, wet,Birky and others. This is the one area you really don’t want to think outside the box. Start with a good known and dial it in. Remember as far as ratios go...all the old dogs would tell ya a ratio is a ratio, but we know there are secrets in between as well.
 
I believe the main reason we have all gone to jackshafts was to get away from 9t and 10t drivers on the engines. MX guys told me that anything smaller than a 14t driver will bind the chain turning around the small diameter. Years ago jackshafts used chain primaries and it would eat chain from spinning so fast and build heat, this is why we now use belts. The original Buller drive that I bought was a 19/57 that was a 3:1.
 
Theshadow05 said:
Why such a high stall? I believe that in itself will creat unwanted heat and could also lead to heat sink in the engine alone. The average in clutch lockup I’ve ever truly seen has been from 9.2 to 10.2 from axel, dry, wet,Birky and others. This is the one area you really don’t want to think outside the box. Start with a good known and dial it in. Remember as far as ratios go...all the old dogs would tell ya a ratio is a ratio, but we know there are secrets in between as well.
Click to expand...

my motor makes peak torque at 10.8, which is why I want my engagement to be that high. It is a Vortex RoK TT with a sudam stroker crank, big bore and on dual alky carbs at 143cc. One off custom piece built by Mike Collins and Judges Performance.
 
Truthfully if it adds driveability on these tight indoor tracks I don’t mind lowering the stall below that number. As long as it’s not lugging and making the motor build up heat I’m open to anything.
 
You must log in or register to reply here.
Back
Top