Longer jackshaft: any concerns?

[BoogieBob]

My current setup (10hp Subaru Robin EX27) has the Comet 40 driven mounted with the spring outboard (yellow spring).

The kart runs ok, but does seem to go through belts faster than expected, so I'm considering ways to attain better belt alignment.

I can't reverse the driven (with red spring inboard) because the sheaves will be so far outboard that I would need to move the drive outboard as well (maybe an inch).
But the drive unit cannot be moved outboard at all.

I'm thinking I could replace the current jackshaft on my kart with one that is approximately 2" longer (if the driven will clear the frame, gotta do some more measuring) so I can flip the drive member and allow better belt tracking.

Mechanically, the drive unit will function reversed.

But... I'm a little concerned about the the driven unit being moved outboard via a longer shaft.
Optimally, the driven would be as close as possible to a support bearing, but this mod would move it further away from the bearing, and the end of the shaft would still be unsupported.
Will the 5/8 shaft properly support the driven?

Many thanks,
Bob

 

[ezcome-ezgo]

Time to post a picture if you can. 10hp could pretzel a 5/8 shaft in a jiffy.

 

[BoogieBob]

Yikes!
That is exactly why I asked you experts: I lack experience with karts.
Good to know my prior mechanic's knowledge is in the ballpark (recognized the potential issue).

Ok, here is a picture...not the best really.
I'll try to attach a pic of the Manco schematic.

Stock might be the best way to go...

Bob

 

[ezcome-ezgo]

Those rusty sheaves will be unkind to belts.

 

[BoogieBob]

Haha, I thought the same...and that pic was before I pulled them for service.
They look much better now.

My brother in law had it stored at his place, but the tarp apparently blew off it at some point, and he didn't realize it.

Bob

 

[ezcome-ezgo]

What are the tooth counts of the sprockets?

 

[BoogieBob]

Umm..more than zero, less than 400?


I see where you are going with this.
For some reason the number residing in my dusty brain is 9x60, with 22" tires. Final ratio 6.6:1 or so?
But, the tires are run a little soft, so the actual ratio may be a bit lower.

I don't recall for sure, but I'll take a peek this weekend.

Bob

Edit to add: I studied some of the info on this site, and used some of the calculators.
Also considered how the kart performs.

It looks like I need to change my gear ratio, as it is currently set up for top speed over acceleration.
The kids don't need to hit 35mph, and the current setup, especially with 2 adult-sized teens in the kart, is stressing the kart and hurting performance.

I'm thinking about changing to a 72 tooth sprocket, which would give a better 8:1 ish ratio, and around 29mph.

Maybe set up the converter to engage at 2200 or so (I have the parts to allow 2200 or 2600).

I was confused when I looked more closely at the "stock" converter setup of medium rollers and yellow springs (2200 engagement). Now I understand (built not sure if this was stock or if the prior owner modified it).

Still interested in optimizing the Comet setup so the sheaves move in the same direction, but maybe it isn't important.

Too much info here, sorry...but I also discovered something else: the governor is disconnected.
I know that optimally the gear should be removed, but I'm not sure if it has been. We've been running the kart for 5 years with no issue.

I could gently hold the linkage arm under tension, rev the engine and see if the gear pushes against the linkage. If not, it was removed or blown apart.

 

[Darren]

Torque converters work really well with governed engines, max torque is around 3500rpm.
The torque plates should be polished by the belt and therefore shiny.
The torque converter pros will chime in, belts wear, buy the best belt for your TC is -comet-
I'm not a TC man, but the guys always point out the alignment, correct distance and the discs & belt on the correct way. Is there a setup manual for the comet 40 series?

 

[BoogieBob]

Thanks Darren.
Yeah, I've followed the diagrams carefully regarding setup (measured to the millimeter), and use real Comet belts.
I'm meticulous.

It doesn't chew belts, but engagement has always been a bit inconsistent, and is surprisingly worn after only 3 or 4 one to two hour sessions.
My sheaves are a little pitted in areas from prior rust, and I've made them fairly smooth, but there is still a little texturing here and there.
Glass smooth may be optimal, but I doubt the texture is causing severe wear.
I'm thinking the combo of "not 100% optimal TC setup," heavy kart and too-tall gearing is the issue.

Now that I think about it, I should measure the wear on the old belt. What I assume to be wear may actually be stretch causing belt slip.

Also, I did purchase additional parts so I can choose to restore governor function (though still not sure I have everything).

I'm a former master wrench, but don't have a lot of experience with small governed engines.
I do understand them generally, and kicked myself for not noticing the modded throttle linkage. Took me a bit to refresh my understanding of how it works, which was especially challenging because the stock linkage was not intact.

I've heard many people do fine with ungoverned engines, but it makes me a bit nervous, especially not knowing if it the gear was removed.
The Subaru seems pretty stout, and hasn't thrown a rod yet, but...
Hmm...if it did grenade, it would be a great excuse, er, I mean reason, to upgrade to an 18hp engine...

 

[JimD]

After you cleaned up the unit is the belt still that loose? It should be down at the bottom of the driver unit on the engine. When it is that loose it slams into engagement and you don't want that. The sheaves should have a mirror finish on them which the belt will do but all the rust and pitting should be smooth up with some emery cloth 120 grit and steel wool. Manco did have the miss alignment on several karts at neutral which they said who rides in neutral we need it to be aligned when driving which is true. Their design and they just left it alone.

 

[BoogieBob]

That picture was before removal and service. It was sticking for sure, and that was the old belt (worn, maybe stretched).
The belt will definitely sit on the bronze bushings at idle now

After cleaning and reassembly, I noticed the driver sheaves are farther apart than the Comet diagram spec. I converted to metric for easier comparison.

Total driver width should be 100.12mm. Mine is 102.88mm.
The distance between the static sheave and movable sheave assembly should be 44.56.
Mine is 48.02

The diagram makes it look like the exposed axle (where the belt rides at idle, with bronze bushings) should be slightly wider than the belt, which makes sense.
Mine is a few mm wider...

That means that the belt should engage at an even higher RPM because the sheave faces are farther apart at idle.
Oh boy.

I will pull the unit back apart and inspect everything to assure there isn't something preventing the assemblies from mating properly.
I can't think of anything that would take up 3mm...

 

[JimD]

On the driven unit you can always put a shim washers under the snap ring to push the ramp down for more contact to the wear plugs. Just hold everything together with a "C" clamp so when you remove the snap ring it doesn't come apart and you are looking for pieces. As far as size of jackshafts we have only used 3/4" because it holds up better but Comet used the 5/8" but everything is very close to the plate to start with so they were not leveraging it very much.

 

[BoogieBob]

Sorry for the long delay, been super busy.

JimD: thanks for the advice. Not sure why I would perform that mod.
My issue is that the driveR is a little wider than spec.
Maybe I can add a spacer to move the sheaves a little closer together at rest?

But I have bigger problems (kinda).
The driveN sheaves are pretty badly pitted (much worse than I thought ). I will have to put in a lot of work (literally removing metal) to get them glass-smooth.

So... I'm thinking about another path which could solve several issues at once:
The 44D driveN.

The kart is clearly over-geared. I can attain a larger sprocket, but would have to drop the axle to get the old one off. More work, darnit.

The 44D would start lower (2.83) and final at 1.24.
When I ran the numbers through the the kart gearing calculator, the cart would go from 6.6:1 to 8.18:1, much better, I think.

So: a new 44D would solve the potting and gearing issue.
The problem? Finding one with a 5/8 bore.
Per the Comet site, the 44D is available in 5/8 bore.
I sent an email asking where I might find one.

Yeah, I could try to change to a 3/4 jackshaft, but that adds a lot more complexity, expense and time.
Admittedly, this might allow reversing the drivEN (IF I can fit a longer shaft) to allow better belt alignment...but it would also be a huge PITA, so I've given up on that quest.

Many thanks,
Bob