Newbie with Questions (clutch and drive ratio)

[camaroman7d]

Hey Guys, I have been lurking on the site for a while. I have read all the stickies that I think apply to me and still don't have the answer to my questions. Very cool site by the way with lots of good info, hopefully I will be able to add to it in the future.

I'm building a motorized "bar stool", basically a shorter narrower go kart with a higher center of gravity.

I am using a 13HP GX390 engine. I see there are several "extreme" duty centrif. clutches that are rated up to and above 13HP. This of course is the simple route. I have looked into TC's and I don't think they are my best option for this project. Limited space and more than what is needed for this project.

I will be removing the governor from the engine. What type of gear ratio should I be looking for? Just an all around fun gear, decent take-off and maybe 40mph top speeds. The "kart" will weigh ~150lbs and I'm 235lbs.

I also have an axle clutch with 8mm belt drive pulley, but I can't find any info on it at all. It came off the donor kart which was an old Invader lay down race kart. See pictures. Do any of you know anything about this? Would this hold up to the GX390? It is an 80 tooth pulley, I can order a 8mm pulley for the PTO as well. The kart had a small 2 stroke engine guessing 100cc, so I know it didn't have the torque of the Honda that I'm using. Would like to order my drive today ASAP so I can have this thing ready for a test run next week.

 

[anderkart]

I think that might be an early/prototype version of one of the Horstman brand axle-clutch models.

It should handle the torque of your GX390 fine, but being it was used with a 100cc./2 cycle, its probably setup to fully lock in somewhere around the 10,000 rpm range, instead of somewhere between 1800 to 3000 rpm you'd want for a lower revving 4-cycle on your barstool racer.

I'd suggest you send your pics (or even a link to this thread) to Horstman: https://horstmanclutches.3dcartstores.com/crm.asp?action=contactus to see if they can ID it, and also ask if it can be adjusted to lock-in somewhere in the 2000-3000 rpm range

 

[anderkart]

I will be removing the governor from the engine. What type of gear ratio should I be looking for? Just an all around fun gear, decent take-off and maybe 40mph top speeds. The "kart" will weigh ~150lbs and I'm 235lbs.

How tall are your rear tires?

Will you be doing any other engine mods, besides removing the governor?

 

[camaroman7d]

Rear tires are ~10" tall typical race kart wheels and tires. I have a 40 and a 54 tooth sprocket on the way along with a 14 tooth clutch. #41 chain set-up.

I found an online calculator and it looks like I can keep the RPM's down to 5000 and still reach 40+mph which is probably more than enough on this contraption.

As far as engine mods, I will be changing the carb to a Mikuni, and building a header (mostly for sound and looks). I have a pulse pump and intake with a pulse fitting. For now those are the only mods I have in mind. If I decide to go crazy, then I will upgrade the flywheel, connecting rod, cam, etc... Honestly this is just a toy I'm throwing together for the fun of it.

How thick of a plate are you guys using to mount your engines? I'm thinking 1/8" should get the job done since, the span is only 8". I will post pictures once I finish the rolling chassis. Should be tonight

 

[OzFab]

Rear tires are ~10" tall typical race kart wheels and tires. I have a 40 and a 54 tooth sprocket on the way along with a 14 tooth clutch. #41 chain set-up.

Hmm, you're sort of, inadvertantly limiting yourself; with the small tyres, the rear sprocket can only be so big, same problem with the clutch but, the two combined only give you a ratio of 3.8:1 which is waaaay too high, you should be aiming for somewhere around 5:1. I'm thinking you may need to incorporate some compound gearing...

I found an online calculator and it looks like I can keep the RPM's down to 5000 and still reach 40+mph which is probably more than enough on this contraption.

As mentioned above, your final drive ratio will only be a maximum of 3.8:1 which will give you a top speed around 40mph but, you'll most likely destroy your clutch trying to get there

FYI: We have our own speed calculator

How thick of a plate are you guys using to mount your engines? I'm thinking 1/8" should get the job done since, the span is only 8".

IMO, it should be double that; have you ever tried running that engine on your shop floor? The amount of torque those things put out will bend 1/8" plate in seconds...

 

[camaroman7d]

I think that might be an early/prototype version of one of the Horstman brand axle-clutch models.

It should handle the torque of your GX390 fine, but being it was used with a 100cc./2 cycle, its probably setup to fully lock in somewhere around the 10,000 rpm range, instead of somewhere between 1800 to 3000 rpm you'd want for a lower revving 4-cycle on your barstool racer.

I'd suggest you send your pics (or even a link to this thread) to Horstman: https://horstmanclutches.3dcartstores.com/crm.asp?action=contactus to see if they can ID it, and also ask if it can be adjusted to lock-in somewhere in the 2000-3000 rpm range

Thanks, I sent them a message and once I hear back from them I will post the information.

 

[camaroman7d]

Hmm, you're sort of, inadvertantly limiting yourself; with the small tyres, the rear sprocket can only be so big, same problem with the clutch but, the two combined only give you a ratio of 3.8:1 which is waaaay too high, you should be aiming for somewhere around 5:1. I'm thinking you may need to incorporate some compound gearing...



As mentioned above, your final drive ratio will only be a maximum of 3.8:1 which will give you a top speed around 40mph but, you'll most likely destroy your clutch trying to get there

FYI: We have our own speed calculator



IMO, it should be double that; have you ever tried running that engine on your shop floor? The amount of torque those things put out will bend 1/8" plate in seconds...

Thanks for your input. Looks like I'm limited due to the tire size, but on the other hand the tire being shorter should also help with the lack of gear ratio. i'll have to give it a go and if I have to modify it later I will come up with something, I would think this engine would have enough torque to pull that ratio. I guess we'll see.

Now as far as the engine mounting plate. My frame rails are only 8" apart (side to side), I have run the engine on the shop floor and it does hop around, but with a little gusseting I'm fairly certain I can keep the plate from bending. Remember this is a bar stool not an off road cart, bumps will be minimum. I am trying to keep this thing as light as possible, so 1/4" plate won't help. I will see what else I have laying around, I might step it up a little.

 

[landuse]

Personally I would not want any barstool to go 40mph. It is too dangerous with the high COG that these things have. You will be popping weelies all the time when you turn the throttle

I also think the ratio might be a bit high. For the price you will pay for a replacement clutch, I would not take the chance of burning the first one up

 

[camaroman7d]

What options do I have as far as clutch/gearing? The Hilliard extreme, is supposed to have overload protection. Once I have my hands on it I will see if i can modify it to accept a smaller gear. Are you suggesting a TC? It's not out of the question. I'm just worried it will feel mushy.

Yes 40mph is probably unsafe, but that's what makes it fun.

 

[landuse]

What options do I have as far as clutch/gearing? The Hilliard extreme, is supposed to have overload protection. Once I have my hands on it I will see if i can modify it to accept a smaller gear. Are you suggesting a TC? It's not out of the question. I'm just worried it will feel mushy.

Yes 40mph is probably unsafe, but that's what makes it fun.

No, I am not suggesting a TC, but you might have to use a jackshaft setup with your centrifugal clutch. You can then use smaller sprockets to compensate for your small tyres

 

[camaroman7d]

Ok I will look into making a jack shaft. I should have most of the stuff laying around other than sprockets. I will search here to see what type of setups people are using.

 

[OzFab]

I will see if i can modify it to accept a smaller gear.

I don't think you'll have much luck with that, you can only fit so many teeth around a 1" shaft

 

[camaroman7d]

I don't think you'll have much luck with that, you can only fit so many teeth around a 1" shaft

True but they do make a 10 tooth sprocket. Also there is nothing saying it can't be off the end of the shaft.

 

[OzFab]

True; if you can figure out a way to attach it to the clutch drum, go for it...

 

[camaroman7d]

The axle clutch above was made by Horstman back in the 1986 - 1990 range. They said it will handle the power of the gx390 and was originally designed to engage about 2000 rpm. I will hang on to it for a future project. I already have a clutch and sprockets on the way. Getting parts together to make a jackshaftas well. I already have most of it.

Thanks for the input.

 

[anderkart]

The axle clutch above was made by Horstman back in the 1986 - 1990 range. They said it will handle the power of the gx390 and was originally designed to engage about 2000 rpm. I will hang on to it for a future project.

Cool, but keep in mind that axle clutches are rated at axle-rpm, not engine rpm...

So for instance:
On a kart with a 5:1 gear ratio, your axle clutch, (with it's 2000rpm engagement) would engage at the exact same engine-rpm, as an engine-mounted clutch with a 10,000rpm engagement rating.

 

[camaroman7d]

Cool, but keep in mind that axle clutches are rated at axle-rpm, not engine rpm...

So for instance:
On a kart with a 5:1 gear ratio, your axle clutch, (with it's 2000rpm engagement) would engage at the exact same engine-rpm, as an engine-mounted clutch with a 10,000rpm engagement

Correct. Here is exactly what the response to my question was:
3/7/2014 10:11:27 AM "The clutch is model AX-4056 made back in 1986-90 range. The clutch appears to have aftermarket external stall speed adjusters. Assuming there are no internal modifications the clutch will engage at about 2000 axle or jackshaft RPM. The clutch must be filled a little under half full with Horstman clutch oil. A GX-390 for racing uses an engine mounted clutch set to engage at 2800 RPM. The Axle clutch can not be mounted direct to any engine shaft. If you wish to try the Axle clutch you will need to make a jackshaft to mount the clutch. Gearing from engine to clutch should be about 1.4:1. Then the gearing from the output side of the jackshaft to the rear axle would be of your determination. ."

 

[camaroman7d]

Ok I have my jackshaft made and mounted. I also have more gear ratios than I know what to do with. What gearing is a good place to start. I can pretty much cover any and everything. I assume 5500 will be max for a non governed gx390 with that in mind 5:1 should put top speed about 35 @ 5000 rpm. I know how I am and a see modifying the engine in the future. Still a ways from running, so unless I hear otherwise I'm going to set it up with a final of 5:1

 

[Half-breeder]

Should be no issues(I usually run a 5:1 on my karts) if your wheels are 10"-11", and if the over all weight(w/ you on it) isnt too much for the motor to push.

I would personally run a 6:1-6.5:1, for a barstool, for starters... the center of gravity(CoG) is so high youll more than likely tip over your first turn(unless you make it about 40" wide and equivalently proportioned correctly... but then, thats a go kart w/ a stool for a seat).

 

[anderkart]

Yep, 5:1 should be fine.