GS850 shaft rpms?

[Guest]

If I remove the rear wheel and final drive, what is the max rpms I can expect out of the driveshaft?

This parts bike may become a powerplant for a project- if the driveshaft rpms are high enough.

Ideally, I'd like to get 5k-9k rpms. I can live with 2.5k-4.5k worst case.

Thanks.

- JC

[bwringer]

Since the bevel gears have the same number of teeth, the RPMs you'd get at the driveshaft would be in the same range as the countershaft on a chain drive bike.

I think this is a lot lower than what you're looking for. If you can find gearing numbers for the early GS750 or GS1000, you should have a basis for the math.

[Steve]

My 850s turn about 5000 RPM at 70 MPH. You can figure the shaft RPM if you figure the wheel diameter (130/90-17 tire) and the final drive ratio of 3.09 (I think).

The 1000 turns about 4500 RPM at 70. It has the same final drive ratio, but a different secondary drive ratio. Shaft speed will be the same.

I would do the math for you, but I am heading to bed.

.

[Dogma]

I've seen a table of all the ratios somewhere. Maybe in the repair manuals?

I'm curious what you're up to.

[trippivot]

driven gear divided by drive gear = ratio

start this arithmitic problem by figuring the crank to clutch basket ratio, and use a rpm of 1000 for ease as a working multiplier for this problem.

then take the answer from the crank to clutch to use as the input -transmission shaft rpm, and that sum can be used with the specific transmission gear ratio -which will get you to the trans. output shaft rpm going to the final drive...

using 1000 rpm as a basic reference-- you can easily figure the output shaft rpm for the specific crank rpm. Do it 5 times
and you'll have the shaft rpm range for every gear

[Nessism]

Engine rpm / Primary gear ratio / Transmission gear ratio/ Secondary gear ratio

If you want rear wheel speed multiply by the final drive gear ratio.

Here is a little chart I put together...

[Steve]

I would do the math for you, but I am heading to bed.

.

OK, I'm up now.

Most shafties have either 16" or 17" wheels, usually in a 130/90 size.
There are two final drive ratios available, according to Nessism's chart, 3.09 and 2.91.
These two factors will affect the driveshaft speed at any given road speed.
Plugging these numbers into a quick spreadsheet, you will get these results:
... Tire ........ Ratio ... Driveshaft revolutions per mile
130/90-17 ... 3.09 ...... 2377
130/90-17 ... 2.91 ...... 2239
130/90-16 ... 3.09 ...... 2472
130/90-16 ... 2.91 ...... 2328

My 850 has a 130/90-17 tire and the 3.09 ratio. The engine turns about 4300 RPM at 60 MPH, so the shaft is turning just over half of engine speed in fifth gear.

.

[GS1100GK]

All great information, thanks!

So if I use 5th gear and a 14 tooth sprocket (vs 7 tooth standard), I can get about 4000 rpm? Any idea what HP I can put out in 5th gear at, say, 6k-7k engine rpms? Ideally, about 8-10hp minimum, but more is okay.

A Husky 3120 chainsaw runs at 9000 rpm with a 7 tooth sprocket runs the chain at about 4000 fpm (feet per minute) with 8.4hp. I can get up to a 14 tooth sprocket, so I'd need 4500 rpms on the sprocket to get 4k fpm out of a .404 chain. If I attach a sprocket to the driveshaft, I'd then need about 6.5-7k engine rpms to get 3k fpm, right?

Unless someone knows an inexpensive way to get a 1:2 ratio (double the output speed) in a ready-made (inexpensive) gearbox that would output about 4k-5k rpms @ about 10-12 hp (so input is 18-20hp) without exploding?

I figure it would need oil circulation for lubrication and cooling, and possibly a clutch to cushion any shock (optional). Yes, of course I will remote the throttle and clutch! I can't be sitting 6 ft in the air on the bike! :-)

I'd prefer to use a gearbox to double the output of a hydraulic motor, but I am also sort of keen to run it from the bike and have it all "WWII-Russian" looking

- JC

[Dogma]

... about 10-12 hp (so input is 18-20hp) without exploding? ...

A gearbox changes torque and rpm, not power (apart from efficiency losses). If you step the RPM up by 2:1, you've cut the torque by half, but the power is the same.

You need to calculate the Husky's torque from the HP and RPM, then from the radius of the sprocket, figure out how much force it's putting on the chain. Once you know that, you can work the numbers back to the 850 to see what ratio and sprocket you need to achieve the desired force and speed. My guess is you want the same force but higher speed, since the same chain probably won't take much more tension. The 850 is going to have monstrously more torque than the chainsaw, so you'll probably end up with a large sprocket to keep from breaking chains, which will result in some serious cutting speed.

[tkent02]

A GS 850 powered chainsaw? Note to self; Do not ask to borrow a chainsaw from GS1100GK

[Dogma]

Another thing, dunno if you thought about this yet: The output shaft of the engine wasn't designed to take the side loads of mounting a sprocket. You'll have to mount the sprocket on a jack shaft, then run a drive shaft between it and the engine's output shaft. Otherwise the bearings in the engine's secondary gearbox will be destroyed in short order.

[GS1100GK]

Thanks, Dogma, for the warnings.

Yes, I should have said something like, "The same delivered torque to the sprocket as 8-10hp." I figured 1:2 gearbox, ignoring efficiency, would need double the input torque, which I spoke as hp.

Not sure how wrong that may be or how much it would affect my project, but I had to start somewhere

As for the driveshaft, I was thinking about a pillow-block style setup for the driveshaft, passing through a steel housing with bearings and steel shield (chainshot anyone?) and the sprocket welded to the end.

There should be very little sideload? If I need a longer shaft for reach (clear the fender) and beariing surface, maybe weld an additional length to the end (machine shop buddy).

Do you think that will be trouble?

Oh yeah, and I was thinking about remote handlebars for throttle and clutch with foot pedal for tilting the saw.

- JC

[Dogma]

I don't know about welding the sprocket on (sorta permanent), but the rest sounds reasonable. Maybe a throttle lock and a limit stop to keep you from accidentally over revving and slinging the chain. I don't know if the clutch will be necessary. I wouldn't trust it to keep the chain stopped. I'd rather kill the engine. That'll help keep the engine cooler too.

You've got me thinking about this now. What do you have in mind for tilting the saw? Stand on the foot pedal to keep it in the cut?

[Normk]

You may wish to check out some of the logging competitions on line as they have some monster powered chainsaws using snowmobile and such engines. Some would seem to run in or above the horsepower range of a GS. The ones I can recall used very large chains as there is a practical limit to chain speed and so to the ability to remove wood. Simply adding tension to the chain (torque from the drive) fails to increase cutting beyond a certain point.

There are likely web sites which will provide some useful insight and so avoid missing important aspects of the problem.

Keep up posted, especially with pictures/video links, of your progress!

[GS1100GK]

Picture this kind of setup, but substitute GS850 instead of hydraulic drive.



Our setup will be a bit different, but there are more pics of this fellow's firewood processor here: http://woodprocessor.blogspot.com/20...-pictures.html

The pivot for the saw will likely be a small hydraulic cylinder with a two way foot pedal for control. Sort of like a hydrostatic drive foot pedal. We'll have to manually control the feed of the bar based upon wood hardness and if the saw bogs down at all.

We have 2 issues that make a hydraulic drive for the saw troublesome. First, the "right" motor is about $1k. Second, the motor needs about 5k psi to be optimum, but my dozer is only 2500 psi @ 28gpm. Funds for building a dedicated 5k psi 25+ gpm powerplant are...low

So it seems possible the GS will become the initial powerplant for the saw. The rest will run from the dozer hydraulics.

And, yeah, I almost forgot that we will have to add a cooling fan for the motor

I could strip it all down, but there's just something about having a mostly intact bike 6ft off the ground that is sort of cool- in a redneck kind of way!

- JC