twin 125cc bike engine setup help

[andrew k]

Hi, Ive run into a problem with my engine setup. Ive two 125cc 4 stroke pit bike engines. Their off two chinese pit bikes me and my brother owned. There made by zongshen are produce about 8-9hp each and there 4 speed with manual clutches. There going to go on my mini buggy that I've just started building again after leaving it sit for a year or so unfinished.

I am running a live axle on a rear swingarm and planning to connect both engines by lining the sprockets up with the rear swingarm pivots as close as possible and then connecting a chain from each engine to 2 sprockets on the axle. Is the setup ok or could one engine be holding the other back if the throttles are not exactly synced? Having a jackshaft is not really possible as 1 engine is on the lefthand side of the kart and the other is on the rhs. Any help would be greatly appreciated as i'm sure some of ye have done this before.
regards,
andrew k

(here are the engines http://www.dragos.fr/eshop/images/images_big/12531.jpg )

 

[Doc Sprocket]

The throttle thing will work out okay but I will warn you of this- If for some reason- any reason- one engine misses a shift, Something WILL go off like a grenade.
Whether the linkages aren't perfectly aligned, or a fastener fails, one tranny jams slightly, whatever. If you're flying along and, say, upshift into fourth, and yet one tranny stayed in third, the moment you let off the clutch, you're going to scatter parts like Hiroshima. If you're very, very, very lucky, you'll only snap a chain. You probably won't be that lucky. Think about it.

 

[andrew k]

Ok thanks for the advice. I can see what your saying. I'll connect the two shifters with a solid bar running the with of buggy and weld brackets onto either end to attach to the splined shifter shafts at the engines side. This should be rigid enough to make sure one tranny shifts with the other every time.

 

[DirtTrackRacer_07]

I would say two seperate clutches on your live axle so that it will give a little space for the engines just in case something bad was to happen but im not sure if it would work.

Another possibility is a jackshaft connecting the two engines with clutchs and a single chain running to the rear end.

Do you have any pictures on what your working with?

 

[Doc Sprocket]

Ok thanks for the advice. I can see what your saying. I'll connect the two shifters with a solid bar running the with of buggy and weld brackets onto either end to attach to the splined shifter shafts at the engines side. This should be rigid enough to make sure one tranny shifts with the other every time.

That would seem to be about all you can do to reduce the possibility. But in a case like this, an engineered weak link might be a good idea. Maybe converting to belt drive would do it. Or perhaps, attaching the sprockets to the jackshaft and axles using shear pins could work. Whatever it may be, you really should examine ways for the two engines to "break free" of each other in case of a sudden shock load.

 

[andrew k]

Do you have any pictures on what your working with?

I dont have them with me Ill take some more tomorrow and put them up.

Im not exactly sure what you mean with 2 seperate clutches on the live axle. Are you talking about centrifugal clutches and if so how would they work attached to the live axle?

 

[andrew k]

That would seem to be about all you can do to reduce the possibility. But in a case like this, an engineered weak link might be a good idea. Maybe converting to belt drive would do it. Or perhaps, attaching the sprockets to the jackshaft and axles using shear pins could work. Whatever it may be, you really should examine ways for the two engines to "break free" of each other in case of a sudden shock load.

thanks for the advice never thought of that and better to air on the side of caution. Actually regarding your earlier point about major damage being done if one engine upshifted while the other not shifting. Im not sure I understand why it will be so dramatic. Lets say im accelerating and am at full revs (say 8000) in 3rd gear doing 30mph. I release the accelerator, press the clutch, upshift (with one engine missing a shift) and release the clutch. Ill be still doing 30mph whilst engaging the clutch so the first engine will be in 4th, the second engine in 3rd. Will engine no 2 not just quickly return to 8000rpm and not let the buggy accelerate any further because its at max rpm. Or are you saying as the clutch is released the quick change in rpm will snap a chain etc. ?

 

[DirtTrackRacer_07]

You would have two seperate chains running from both the engine down to the live axle.

On the axle, you would have two seperate centrifugical clutches instead of just a solid mount which your sprokets would mount to.

On the centrifugical clutches you could weld your sprokets onto them or make a mount on the outside of the cluthes so that you could change sproket if needed be.

That way it would give some room in the rpm range, but im not completely sure if it would work.

I just thought of it and put into the table.

Another possibility could be a belt drive of some sort as toystory_4wd was saying.

 

[Doc Sprocket]

It may not be that dramatic- but then again it may. I just keep thinking that there would be a sudden shock in the driveline, as one engine would suddenly be spun up (or down for that matter) into an RPM range it wasn't in a half second ago. It's all theory of course- I've never heard of anyone actually building something like this.

As far as two centrifugal clutches go, I don't think you'd want the additional slippage at lower shaft speeds, AND, if those shaft speeds are low enough, the clutches would never lock, burning themselves up.

 

[andrew k]

It may not be that dramatic- but then again it may. I just keep thinking that there would be a sudden shock in the driveline, as one engine would suddenly be spun up (or down for that matter) into an RPM range it wasn't in a half second ago. It's all theory of course

Its all theory of course. Only thing is when i was driving the bikes i took the engines out of, once in a while you would miss a gear accelerating and release the clutch in 3rd thinking you were in fourth and all that happened is there was a slight jump as the engine was brought back up to rpm. I presume another engine wont really change that but for safety's sake i will try and incorporate an engineered weakness.

I've never heard of anyone actually building something like this.

Ill let you know how i get on

 

[Russ2251]

Start a build log (lots of pics) and keep us apprised.
Having built a twin engined kart I am curious as to the outcome and how you get there.
Sounds to be a linkage nightmare.
Good Luck.

 

[redsox985]

How about both throttles, clutches, and shifters are connected as one, but use a small stub axle on each side. This way if one goes, nothing will fail AS badly.

 

[daniel7250]

mate, do you really need the 2 engines. 1 engine with some simple mods would be far easyer. plus these engines have gears so even tho they put out 8-9hp thats geared so its much better then say a clone or anything. simple mods are things like, a flat side carby, that will give you better bottom end power. a lighter rotor kit. or even a inner rotor kit. iv worked with bike engines for years and it would be a much easyer setup just to have 1 and you will still have sufficient power to get your grin on

 

[Russ2251]

I built/ran single engine karts for years.
I just recently finished a twin build (both engines mildly modded for increased power and RPM) and the difference is day and night.
I love the feel of getting slammed to the seat when the power curve kicks in.

 

[andrew k]

mate, do you really need the 2 engines. 1 engine with some simple mods would be far easyer. plus these engines have gears so even tho they put out 8-9hp thats geared so its much better then say a clone or anything. simple mods are things like, a flat side carby, that will give you better bottom end power. a lighter rotor kit. or even a inner rotor kit. iv worked with bike engines for years and it would be a much easyer setup just to have 1 and you will still have sufficient power to get your grin on

Yes. The kart will weigh roughly 250lbs-350lbs and I want to have enough power to drift around corners etc. as i will mainly be using for off road. The last thing i want is to put all this work into something (not that i dont enjoy it) and be ultimately disappointed with the performance. Ill post some pics soon

 

[daniel7250]

fair point, cos drifting is much fun . especially that close to the ground. aslong as its all set up perfectly it will work , just alot of mucking around to get it perfect .

 

[andrew k]

the is the frame after being cut to extend it about 15 inches in total

this is a block i made on the lathe to extend the axle from 34 inches to 41 inches wide. i will also be making wheel spacers