4 Wheel Bike 6.5 hp horizontal shaft engine


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1:13 AM
Jun 24, 2008
Thanks or this great forum. I have been looking, searching, on here for about 2 weeks and I am still so lost.
I want to mount a 6.5 HP Electric Start horizontal shaft engine with a go kart clutch to a 4 wheel bike for my parents. I would get a kit from one of the vender's, but they don't have electric start, and my parents need it, to old to do the pull start.
The bike has 18" tires and is very heavy.
How would you guy's tackle this ?
At first I thought I could just put a go kart sprocket on the back wheel, size 35# chain to the go kart clutch and send them on there way, but know I know that won't work. Will go to fast and won't be able to take off, be like trying to drive off in a car in 5th gear. Is this true ? I was thinking a jack shaft setup. Go from the small gear on clutch to a big gear then back down to a small gear, then up to a big gear on the back wheel, But I have no idea what size sprockets, or chain to use. ???
Then I was thinking, why don't I just weld a car exhaust pipe to the clutch and drive the real wheel bye friction. Will that work ?
All they want to do is go about 15 mph max at 1/2 throttle.
Sorry for this long post, it's my first time using a forum hope it's ok.
This is the bike I am trying to do this to,
http://internationalsurreyco.com/store/images/Red DX Surrey.jpg
I will post a video of what I am trying to do also.


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Check this link, especially the newsletter archive. These guys build 3/4 to 7/8 replicas of old cars, but they power them just the way you want to. You should find some good info.

I would consider #41 chain rather tha #35 as it is stronger and you are dealing with more than typical go-kart weight. Also, think about using a Comet Torq-A-Verter which is a continuously variable transmission made for these small engines. It would allow a nice low ratio for starting from a stop, but allow a little speed as well. A jackshaft lowering the ration may still be needed due to the size of the wheels. I think a top speed of 20 mph is probably fast enough (which would put it at around 12 to 15 at half throttle), so just use that speed, the engine max output speed to coincide with that speed, and work backwards to see what ratio is needed.

For instance, 18" wheel (if that is the outside diameter) has a circumfrance of about 56.5 inches or 4.7 feet. With 5280 feet in a mile, the wheel would revolve 1123 times in a mile. 20 miles would be 22,460- so in one hour at 20 mph, the wheel would revolve that many time. This equates to 374 revolutions per minute. A normal small engine of around 6.5 hp maxes out at 3600 rpm (adjust for your engine). This means you need to reduce the engine speed by 9.6:1 to get the approximate 20 mph top speed (15 mph cruising). This can be done with a jackshaft that takes a 10 tooth drive sprocket from the clutch to a 16 tooth sprocket on the shaft- a 10 tooth on the other end of the shaft to a 60 tooth on the axle. With a larger sprocket on the shaft from the drive, you could have a smaller axle sprocket.

Here are jackshaft kits:


You may have to look a bit for the right combination of sprocket sizes.

OK- if you want decent low end pickup and to cruise comfortably, let's look at the Torq-A-Verter. Here is amodel that has a low ratio of 3.13:1 and a high ratio of 1.4:1. I'm going to ignore the low end ratio for now and bring it up later. If this provides a reduction of 1.4:1 at high speed, a 60 tooth axle sprocket gives you a final ratio of 8.4, a little higher than you probably want. However, with a jack shaft that takes the Torq-A-Verter output (already reduced 1.4:1) 10 tooth to a 16 tooth, then with a 14 tooth to a 60 tooth on the axle, you get about 9.6:1 (1.4 x 1.6 x 4.29 = 9.6). Back to the low end- remember the low ratio is 3.13:1. This means that at this ratio, the engine hits its power peak (assuming 3600) at a little under 9mph, meaning it would have decent pickup from a dead stop.


I don't necessarily endorce these vendors, it's just that they have the stuff.

Good luck! Sounds like a neat project.
You might want to spend some time studying the various drive components offered by Staton_Inc http://www.staton-inc.com It seems to me that they have the parts/kits you will need to make your project go smoothly.

I just watched your video and again I'd look into the Staton 18.75 :1 gearbox, mounted to your engine and then a chain to the freewheel on your rear hub. Staton's prices are higher than other vendors but their quality is worth it.

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You mention electric starts - it is possible to find electric starts on this class of engine - you just need to know where to look.

Look at pocketbike sites - they often do a electric start conversion kit for 49cc engines.

However since it sounds like you are doing a two-up idea it would be well to know that while you can get a small engine to put out 10hp without too much issue it wont have that high a torque output which for shifting weight is what you need. In the case of my bike I have 1.6hp and something like 1.2nm max torque - this is fine for a bike plus 130-odd lbs of rider, but it would labour trying to do anything but flat roads or surfaces with more than twice that payload

I would say that your best bet might be to indeed look at the staton bits... they have a lot of transmission parts and knowhow and this is pretty much a custom build..

There is another option I have been playing with in my mind

its a little complicated but basically as follows...

Get hold of two hardtail bikes of the same model and two of the same engine kit - this in standard form should give with a 2 x 67cc maybe in the region of 5hp power (more if you fit up tuned pipes). fit a jackshaft to both and use the rear bike gears to provide transmission ratios (left and right shifters both x-speed or fit a cable splitter). You would need to pipe the exhausts through to a large resonator silencer (maybe a car type) and fit a larger fuel tank. You'd need to route the braking to either a pedal or dual twin levers and therefore a cetrifugal clutch would be needed (these can be obtained for chengines).

The bike pedals cranks would need to be removed on the inboard side as you would then weld the bikes together with enough space for driver/passenger in the middle (sort of like a cyclecar arrangement - one behind the other) and the exhausts (tuned pipes) routed to the outside of the frame.

This system gives some advantages in that the drivetrain is duplicated so if one engine has a problem you have a 'limp home' facility on the other and using premade bike-frames means you are sure of dimensions and such and given that you would be sitting between the frames gives a measure of protection from debris and idiot BMW drivers lol. With a bit of ingenuity it could also be fully weather protected as well.

It would be a complicated build but would certainly result in an interesting and unique machine :)

Jemma xx
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note to self - look at the video *before* you come up with something.

However alot of the previous post still stands - what you could do is set up the engine underneath the rear set of seats either to one side or the other or centrally. You then run a chain to a derailler cassette on the driven wheel which gives you anything up to a 9 speed transmission although 7 speeds would be better (more space for a beefy chain) which is controlled via hand lever or even a floor shifter (make an lever extension to a standard click-lock shifter).

are you going to use this as a one up or two or more up machine? because you will need to think about ratios. If you gear the bike to max out at 30mph on the flat with one rider you will get adequate performance with 4 people on it. If you assume a person to be about 70kg then 4 times that is a significant difference even if the bike is light and as you say that is the last thing you can say about it.

Since I am assuming that the persons who will be using this will be older people it might be an idea to pipe the exhaust to a secondary silencer because a motor like that buzzing away is the quickest cause of a headache and also the best way to interrupt a conversation.

I would suggest that a centrifugal type clutch is the way to go but would also suggest that a heavy duty design would be a good idea - a fried clutch is not a good thing.

Also considering the size of the rig and the people using it - it might be worth fitting up some hub motors and batteries for a 'get home mode' since trying to ride my cycle back 2 miles completely exhausts me and I am young...

Jemma xx
why not electric

Given that your parents are unable to handle a pull starter, the intended low speed and all of the complications of designing and building a gasoline powered system for this bike, why not just add an electric motor? The batteries can even help lower the center of gravity.
Thanks everyone for your help. HoughMade you seem to know your stuff, thanks for all the infor on gearing, but I am still very lost doing the jackshaft !
Today I tryed to drive the pasenger tire with the go kart clutch, the bell part, "friction" and it worked, but it is still to fast, 1/4 open on the gas and it is going about 22 mph, So I think I need to go up in size maybe make the bell 5" or 6" round. some type of steel pipe ? One big thing that is not good, the clutch gets very HOT on the tire.

Or my friend has all the parts to a Razor MX500 with a 500 watt motor, and the Razor Razor Dirt Quad has a 350 watt motor w/ gear reduction, http://www.razorama.com/razor-dirt-quad-motor.html
I am still so lost.