View Full Version : st2 build, 650 cc 4x4 inquiry

04-07-2015, 02:05 AM
hey all.

Got the plans and the tools plus some know-how and go-to.

I'm building a standard st2 from badland buggy, all the fixings as usual. Planning to use a yamaha 650cc 4 cylinder 4 stroke. Same plans for suspension etc...

My question is are there any ways to go 4x4? I understand that I will be using a chain driven rear axle from the donor bike motor. My idea was a seemingly quick and simple solution to both the 4x4, as well as reverse capability (bikes go one way). This was to keep the 650cc chain driven rear axle for primary power, add a 125,150 or 175 cc or better (of the atv semi-automatic type with reverse) to the front portion centered around the linkage and "a arm" areas. The idea was to chain drive this as well, just as is done for the rear only with a 125 cc or so. I figured putting the rear engine in neutral and the forward engine in reverse would allow for rear movement. As far as steering while supplying power to the front axle, a double "u joint" or universal joint would couple to the wheel allowing for both side to side (steerage) and up and down movement (a arm) while power is supplied.
I'm pretty confident this will work, yet another question arises; would synchronization of the forward and rear engine during forward movement be an issue? Any ideas as to how?

Any thoughts, ideas, comments!!
I'll be posting pictures and continuing this thread once I get some feedback and charge out of this planning phase.

04-07-2015, 03:37 AM
i think you'll want to stay in the planning stage a long time with the concept if you decide to pursue it. With different engines, you'll ideally want to match output rpm through the whole acceleration range so that the rear tires aren't overdriving the front or vice-versa.

with a single engine and a center diff splitting the output as in most awd applications, when the power output is uneven (adjustable diffs or non 50/50 diffs) the feedback from the lower powered tire pair isn't directly affecting the engine. the rev output is constant. changing that can kill things, it's one reason awd cars are strongly recommend to not let tires fall out of wear too far unevenly front to rear and most tire shops will refuse to replace only one end if they're too far gone citing manufacturer requirements. enough of a diameter difference and you have enough rpm difference to start wearing things out.

with two engines the stronger one will be driving the weaker or slower one in a manner somewhat like pushing a friction-drive toy car forward. in a way it would basically be constant engine braking up front if the front tires can't keep up with the back.

not insurmountable with gear ratios and tire size differences, but even with that, it'd be really tricky to match the engines through the full rev range.

04-07-2015, 03:39 AM
Hmmm well thats an interesting idea and although I'm sure if would work I think there's some better options. I imagine you would have problems synching the drive speeds under load/shifting, and you'd have to deal with gear double fuel, oil, air, cooling, controls.

I'm in the planning stages of a 4x4 buggy and my currently favored method is to use the motorcycle engine to drive a Suzuki Samurai transfer case, which drives a driveshaft. The driveshaft is connected to 2 identical differentials (then CVs). It seems relatively straightforward to modify the Samurai transfer case to replace the low gear with a chain, giving you reverse as well as 2wd/4wd selection. Details are here. (http://www.dtsfab.com/index/index.php?topic=4128.0)

04-07-2015, 03:45 AM
I would rethink your plans. Your opening a can of worms with that build. Back in the late 80's I built a demolition derby racer out of a olds tornado that had a engine in the front and one in the rear. When I first built it I was running the back engine off a hand throttle. What a pain in the hind end to operate. One was always pushing or dragging the other one. I finally did get them on a common throttle but one engine was pretty wore out, it was much better than having two throttles but still a bugger to drive. The stuff a crazy 16 year old will do. I would look into using a large 4x4 quad engine and drivetrain if I was building a 4x4 rail. good luck and post a build thread if you try it.

04-07-2015, 05:11 AM
thats the kinda feedback i was hoping for, thanks for everyones senior opinions i value them. it does seem a little out of scale and the reach for that is far because id throw money into an engine and parts to make that all happen; whilst i might as well look into a differential with reverse capability.

4x4 capability as well as reverse are not negotiable for my build. ive seen buddies with buggies getting stuck and winches that were either too short in cable or too weak to get em out of the spots you find yourself in i.e. slopes and ravines. is there a source anyone is familiar with for sharing' that will help me out? maybe a link?
i know theres quads with 4x4 and reverse diffs. is this what i should look for?

i also have a driveshaft assembly that the donor bike utilizes. no chain just a driveline to the rear tire. i was thinking this could possibly be utilized.

but where do i stand with the u joint idea, because it seems like it would work for 4x4 front applications. necessary?
thans for everyones time.

04-07-2015, 05:34 AM
XBIRD, i understand completely what your saying. good analogy.
what options do i have with 4x4 and reverse for a mini sand rail like the st2?

04-07-2015, 01:43 PM
Buy a utv and use your fab skills to upgrade it over time.