I like your enthusiasm!!!! How did you to mount that huge engine of yours? Pics or details? I'm open to ideas, as you can't beat the HP/$ ratio of the bigger engines! Feel free to fill us in how you are using your 5.5HP engine!!!!
My 6.5 engine is modified, and ungoverned, so it's a bit faster than stock, probably about 8HP @ 5000 RPM
The 196cc 6.5 is 38lbs, compared to the 24.8 lbs of the 79cc 2.5. I weigh only 138lbs myself. If I rack mounted the 6.5, do you think I'd be able to balance the bicycle properly? Maybe putting the gas tank lowered on the bike somewhere with a fuel pump would help the center of gravity a bit. Also, I want the engine to look smaller, so it doesn't appear to have nearly 200cc's.
(This is crazy heavy mounted in that location for off road use)
My new Staton/Skyway plastic tuffwheel would handle it for sure, but I'm concerned with my safety of harnessing such a beast. However, I'm open-minded, and I REALLY love the torque of the 6.5, and I was thinking of how I'd really miss the HP by going to a smaller engine.
(You have Pedals, AND a motor!)
Heck, I may just do that, well see.
As a Downhiller and freerider (MTB) myself, I just want to bring up a couple of points regarding your proposed build. The Skyway mag you are planning on using is, at any given diameter (20', 24', or 26') VASTLY less stiff and strong than a spoked wheel alternative. This is a fact, and has been proven over and over again by everybody from ten year old kids jumping plywood jumps in abandoned lots, to pro BMX riders who never use them as a primary wheel, except in a few cases of tech oriented riders who don't go too crazy on jumps, and then only EVER as a front wheel. The skyway mag looks Tuff, and IS cool, but it's NOT a good wheel for off-road riding at all. In addition, the braking surface it provides for a rim brake is vastly inferior to metal rims for pad contact friction and in the wet will sound horrible and work even worse than it sounds. It's also important to note that a 24" rim will feel vastly less smooth rolling over uneven terrain than a 26" wheel, and lowers your final drive gear ratio noticeably as well. (Hence the rising popularity of 29" MTB's lauded for their smooth rolling characteristics and inertial qualities). Do yourself a favor and forget the skyway mag- it will cost you weight and certainly performance on a number of levels.
Also- there's a reason you don't want a 6.5hp motor chilling on the rear rack of an MTB- aside from the obvious (I would think) concern of screwing the balance point of the bike up to the point of making it handle like a wheeled turd, especially in off camber, uneven, and uphill terrain, there are very few racks made than can handle the weight of that motor, in this application. What I mean by that is- a rack that is rated to hold even 40-50lbs on pavement will quickly fail when taken off road for even comparatively mild backroads riding. Take an average fire road or forest service access road (never mind the legal implications of taking an unlicensed, non spark arrestor equipped rig on these types of roads) the average randonneur or adventure cyclist routinely breaks rack support struts just from the gentle rippling road surface of these gravel byways. Now you'll probably tell me you're going to fab up your own rack that is way stronger than the off the shelf products- and that's fine. That still doesn't address the issue of an absurd center of gravity and the complete inability to EVER bunny hop the bike over even very small irregularities in the trail, something any mountain biker must do numerous times a day on an average trail ride if they're really getting it done out there. This motor (rack mount) will also have further to fall in the event of a crash, will be tenuously mounted to a fragile rack that will likely damage the bike on it's way to failure, and most certainly leave you stranded just when you've gone further into the back country than you've ever ranged before.
I'm not poo poo-ing the idea of an off road capable MB, but I do believe there are some things that true off road riding will bring to light very quickly should you proceed with your build as planned. I would advocate the use of a steel framed hardtail bike, with a decent front suspension setup, and disc brakes front and rear. A suspension seatpost is fine if you just HAVE to have rear suspension of some kind. (At your stated weight of 136lbs this is totally elective IMHO) Also- I believe that a MB ridden in the woods will do well on fire roads and also smoother singletrack trails, but probably not hold up very well long term in truly rugged trail riding scenarios. I've seen handlebars snap in half out there, forks let go of wheels, frames cracked, stems shattered, wheels taco'd, the works. This is why lightweight trail bikes from the major manufacturers are all vastly heavier weight than even the beefiest DH bikes built to run under human power only. Check out Sikk MX, or FX Bikes to see two more good examples of featherweight class trail bikes that are rugged enough to be jumped, and CRASHED- and still have a chance of getting you home. You will see thoughtful application of some DH bike technologies and components backed up by a lot of motorcross derived technology in wheels and frame especially. Without exception, these bikes and any others I've ever seen for off road use utilize an in-frame motor mounting architecture. This is both for protection of the very important motor, but also for a safe and functional center of gravity.
If your goal is to access your fave fishing or camping spot, or a distant peak off in the hills, and you're willing to do it at a safe and sane rate of speed, I think you'll get there smiling. If on the other hand you're envisioning jumping culverts, fording streams, and riding skinnies out there, well, I guess all I can say to that is- Pics or it didn't happen!
Don't let me rain on the parade, I'm just bringing a little pure MTB rider's ethic to the criteria I think a bike like that ought to be held to if expected to work..
