New to Motorized Bikes!

[TABrinn]

Hello to all! A few weeks ago I came up with the crazy idea to put a Briggs and Stratton lawn mower engine on a bicycle and use it as uber-cheap transportation to work. I can park my truck, cancel its insurance, and get upwards of 100 mpg, if not better for a savings of nearly $200 a month! I wanted the bike to resemble a Board-Track racer from the 20's. I googled to come up with some inspiration and ended up here. Turns out, I'm not quite the pioneer I thought I was. But... there are some truly great builds on here and I look forward to learning and sharing with yall.

Naturally, I've got some questions. The only part I currently have for the build is the motor, a 4.5 hp Briggs and Stratton engine. I've been researching various performance mods on the Go-Kart forums and figure I can pull about 6000 RPM's. At a 8:1 gear ratio on 26" rims should put me just shy of 60mph. Is this speed safe on a bike frame? Will the frame hold together, welds crack, tires fly apart, etc? Thinking of using moped wheels and tires that should safely handle the speed and give adequate brakes, right? I'd like to keep the pedal drive and figure out how to pedal up to speed, and dump the clutch to start the motor. My commute is 24 miles round trip with stop lights, stop signs, and 1 overpass to climb. I would like some sort of Gear Box (Ride Mower, ATV?) to help with excelleration, climbing the overpass, and down shifting to slow down. I will need head and tail lights, turn signals, horn, etc to pass tech inspection to get onto the base where I work. Look forward to working this out with you guys!

 

[butterbean]

60 mph is not safe unless you build for it. What I mean is, bicycle tires are rated for 35, 40 max. At 60, you will be leaving a lot of rubber on the road. The tires will get very hot and they won't last long, if at all. If you're not mechanically inclined/experienced, then I don't think this is a good starting project. If you are, then have at it but be careful. 8:1 is not sufficient reduction for 6k rpm's. I run 5.2k tops and I'm geared at 12.44:1, I will redline at 32mph. If you're not taking it on the highway (which is not recommended anyway, in most states you would have to pass a motorcycle inspection to be allowed on the highway anyway), then you don't need to be able to do 60 and you might not have as much torque for hill climbing. 35, 40 mph is plenty fast. At that speed, I still recommend moped or motorcycle tires. If you look around, you may find some 26 x 2 motorcycle tires that will fit a standard bicycle rim. I believe there are also some moped tires that will fit 24" bicycle rims. Yes, moped hubs will have pretty good brakes, most likely drums. I am not sure if the lawnmower engine can easily be adapted to the bike or not, because if it has a vertical crankshaft, it will be much harder to fit a transmission on it. If it has a horizontal crankshaft, you're good to go. But there are also engine kits available, and with a 4 stroke engine and a mountain bike, you can install a shift kit and use the bike's gears as a transmission. The 4 stroke kits are a bit more expensive, but probably cheaper than a non-kit bike. Even with an American made transmission (the 4 stroke kits come with several different transmissions), they are still about $200 less than what I spent on my custom bike (about $800 to get her rolling). I can help you with setting up a battery and charging system for your lighting system, and it's not very expensive (about $75 or less), but I have no experience with turn signals except for the crappy cheap battery operated ones that break very easily. I just use hand signals, not sure if that's an option for you. One piece of advice I can give you. I know you're excited to build your first bike, and that's completely understandable. Take a breath, slow down, take the time to gather all the info you will need (it's going to be quite a bit, if you do build a custom bike it's not going together in an afternoon), and find out what parts you will need, what it will cost, and whether you have the tools and skills needed to build such a beast, then decide if you want to go that route or if getting a kit would be more suitable. If you do decide to go forward with the 4.5 hp engine, build it like you're building a mini-motorcycle, because you pretty much are, and to not build it as safe and reliable as possible is not only risking your life, it's risking others lives and the reputation of this hobby as well. Good luck.

 

[butterbean]

If you gear for 60mph, you're also going to need hydraulic brakes.

 

[Donavan321]

If you gear for 60mph, you're also going to need hydraulic brakes.

Even with hydraulic brakes, I wouldn't want a bearing blowing out at that speed or a hub seizing. You'd be in for it for sure! A front disc would be the best ideal brake to use. 60mph may be fun, but just be very careful! Have any pictures, or have you not started the construction yet?

 

[Fabian]

If you gear for 60mph, you're also going to need hydraulic brakes.

If you gear for 60mph you're also going to need good health insurance.

 

[butterbean]

If you gear for 60mph you're also going to need good health insurance.

I agree.

 

[butterbean]

Oops lol only meant to put that once.

 

[Fabian]

If you gear for 60mph, you're also going to need hydraulic brakes.

If you gear for 60mph you're also going to need a high pain threshold.

 

[Fabian]

Will the frame hold together,

maybe

welds crack,

possibly

tires fly apart,

Quite likely

At a 8:1 gear ratio on 26" rims should put me just shy of 60mph.

and just shy of a white knuckle flirtation with disaster...

Is this speed safe on a bike frame?

Only if you are wearing a super hero costume.

I came up with the crazy idea

It is not a crazy idea if you don't value you life, however, if you "do" value your life, the idea is complete lunacy!

 

[TABrinn]

I'm well aware of the potential hazards. I do want to build this properly, strong, safe, reliable, modestly budgeted, and be able to cruise along at least 45mph. Most of my commute is at 45mph but there is a short stretch of 65mph. Riding a motorcycle is dangerous. Riding a home built one is down right suicidal, if not VERY WELL executed. Please don't flame, I'm here to learn all I can from the collective knowledge here. In time I'm sure I will have something of value to contribute. I am an A&P Mechanic by trade but primarily work on Aircraft Electrical and Avionics. I am in the middle of building a 67 Ford Ranch Wagon and a 71 Datsun 240Z. I know Aircraft and cars but my knowledge of Bicycles or Motorcycles is sparse. Both car projects are stalled due to lack of funds and I hoped this would be a fun means of freeing up roughly $200 per month from gas and insurance.