Anyone every use a 4 stroke trimmer motor on a bike?

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Mar 8, 2022
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There's been guy on CL selling Briggs & Stratton four stroke trimmer motors for $100, brand new. They appear to be Honda clones, the kind used on the small Honda power equipment.
Size wise they seem ideal, but have never messed with one before other than to use one on a trimmer.
Here's what I'm talking about:

I'm thinking its likely a pretty high torque little motor considering I've seen these on small rototillers.
I have only used 4stroke motors with friction drive setup’s. The Honda GX series clone motors with centrifugal clutch work well for me. I can only speak for myself. I have bought other 4stroke motors similar to the motor you are talking about. Personally they have been more problem than they were worth. Replacement parts can be a nightmare tracking them down. I have had better luck with clone motors ( from a vendor on eBay) than actual Honda motors. I live in western Washington state, and steep hills are everywhere. And yes WET too. To make things work I find that it takes a lot of things other than the motor. Tire pressure, roller size and shape, steel or aluminum frame. But when it all comes together I can go anywhere, and pull a trailer with who knows what on it. Such as (I have pics ) a 3bay stainless sink, a 12’ I beam, a small Refrigerator etc… but the one thing to remember is it’s a bicycle not a motorcycle, and 50mph tho obtainable will beat you up. The staton friction drive setup with a GX 35 works well I use a GX 50 but if it were not for the hills the 35 would be my choice. With that a good aluminum disc brake frame with a Steel fork, and good quality tires I bet you might like it. I do . But I would like to mount one midship if I knew how I could make the Honda GX fit I have seen the GXH series but they are a lot heavier
Thanks like I said I can only speak for me, I’m old but not too dumb not to listen and learn to other ideas
If the Briggs motors are like most Honda clones, all the parts interchange.
I don't know of any Honda dealers in this area, I went nuts trying to find carb parts for a Honda FG110 tiller a few years ago. I had to order everything online. I paid more for the parts than I could have bought one of those Briggs motors for on CL.

The ticket to use one of those trimmer motors would be to build some sort of metal outer drive or bearing so you could ad a sprocket or pulley and have both sides of it supported so you could run a jack shaft and some sort of gear reduction system. The driven gear would need to be fairly large since those motors turn up to 3600 rpm or more.

I played around with a pusher sytem using what was pretty much a skateboard with one mini bike rear wheel and sprocket, a 24v truck starter with the nose cone cut off and a sprocket added, and a chain, with two group 27 deep cycle batteries. I used a universal joint and an arched tow bar connected to the base of the seat post. I used a single wire and a home made thumb trigger as a throttle. The battery weight gave the 'trailer' traction. The fact that the 'bike' wasn't powered got me around the old Motorcycle laws here, (which have since changed in regards to electric power). It was scary fast, it seemingly never stopped accelerating. I think i had it up over 50 a few times but never did clock it other than I knew I was blowing by car traffic pretty fast. The drag of the motor meant that bailing off the throttle helped to stop it, since the bike had only standard caliper brakes. (80's Raleigh MTB).
The bad part was that if it did run out of charge, the only way to pedal it home was to remove the chain so it would freewheel and not drag the bike down trying to spin the motor at a mechanical disadvantage.
I messed with it for most of a summer, the batteries were used and not 100%, and I never bought it good batteries and it got set aside. I think I still have the motor but stole parts from it to fix my backhoe's starter. A buddy was playing around with using a 2hp trolling motor to power a bike using a mid drive set up but I don't think he ever finished it. The idea was sound but I'm not sure a motor like that has enough torque and it would need a pretty serious battery pack to run it for any length of time. Maybe it would work better with a proper pulse type speed controller.
I had several string trimmers stockpiled for such projects, all curb finds for $0. Then my wife bought an electric trimmer and gave all the gas models away. Oh well, plenty of other projects to do.

You can buy a brand new motor kit for bikes for $100.
I've looked at those motor kits but never liked the way the rear sprocket attached to the spokes or hub. I also see a lot of them for sale on CL not running or with bad motors. I sort of figured those motors aren't long lived over time. I figured using a name brand motor would make it reliable.
Driving it off the crank sprockets would also mean not having to deal with some messed up clamp on rear sprocket that either hangs on the spokes or clamps to the hub and goes through the spokes.
I had a rear wheel here that a guy brought me where the rear clamp slipped and it slowly pushed itself against the spokes near the hub. Since it was a dished rear wheel the left side spokes were looser right and the torque was messing with the wheel dish.
I just figured there has to be a better way. Too many of those kits look like a Rube Goldberg rig to me. Too many clamped on parts that can move or do damage if they work loose at speed.
If i were to build one I think I'd be stripping down the frame and brazing on permanent motor mounts and some sort of solid chain drive set up without clamp on parts.
I can just picture a chain coming off at speed and getting wrapped up around something locking up a wheel or worse yet, smacking your leg as it lets go.
As a kid, I bought a bike where someone had built a home made motor mount a top the remains of an old baby seat frame. They used a three speed freewheel on a Sturmey Archer three speed hub. The motor sat on the rack bolted to a piece of plywood, the chain ran out in the open down to the sprocket via a jack shaft. They aligned everything just fine but didn't account for wear or possible failure. The jack shaft was off some old machinery and had bushings not bearings. The guy I got it from was riding it when the bushings wore through, which let the chain come off, and being a long length of bicycle chain, it got wrapped around the freewheel and locked up the wheel. In doing so it swung around a few times giving him some nasty hits from behind. He showed me the stitches when he sold me the bike for $10. I remade the rear rack, used pillow blocks and sealed bearings for the jack shaft, tossed the old Clinton motor in favor of an easier to start Briggs and Stratton, used a varibble Comet clutch off a golf cart between the jack shaft and the motor, and ran heavier chain down to the hub. I also made up a few brackets that let me mount up a simply chainguard around the chain using a couple of plastic chainguards off a 400 Honda motorcycle. The only problem Ihad after that was stopping it at higher speeds with only two calipers. That was about 45 years ago or so.
The rear motor worked but building a suitable rear rack mount meant a lot of added weight, and the motor being right behind you like meant being careful of the hot motor right behind your back. It also sat too high making the bike less stable.
Things I learned back then were that if your chain is going to take piecing three chains together to get the job done, its too long, and that caliper brakes don't work well at 50mph, especially not going down hill on a 90 degree day.
I totally agree about how cheesey those motor kits can be. There are slightly better kits where the main sprocket threads on to a hub instead of bolting to the spokes. But those kits require spoking up a wheel which is beyond many builders abilities. Another option would be to use a disc brake hub and bolt the drive gear to the disk mounts. The biggest danger I see is the chain tension device. Some are a single jockey wheel on a short metal bracket that only attaches to 1 stay. Those can turn and go into the spokes. A better design is a long bracket that spans both the seat stay and chain stay. Better than that is a motor bracket that allows for moving the motor to adjust the chain slack.

Kit motors are available in 49cc (legal everywhere) and 88 cc. And in 2 cycle or 4 cycle.

Brakes or lack of. At my local bike coop I get guys looking for rear wheels after they ripped them out. The single common factor is no brakes on their motorized bikes. The brakes are in the way of some kits so they just take the brakes off. I've had a couple customers switch to electric drives of various types.

There are some vids on youtube where they use cordless tools to power bikes.
I'd like to see a drum brake hub, or a pair of hubs with a real sprocket set up. The problem then would be anchoring the brake arm somehow that won't move or eat up the frame.
I've got a few prewar ballooner frames that would be good candidates for a 'Whizzer' kit but I'm not a fan of the super cheap looking Chinese kits.
I do have an opposed twin trimmer motor out in the shed, its a Japanese brand motor from the 70's but its a two stroke. I'd highly rather do a four stroke motor. I hate mixing gas and hate the smell and mess of a two stroke even more. Two strokes belong on dirt bikes, boats, and chainsaws.

Legal wise this is what I found online here for NJ:

Low Speed Electric Bicycles:
  • 20 mph max speed
  • 750 watts or less
  • May be parked on sidewalks without blocking pedestrian traffic
  • No license, insurance, or registration required
  • Must follow all laws applicable to bicycles
  • Helmet use is required for those under age 17
  • May be pedal assist or throttle powered

Motorized Bicycles
  • Electric pedal bicycles that can achieve speeds between 20 and 28 mph
  • Gas pedal bicycle with maximum piston displacement under 50 cc
  • Require driver license, registration, and insurance
  • Helmet use is required

The problem is the Insurance requirement for gas powered bikes. I priced insurance for a bike with a Whizzer kit and they wanted $900/yr. Some were even higher.

For the electric option, 750w is about useless on a 75 lb bike with a 6ft 4in 330 lb man on it.
They wrote the law assuming a 170lb rider. and a max motor size as stated.

Any more and it needs insurance. I don't care about the license requirement, I have an MC license, but if I have to pay for MC insurance, I might as well just ride my motorcycle.

In my opinion the main reason for having a motorcycle is to save money vs. running a gas guzzling car, but here they make it more expensive to ride on two wheels than on four lately. I parked both my motor cycles when the insurance jumped from $75/yr to nearly a grand for each bike about 20 years ago. What's worse yet is I've got a perfect driving record and I'm not some teenager who just got a bike, I've been riding all my life.
I refuse to pay more for bike insurance than I do for my car or truck. Its bad enough my two vehicles cost me $225/mo for just liability here, they said once you get older your insurance should go down, that's BS. My insurance has gone up every year for the last 42 years. I paid the least when I first got my license at 16. ( I had a MC license a year before I got my car license too). MC insurance then was $24/yr. It went up to $75/yr by 2001, then in 2003 it went nuts. I got a bill for $1,2000 for my HD then. I sold the bike soon after. I put my GS1100GK back on the road in 2016 for bit after getting a decent quote from one company then 6 mo. into the policy they tripled the price, so I dropped them and parked the bike again.

If insurance was a few bucks a year, then fine, but I'm not paying a month's income just to ride a motorized bicycle these days.
So long as people just pay these crazy rates they'll keep charging them. If everyone just said enough is enough and refused to pay they'd have to fix the rates or folks would just ride without insurance.
Motorized Bicycles
  • Electric pedal bicycles that can achieve speeds between 20 and 28 mph
  • Gas pedal bicycle with maximum piston displacement under 50 cc
  • Require driver license, registration, and insurance
  • Helmet use is required

The problem is the Insurance requirement for gas powered bikes. I priced insurance for a bike with a Whizzer kit and they wanted $900/yr. Some were even higher.

For the electric option, 750w is about useless on a 75 lb bike with a 6ft 4in 330 lb man on it.
They wrote the law assuming a 170lb rider. and a max motor size as stated.

Any more and it needs insurance. I don't care about the license requirement, I have an MC license, but if I have to pay for MC insurance, I might as well just ride my motorcycle.

Apparently it's quite different here in Illinois. I found this legal opinion on a law office's site:

"...., none of the classes of motorized bicycles requires the operator to hold a driver’s license. In contrast, moped riders must possess a regular Illinois driver’s license to operate a moped.

Lawmakers took special care to insist that motorized bicycles are not mopeds and set specific laws for mopeds. They have a top speed limit of 30 mph, ten mph higher than motorized bicycles. In addition, mopeds have an engine no larger than 50cc, producing no more than 2-brake horsepower. Mopeds must also have an automatic transmission.

If your motorized bicycle has an engine exceeding 50cc but below 150cc you have a motor-driven cycle and must have a class L driver’s license. Motorized bikes with engines larger than 150cc, utilizing manual transmissions, or with more horsepower, are motorcycles. To ride a motorcycle, you must have a class M motorcycle license. In addition, you must register mopeds, motor driven cycles and motorcycles with the Illinois DOT."
It's particularly odd here in Cook county ( Chicago), because we never had a Moped designation. Previously any 2 wheeled motor vehicle was considered a motorcycle. Now I see the gas powered bikes whizzing around and the authorities don't seem to hassle them. It seems like, if it looks like a bicycle, it is a bicycle, but I haven't personally tested the limits of this, .... yet.

The gas powered bikes are considered to be bicycles, and as such, require no license, no registration, and no insurance, same as the E-bikes. I've heard them referred to as "DUI bikes", for this reason.

Speaking of the E-bikes, I wouldn't rule out the 750 watt ( or smaller) motors. The torque is available at low rpms, and is available consistently throughout the rpm range, unlike the peaky power band of an IC motor. Also, E-bikes are being supplied nowadays that rely on the controller to limit power. Some makers are putting 1.5kW or even larger motors on some of their bikes. In some cases, more power can be readily unlocked by a firmware update.

BikeBerry has been in the motorized bicycle business for 15 years, and are a good resource for parts and information on what's available nowadays:

Last, there are a few options in drum brakes on the market, Sturmey Archer offers a few and Shimano has variations of their roller brake for front and rear hubs:
I haven't seen any rear hubs yet with sprockets on both left and right side, which it seems is what one would want.
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