Weed-whacker DIY build

[Ludwig II]

I take this as an confirmation that my concept of a bearing at the other end of the shaft will offer some relief to the crank bearings. Therefore, if the engine is used for a bike, that is what it will get.

 

[CaptainObvious]

Back in the 90's a local home DIY guy in our bike shop's neighborhood was building friction drive weedie engines for kid on BMX bikes and other bicycles.

And these were little Homelite and Ryobi overhung crankshaft engine conversions. Most of the kids with these came in for various repairs, parts, and what-not, and with my just casually looking over these, I noticed that on most of these installs the engines had some slop and looseness axially in the crank main bearings. Some way worse than others. Despite what you believe, most ball bearings, if they are in a side loading use, should be well separated. And yes these are ball bearings, a look inside my Ryobi 825r confirms this.

It's not a matter of if, it will happen with this use that they weren't designed for. Most cheap weedeaters drive either a solid or flexible shaft that applies little to no side loading. On an unsuspended rear wheel mount this engine will bounce up and down on the tire with every bump, not to mention the twisting force the drive roller will add.

The other downside of this is that the rod bearings will have accelerated wear from the mis-alignment.

I've seen it, it does happen.

I don't disagree that the moment on the bearings is not ideal in this context. As you say, they are designed to run with little to no side loading other than the force of combustion pushing down on the crankshaft a few degrees after TDC. Fortunately the loading from bumps in the road is significantly damped by the tire, tensioner, and the rubber spacers you can see in the original set of photos.

Most people don't appreciate the difference between a completely rigid mounting and one that incorporates shock absorption. The difference might be tens of Gs versus hundreds, and that difference is quite significant.

The contentious point is the actual durability of the bearings in this use-case scenario. Without a data sheet on the bearings and engineering data on the strength of the crankshaft housing it is impossible to truly know what kind of longevity I will see. As my motor is all but brand new if I may judge by the quality of compression, I am in a good position to find out. So far, I am still breaking in the engine anew on the basis that the loading profile in this scenario is very different from how it was used when it was a trimmer. Time will inevitably reveal what occurs. I will surely remember to post a note if it blows up or if I discover play developing where it should not.

 

[CaptainObvious]

I take this as an confirmation that my concept of a bearing at the other end of the shaft will offer some relief to the crank bearings. Therefore, if the engine is used for a bike, that is what it will get.

Yup. In the ideal case the bike tire will contact the roller at the exact mid-point between the crankshaft bearings and your end bearing, assuming bearings of equal strength. My physics is a little rusty, but I think the loading varies linearly as the tire contacts the roller closer to one of the bearings so exact centring is not so critical. By the same token I have positioned the motor as close to the tire as is practical to minimise the moment on the crankshaft bearings and their housing.

As a final comment I should note that there is considerable dust accumulation on the motor and inside the roller after only a few tens of kilometres. Any bearing you use as an end bearing should be sealed.

 

[Ludwig II]

Sealed is a very good point.

 

[curtisfox]

I have a 'K&S Bike Machine motor' and it has a seperate cast frame with beraings on both ends,also has a clutch bell mounted to the roller shaft. The motor just fit in and bolts to that frame,so the only load would be to the clutch bell ( about 4" diamitor ) Also a pocket bike engine fits right on it. The pocket engine i have is 50cc and i think the K&S is around 30 does a good 20 to 25 mph. It has a small roller like the one that Statin sells but the groves are V shaped and go from the out side into a V toward the center,doesent seem to eat up tires. as you think...........Curt

 

[cannonball2]

Another factor to consider with the newer engines is the design life span. I have been out of the business for several years now but one of the last manufacturers schools I attended the instructor was introducing a new Pacific sourced engine for hand held applications. I forget the exact span and Im sure it can be researched, but it was something in the 50hr range. The logic was the average user uses the unit only several hours per season, so this translated into several years. Price point is the determining factor on what you get for your $$. I doubt there are high end bearings specified. The older Homies, Poulans and such ran decent domestic bearings. It would be a simple matter to upgrade to better bearings, Boca Bearing has anything you want all the way up to ceramic(BIG$$). This defeats the idea of a cheap basic FD though. Though complicating the build a bit, running a belt relieves the problem somewhat. There was a build just done here with that design.

A step up to a well designed double supported crank engine could be at the next yard sale for little $$. If I was going to build a small CC FD that's where I would be looking.

 

[curtisfox]

For sure, the best way to spot them is most have a split crank case and the newer ones have a one piece with a cover on one end and most new ones drive off the flywheel end. A lot of the real old ones have points but there is a CD conversion you can get if they are bad..............Curt

 

[a_dam]

I've been using Homelite 25cc motors on my Continental since 2008, except for one year when I ran a 28cc McCulloch just to test out the motor (ran like a spanked cheetah).
My original Homie was from a garage-sale leaf blower for ten bucks. That thing lasted 4 years, and I always ride at least 2000 miles a year. It was a "one-side crank" type and the piston seized in the cylinder before I ever had any indication (noise) of crank bearing problems (wish I would have changed the ring before it froze up). Afterwords, by yanking on the shaft with my hand, I could detect some slop in the bearings.

Then I found another old 25 homie. I like them because they are all pretty much identical and fit right up to my setup. That one lasted about a year and a half before making noise which I do believe is from a failing crank bearing.
Found another and have it on bike now (in pic).

I have ALWAYS used a pillow block bearing on the end of the shaft. Around here we have "Blain's Farm & Fleet" where a cheap stamped-steel pillow block housing is 5 bucks and the replacable bearings (5/8 shaft) cost 8 bucks. Use only enough pressure to keep roller from slipping. If the road is even slightly wet, I just give up on the idea of using my friction drive. I have tried a grit roller once; like a belt-sander on the tire.

I recently found a few "quality" garage-sale wackers (stihl and echo) with double crank bearings and split cases. Will show them in next post.

 

[a_dam]

I have tried a grit roller once; like a belt-sander on the tire.

What I meant is that it wore the tire down excessively, like a belt sander.

Anyway, besides the latest 25cc homelite on my bike now, I have a couple of 28cc McCullochs, one which was already used on my bike and is mounted on a plate, and another running good on a weedwacker for the purpose of a spare.

I also got a Stihl and an Echo wacker for 25 bucks for the pair. Had to do usual stuff to get running - fuel lines - clean out carbs. Both run good. But I found out the Stihl fs55 is only <20cc. Don't know if I would bother using it for a bike.

The gray Echo was obviously bigger than the Stihl. Thought it might be about 30cc (can't find model number). But even the biggest commecial grade Echo wacker I saw online was 28cc. Mine turned out to be 25. That's ok because it is solid.

Then someone gave me an OLD orange echo wacker. Said he couldn't keep it running. You could see right away that it had a fuel shutoff valve in the off position. Valve was really stuck. Tried carb cleaner and penetrating oil. Had to take apart and heat a bit with a heat gun to free it up. The thing fired in two pulls and runs strong.
Just by looking you can see that its a tiny thing with a tiny carb throat. Measured bore & stroke - 14cc! The model number is srm-140da, but model nums often don't have anything to do with displacement. When I looked it up online, I saw someone had put two together to get a 28cc twin engine. It's a shame I can't use it for MB; really is a solid little wacker.

Notice in the pics, the little Echo spins in the opposite direction. That's the only motor I've got that spins that way.

 

[cannonball2]

That's the problem with Echos. They are low displacement generally. The earlier ones were as you say less than 20cc some being 18cc I believe. Homelite made 30cc variations for the 25cc. They were generally in blowers, the little pressure washer, and maybe a few other higher loaded pieces of equipment. They can be identified by not having the bridge in the exhaust port. The blowers have a longer crank which I preferred. It allowed more options of what could be done with it.

A grit type roller is very good in the wet and not too bad on the tire when wet. Has a voracious appetite for dry tread though.

 

[CaptainObvious]

What I meant is that it wore the tire down excessively, like a belt sander.

You don't need grit on the roller to wear down a tire excessively.

The small city in New Brunswick I was living in wasn't working out for me. At first, the motor on my bike was to be a convenience but then I got to seeing the possibilities. Halifax is only 300km from Moncton and what with the impending onset of winter I made a snap decision to get the **** out of Dodge. Winters in Moncton are BRUTAL. Daytime high temperatures of -20C or lower are not uncommon by any means and the snow just keeps coming and coming. Last year was the Endless Winter, and I seem to recall getting snow in mid April. Add in a lukewarm at best employment situation, a constabulary that distinguishes itself from organized crime only by virtue of wearing a uniform, and the decision to go was a complete no-brainer.

I made a little trailer with more of the angle-iron I used to make the motor mount and bolted on a couple of wheels from a kid's bike. Everything I own fits in a large MEC duffle bag and a 60-litre backpack (plus a little bit). I strapped all that crap (more than a hundred pounds) on the trailer and set out last Tuesday. 25km out I figured out that my trailer hitch was too weak so I headed back to make a stronger one. That one held.

I got about 170km in two days before one of the trailer tires wore through and popped the innertube. As it happened, the back tire of the kid's bike had a flat spot where it would skid consistently when the original owner braked. Lesson learned. Kind strangers nearby helped me find a replacement which turned out to be a similarly sized wheel taken from a commercial vacuum cleaner. It had a brass bushing to fit on its axle, slightly larger than the bike wheel axle, but it actually lasted long enough to finish the trip. I spent three nights in a park on account of wet weather, but otherwise the trip was successful. I expect to rehabilitate the trailer with better wheels and will use it to go camping along the coast in the summer.

All of this is to say that the 31cc motor made the enterprise possible. It required assistance to get up hills and a headwind would slow me down considerably, but without the motor the trip would have been impossible with the baggage. My roller is smooth and had a tendency to slip a little when the revs were in the power band. This ate most of the tread from my back wheel over the course of the trip. I think the tension was ok for the most part: Two small bed springs with a length of rope between. Attaching the tensioner with one hand is difficult; any more tension and thrust was reduced.

On level pavement the thing easily hit 40km/h. It climbed modest grades without assistance once it was in the sweet spot of the power band. Steeper grades were attacked at slow speed, and I wonder if advancing the ignition timing was a factor here as there were only a couple of hills where I had to stop and push everything to the top by foot. Total fuel consumption was about four litres of 30:1 mix.

Tire wear should be much more modest without all the extra weight and a lighter touch on the throttle. I will be swapping the front and rear tires before long and will use the bike extensively here in Halifax over the winter as weather permits.

The gray Echo was obviously bigger than the Stihl. Thought it might be about 30cc (can't find model number). But even the biggest commecial grade Echo wacker I saw online was 28cc. Mine turned out to be 25. That's ok because it is solid.

Then someone gave me an OLD orange echo wacker. Said he couldn't keep it running. You could see right away that it had a fuel shutoff valve in the off position. Valve was really stuck. Tried carb cleaner and penetrating oil. Had to take apart and heat a bit with a heat gun to free it up. The thing fired in two pulls and runs strong.
Just by looking you can see that its a tiny thing with a tiny carb throat. Measured bore & stroke - 14cc! The model number is srm-140da, but model nums often don't have anything to do with displacement. When I looked it up online, I saw someone had put two together to get a 28cc twin engine. It's a shame I can't use it for MB; really is a solid little wacker.

Notice in the pics, the little Echo spins in the opposite direction. That's the only motor I've got that spins that way.

That little 14cc motor would be ideal for a light-duty 12V gen-set. Get a used starter motor from a small car and find a way to link the shafts. I'm into microcontroller programming, so what I envision for such a thing is a motorcycle battery or similar as an energy storage device, and use the gas motor on demand to charge it. The starter motor ought to be able to turn over the gas engine, and in fact that is a factor in the design. Common practice is to drive electric motors using pulse-width modulation, and if your MOSFETs / IGBTs are beefy enough they ought to be able to turn anything. Once the motor is running, use a solid state relay to switch over to a charging phase. To control the chargeing load on the motor, a MOSFET can be driven by a DAC or any of a number of configurations to vary the load. These little motors use tiny amounts of gas at moderate speeds in contrast to COTS gen sets designed to run 110VAC loads. Such a contraption could power lights, radio, laptop computer, and charge batteries in any sort of emergency scenario and would be much more economical than any conventional gen-set.

Such a thing is on my to-do list.

 

[a_dam]

This is going off-topic "bikewise".

Every time the power goes out and we fire up the generator, there is not enough gen output to run everything in the house, but way more motor/gen output for the constant little stuff we use.

We don't have a power storage (battery) system, so all the excess power the generator motor makes is wasted.
It would be way more efficient to have motors/gens/batts rigged specifically for certain needs.