About ready to give up on MB's

[Wolfshoes]

Using a stronger steel in the mounting studs certainly helps, but not enough to offset the main problem of the studs not fitting the frame tubes at a 90 degree angle. With the side pressure and the vibration almost a guarantee of problems. There is a few things that can help to make the motor the universal fit it was intended to have out of the box.

For the rear mount. The clamps around the seat tube could be nutted on both sides of the clamps with rubber washers like plumbing viton on both sides of the tube clamps to allow the clamps to match the angle of the tube and reduce side pressure on the studs.

If one stud is broken; to delay the tear down and removal of the broken stud or if thee stud is not removable there is a plan B to save the motor.

This picture is from the bottom of the motor looking up. One stud is broken and its removal attempt has been delayed. The stud has been replaced with a bolt to hold the clamp together. The other stud is doing the work. This may sound like a dorky solution, but stay with me.

If you look close, the stock bolt that normally is used to attach the stock chain cover is through a steel strapping material which is bolted around the seat stem. Now there is attachment points for the rear engine mount top bottom to the seat post instead of side to side. This is an option to make the mount more universal to the angle of the seat post. All china girl kits seem to assume the seat post angles to the back at more of an angle than most bike frames. The rear mount of the kits should have been adjusted to a more common fit long ago. I broke a rear mount stud last summer and have many hours on this fix. It seems to be holding up well. At some point, the broken stud will be addressed, but even so the added upper clamp will remain.

What looks like chewing gum near the top of the chain is actually Loctite S10 non sagging polyurethane masonry sealant that lines the drive sprocket housing to prevent the chain from cracking the casting.

This is what worked for me. Safety concerns may not be at a comfort level where it would work for you. As for the front mount; a picture of the situation would be helpful.

 

[FurryOnTheInside]

Using a stronger steel in the mounting studs certainly helps, but not enough to offset the main problem of the studs not fitting the frame tubes at a 90 degree angle. With the side pressure and the vibration almost a guarantee of problems. There is a few things that can help to make the motor the universal fit it was intended to have out of the box.

For the rear mount. The clamps around the seat tube could be nutted on both sides of the clamps with rubber washers like plumbing viton on both sides of the tube clamps to allow the clamps to match the angle of the tube and reduce side pressure on the studs.

If one stud is broken; to delay the tear down and removal of the broken stud or if thee stud is not removable there is a plan B to save the motor.



View attachment 77071

This picture is from the bottom of the motor looking up. One stud is broken and its removal attempt has been delayed. The stud has been replaced with a bolt to hold the clamp together. The other stud is doing the work. This may sound like a dorky solution, but stay with me.

View attachment 77072

If you look close, the stock bolt that normally is used to attach the stock chain cover is through a steel strapping material which is bolted around the seat stem. Now there is attachment points for the rear engine mount top bottom to the seat post instead of side to side. This is an option to make the mount more universal to the angle of the seat post. All china girl kits seem to assume the seat post angles to the back at more of an angle than most bike frames. The rear mount of the kits should have been adjusted to a more common fit long ago. I broke a rear mount stud last summer and have many hours on this fix. It seems to be holding up well. At some point, the broken stud will be addressed, but even so the added upper clamp will remain.

What looks like chewing gum near the top of the chain is actually Loctite S10 non sagging polyurethane masonry sealant that lines the drive sprocket housing to prevent the chain from cracking the casting.

This is what worked for me. Safety concerns may not be at a comfort level where it would work for you. As for the front mount; a picture of the situation would be helpful.

I take your point about the V angle of frames being a variable and it causing some thought to be needed to get the mounting right, but
In my not expert opinion, I think the rear mount is easy to get the bolts perfectly perpendicular to the seat tube if you shim the tube and then bolt it up snug first, before then going to the front and making the best you can of the front mount.
It's only the front where you should have an angle issue to deal with. That would be easy enough too, by bolting a plate to the front mount, with a bend in the plate that allows the plate, not the engine mounts, to be U clamped to the down tube.

 

[bakaneko]

you can try the ghetto chain method if you are really desperate to get that engine back on the bike. it will get you where u need to go until you fix it or get a different mode of transportation. the chain method will cost about $5-7. let me know if you want to know what it is. again, this is super cheap and a band aid. you should also go much slower as well

 

[JunkyardDog]

When you use Chinese frame mount motor kits, something like this is always going to happen. They are badly designed, badly made, out of very poor quality material. First, get your car back now, get a rebuilt alternator, install it, and never ever loan your car to anyone again. I made that once, and something a lot worse than a bad alternator happened. Next, save up your money until you can afford a decent bicycle motor. Get either a Honda GX35 or Robin-Subaru motor, and either a Staton friction drive kit, or a GEBE kit. Stay far away from those cheap Chinese kits. Not only will they keep breaking, but they can get you badly injured or killed. Remember you are pushing it anyway by putting an engine on a bicycle, as they were never designed for that. Keep you speed below 20 mph. If you can't live with that, get a scooter or motorcycle, or just drive your car.

 

[gary55]

Pay no attention to the Dog behind the fence. He is inherently bitter, and delusionally convinced in the sole correctness of his own opinion.

 

[Wolfshoes]

In the other MB forum site Motorbicycling.com; the Russian engines D-4, D-5, D-6, D-8. History thread; has some pictures of the original Russian bikes our China girl design is a copy of. Unfortunately, what we have is too close of a copy with the common early D4 design. The early Russian frames have the far back leaning seat frame tubes used as the rear engine support. The original Russian engines were only 33cc. and therefore probably used a small frame. Is it fair to say a far back leaning seat tubes are normally found on short wheelbase bikes? Therefore since then to this day the china girl were factory made to correctly fit an exactly matched short wheelbase frame. To install the motor on a longer frame is to invite a problem of snapped studs, front and rear. To the despair of serious hobbyists and those wishing to make high RPM racing bikes, the simple mounting design was never made to be a universal fit. It is my opinion that the side stress on the studs cannot be adequately relieved by tapered shimming or by using a better steel.

One of the last of the Russian D models, probably made in the 1980's, appeared to have a different way of supporting the rear of the engine. As pictured in an earlier post to this site taken from the other site, a close look at the fastener placement on a more sturdy than usual motor chain guard suggests the engine is rear supported and adjusted for chain length by the chain guard and not the seat post. This is one way to create a universal fit.

Mid 20th century American made Whizzers, I have not studied but it may be safe to say they were made to fit the commonly available Schwinn and Huffy frame seat post angles. The basic rear support design may be ok if the angle is correct. I saw a picture of a late 1940's Whizzer with a subframe metal downtube for the front engine support suggesting they were struggling with how to cope with not having a consistent angle for a front engine support. Perhaps even Whizzers were not made to be a universal fit in my opinion. The risk of injury from the failure from a complicated universal front engine support was too great so the expected downtube angle has been preset in the engine casting. This important specification has not been published or been part of an installation requirement to my knowledge.

It is possible to take the side pressure off the mounting studs by making a subframe engine support.

With this solution, the engine is front mounted to the exact angle it was cast to receive. The rear mounting studs are not used or present. Instead the subframe hugs the seat post regardless of its angle and is adjustable for drive chain length so no tensioner is needed.

The bike is pictured with my latest mod, a smaller pedal sprocket from mbrebel.com matched to a SRAM two speed rear hub for better low speed pedal performance. This mod also offers the opportunity to lower the engine for a better fit in the frame.

 

[Timbone]

I think it is more than OK to voice frustration with the motorbiking hobby. It's not for everybody. Maintenance is full time. Even if you weld things into forever position, you'd better check that weld everyday! Even if you do achieve Valhalla and built a very dependable unit, thieves can get it in a hurry!

Sure, fixes are cheap and every issue can be addressed in time. My philo is to check everything all the time and address any pending issues. The problem will just fester and cause bigger problems in a very short time.

My recent experiences: Stopped at a coffee house and observed some slight wear in the motor chain. Inside, I ordered a new 415 chain online. About a half mile from home - chain break! The master link broke, so i pedaled home. In the garage I had an extra masterlink, so it was a quick fix.

Riding home the next day, just as I approached the driveway, my clutch cam popped up out of the case - no clutch! The little pin inside the sprocket case broke - no way I could have foreseen that!

About a week before, my exhaust was loose - very strange because I made a killer bracket to support the heavy thing. One of the studs had lost its threads due to vibration. Just be be safe, I'll remove the stud. Thing is so tight! Double nut removal failed. I ground in a slot for a flat head screwdriver; that failed, too. I ground the end into a square and my vice grips couldn't get it. So, I will live with it for a while. I removed the washers and tightened it down to the exhaust. With the remaining metal I used a punch and pushed the metal up to the nut, making it a kind of rivet. At some point it will fail and I will have to EZ OUT it.

On top of that I built a new wheel using a Halo SAS rim. That was quite an ordeal and I am experienced wheel builder.

This hobby is a commitment. A struggle. It's not for everyone. Geez my hands are filthy and stained.

 

[gary55]

The newer engines with the 8mm studs will help this problem. A lot of guys pay more attention to how the engine looks setting in the frame on first builds. Many have little mechanical knowledge when they dive in, and pay less attention on how squarely set the mounts are to the tubes or how well the engine is cradled to relieve constant stress on the poor little 6mm studs. It just takes a while to catch on to our mistakes but eventually they are caught and the probs. diminish. I know a lot of you guys have built a bunch of these bikes, and I'll bet you seldom if ever have problems with the mounting studs on your builds. It's almost always the new guys, as we all were once.

 

[Frankenstein]

The newer engines with the 8mm studs will help this problem. A lot of guys pay more attention to how the engine looks setting in the frame on first builds. Many have little mechanical knowledge when they dive in, and pay less attention on how squarely set the mounts are to the tubes or how well the engine is cradled to relieve constant stress on the poor little 6mm studs. It just takes a while to catch on to our mistakes but eventually they are caught and the probs. diminish. I know a lot of you guys have built a bunch of these bikes, and I'll bet you seldom if ever have problems with the mounting studs on your builds. It's almost always the new guys, as we all were once.

Nice that most people can easily acquire an 8mm tap and an appropriate drill bit, all that anyone would have to do is spend 2 minutes on the mounts and they could have very very reliable mounts..

 

[Wolfshoes]

Regarding the observation that china girl engines are still being cast to fit Russian D series frames; there could be an exception to that. I seem to recall the recent batch of Grubee Skyhawk engines being cast for larger downtubes with a shallower upward slope. The engines may also have been cast to match the more commonly found more upright seat post than the D series frame, but I do not know that. If true, in most common wheelbase builds, the Grubee may be less prone to break mounting studs than the other engine makes. What engine you buy may best depend on the frame you are installing it into. The engine seller should be providing the mounting angles to the buyer to help match the fit with the selected frame.