A few questions for those more knowledgeable than I

[Anybodysguess]

A while back I posted on this forum asking a few questions before building a motorized bicycle.
I have since put together two using Walmart Cranbrook frames and 66cc china motors.
They are almost identical, mine has a Walmart speedometer, and my brothers has a Tabasco boost bottle.
I would say they are broken in now, I have leaned back the oil and they both have about 100-150 miles on them, and they have become quite reliable. They used to run the same, with the addition of the boost bottle my brothers is noticeably smoother and sounds better when running and idling. Ill probably add one to mine this weekend. But the main question I have is why they go so slow. Now I'm not saying they are slow, when riding them they get you around faster than I would have thought, I live about 3 miles from town and that takes a few minutes but once I'm there I can actually beat cars places because I can get in and out of traffic and take side roads and whatnot. But mine cruises at 18 quite smooth, 19 and the vibration is suddenly gets bad. My brothers cruises at about 19-20 I would guess, and then gets bad vibrations right after that. (He can ever so slowly gain on me without experiencing vibration.

Long story short I would like to go a tad faster sometimes when I have a car behind me or something and was thinking about getting 36t sprockets, but I hear of most people getting 24+ stock on a 44t and was wondering if there was something I'm missing? Should I just give it gas anyways maybe its a sympathetic vibration that goes away with speed? I've never exceeded the speed at which it starts to vibrate, except down hill with clutch in.

Otherwise we are loving them, we didn't need them, we don't commute on them, they are just for bumming around town on when the weather is nice and the schedule is free.

 

[bakaneko]

I went from the stock 44T sprocket to a 36T sprocket a while ago and documented my results below. I would say it adds about 1 mph per teeth reduction but of course you give up torque. The vibrations you are experiencing are probably due to the mounts. Check the mounts to make sure they are very tight. I have other options to supplement the mountings but it ain't pretty. Best thing is to just double check those mountings.


=======================================
Rider: light rider
Terrain: 50% flat, 40% low to moderate hills, 10% difficult hills (suburbs Wisconsin)
Bike: 26" standard mountain bike at a slightly below average build quality

44T cruise: 18 to 20
44T acceleration to cruise: 1-2 sec
44T max speed: 24 (at much as I want to push the RPMs)
44T low-moderate hill: 1 mph speed reduction
44T difficult hill: 2-3 mph reduction

36T cruise: 24 to 28
36T acceleration to cruise: 2-3 sec
36T max speed: 30+
36T low-moderate hill: 2-3 mph reduction
36T difficult hill: 5-7 mph reduction and I pedaled (but this is an insane hill)

 

[Anybodysguess]

I went from the stock 44T sprocket to a 36T sprocket a while ago and documented my results below. I would say it adds about 1 mph per teeth reduction but of course you give up torque. The vibrations you are experiencing are probably due to the mounts. Check the mounts to make sure they are very tight. I have other options to supplement the mountings but it ain't pretty. Best thing is to just double check those mountings.

All mounts are very tight, mounted to front tube with an exhaust clamp though a bent metal plate to fit large downtube, but the plates on both bikes were made with extreme care to make sure they are holding the motor perfectly straight.

 

[bakaneko]

Okay, that seems to be a custom front tube mount. Can you give us a close up picture of that mounting? It will help folks here judge if there is might be some issue with it.

 

[crassius]

My first guess is you got the pre-2012 motors with your kits. These are very cheap now because they still come with the 'bone-shaker' crankshafts. My GF has a 5 year old bike that has that and will go right around 40mph, but the vibration will leave your hands, feet, and butt numb when you go that fast.

 

[Anybodysguess]

Okay, that seems to be a custom front tube mount. Can you give us a close up picture of that mounting? It will help folks here judge if there is might be some issue with it.

What you said got me thinking, the mount looks sound, and yes it is homemade, but since the exhaust clamp is 3" higher than the motor mount maybe even though it is thick metal it may be acting like a spring board allowing vibrations.
Ill post a picture soon, I'm going to make another bracket that uses two exhaust clamps, above and below the motor bracket, allowing no flex, if there is any.

Thanks for the ideas guys, I'll report back.

 

[bakaneko]

Yeah, lets see that picture and others can judge. Also, there are universal U mounts for the front tube. The cranbrook is a very popular bike for motorized bicycles so the standard universal U mount should work great. If the source of the vibration turns out to be the custom mounting; just buy the U mount; it will save you headaches and possibly money. Just make sure to double nut the thing or friction nut it. Here is a US shipper for $7

http://www.ebay.com/itm/Universal-M...ash=item1eaa145e16:g:MPoAAOSwQTVV88lD&vxp=mtr

 

[HeadSmess]

the top mount being a piece of bent steel plate doesnt look terribly rigid to me... combined with a "Z" shape, its going to buzz like a ruler on the edge of a desk...

i cant see anything supporting the exhaust, thats going to end up breaking ports, studs, or both...

if you tried "suppressing" vibrations by using rubber under the mounts...dont. it increases vibration.


ive even found certain chains get strange harmonics at certain speeds and buzz around like a blowfly in a window...

mudguards, and all the other trimmings just add to the buzz. keep your rear end clean but add to the buzz.

 

[45u]

Boost bottle is a waste of money.

 

[sbest]

I have a new Grubee Skyhawk GT5A motor and very pleased with the top speed and (mostly) lack of vibration.
Mount the engine as rigidly to the frame as possible to minimize vibration.
There are several rpm ranges where vibes are worse than others, like riding a horse, ride through them.
My top speed varies from day to day but is in the mid to high 30mph range with a mostly stock engine.
Combustion chamber has more squish, and a different air breather with a 44t sprocket, carb leaned slightly.

Steve