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Thread: CVT Transmission?
08-23-2007 #1mppardee Guest
I searched this forum and read every post containing cvt and saw a few people mentioning using a CVT transmission but they don't seem very popular for motored bikes. Can anyone using a CVT comment on its effectiveness?
For those that are unfamiliar with a CVT, it is a simple transmission, usually automatic, that gives you a higher gear ratio when you are starting out. It would give a motored bike a feel more like an automatic transmission car instead of a car that only has 3rd gear.
It seems that most motored bike setups without any "transmission" are severely limited in either hill climbing or flat speed. I'm assuming a power band of a 4-stroke 35CC engine is 5000-8000 rpms, which means if I want a top speed of 30 mph I won't get good power until 18.75 MPH which is a lot of pedaling to get going up big hills.
There are some cheap ~$50 CVT transmissions available for pocket bikes:
Maybe you could get a CVT or other transmission from a moped too, but I had trouble finding moped transmission parts. I think you would need a CVT designed for a 4-stroke engine since the 2-strokes are higher RPM. I can't find any specs on these cheap CVTs gear ratios, etc.
It seems like you could get a real performance boost by adding one of these but GEBE and staton and the other major kit makers don't use them. Does anyone have experience with a CVT and advice for using or not using one?
Staton offers the nuvinci 350% internal hub but I'd rather not spend $400 on a hub that probably has no warranty for use in a motored application and might wear out quickly. A basic CVT might be "good enough" and would also be automatic. According to some laws when a bike engine has manual shifting it is no longer a moped, so an engine tied into the nuvinci hub might technically make it a motorcycle.
08-23-2007 #2just for kicks Guest
I got a project going right now for the wife that will have a CVT on it, I just need time to work on it.
There is a CVT on my Honda PA 50 and I love it! I'm sure there would be a way to put on on a Happy Time engine.
08-23-2007 #4HoughMade Guest
As soon as i get my 4 stroke up and running- the next project is to get a CVT and make it work...there is no such thing as "done".
08-23-2007 #5turkeyssr Guest
I've had the same thoughts about durability, but for me, it's mostly cost that's preventing me from 'pulling the trigger' and buying the Staton/Nuvinci kit. I want the best of both worlds, as do you, to have hill climbing ability and good top end.
Some of the pocket bike engines look interesting, but because I live in an apartment, I don't have the resources to engineer something like this without a full workshop. I think the pocketbike route would be the best bet because it's an all-in-one contained unit.
In short, I think the Nuvinci would be okay. Of course, I don't have any experience yet. Just my $.002.
08-24-2007 #6mppardee Guest
tough to fit
After more research, I'm guessing the reason there aren't more CVTs used for motored bikes is that they're tough to interface.
I found one that actually lists some specs:
The typical gearing of about 7:1 is fine for scooters, pocket bikes, etc. with a 10" wheel, but for a 26" mountain bike wheel you'd need to gear down another 3:1 assuming you want a max speed of about 30 mph. The output sprocket on the CVT could be 1.5-2.5 inches, so you'd need a sprocket on your wheel of about 4.5-7.5 inches.
I don't know of a good way to get a driving sprocket on a wheel other than the staton left-handed hub addition, which has 16 teeth and a diameter of about 2.5 inches. I can't imagine less than 8 teeth on a chain drive so that gives a secondary reduction of only 2:1, giving an overall reduction of 14:1 which means a top speed of 44 mph which means the low end is horrible even with a CVT. It says on the staton site the CVT works for 20" or smaller wheels, using a 20" would get the max speed down to 35 mph which is OK. If we could get staton to make a 24 tooth or even larger lefthanded freewheel we'd be in business and have our max speed of 30 mph + good gearing for hills. I'd think a 36 tooth would be perfect, allowing a 12 tooth sprocket on the output of the CVT.
The GEBE drive ring is about 15 inches from what I can tell, so you would need a 5 inch diameter pulley on the CVT which is way too big to fit as it would hit the engine/clutch attachment. You could probably get a 2.5 inch pulley on there, but now you've got 6:1 reduction which means 42:1 overall which is about 15 mph max speed.
I've also thought about using a disc brake rear hub, but bolt a sprocket on instead of the brake disc. Problems with this approach: 1. my current disc is 6" and barely clears the frame, a sprocket of 4.5" would still maybe not clear enough for a chain. 2. You would have to build a wheel from scratch using a disc brake hub but a standard rim. If you're using the disc attachment for a sprocket you will need to use rim brakes, and disc brake rims aren't meant to be rim braked. Any new disc brake bikes probably don't have rim brake attachments, so you'd need to start with a rim brake bike.
What about the ultimate friction drive with a CVT? You could mount the engine right above the wheel with the CVT pointed downwards connected to a large diameter roller. A larger diameter roller should eliminate slipping, but once again the 7:1 gearing on the CVT screws things up. A 1.25 friction roller gives a top speed of 30 mph at 8,000 rpms, but now with a 7:1 CVT in there we'll need a 8.75" !!! diameter roller to still get 30 mph. That is not going to fit in line with the CVT output shaft.
Bottom line, if we could get a CVT with a gear ratio of roughly 1:1 or 21:1 we could use all the standard interfacing methods that have already been developed, but 7:1 just screws things up. If someone could just develop a small gear box with the right mounting hardware and an additional 3:1 or just a 1:1 with a shaft offset to allow a huge roller or pulley I think we could have the ultimate motored bike.
I sometimes go back to the trailer idea, it would be so easy to take the rear end of a pocket bike or scooter (one with a CVT) and attach it as a trailer to push you. You could basically use it as is and get good gearing and 30 mph top speed. However, the ugliness factor and maneuverability are keeping me from going down that road again.
08-25-2007 #7custom_tecumseh Guest
I bought a Comet CVT aka Torq-a-verter, and I have an excel file that I made to take wheel size, sprocket size into account and give you MPH and RPMS over the varying ratios of the CVT, if you want it I can email it to you.
08-28-2007 #8mppardee Guest
custom_tecumseh, I sent you a PM with my email address, that would be great if you could send the spreadsheet. From what I could find on the TAV2 30 Torq-a-verter the gearing is about 3:1 at the low end to about 1:1 at the high end, which means it could go into an existing setup without changing any other gearing. It seems they cost about $200, but for a quality automatic transmission I think it would be worth it.
08-28-2007 #9HoughMade Guest
I was running the numbers for my setup and I agree- a torque converter that varies from 3:1 to 1:1 would be perfect.
I am using a Honda GXH50 with a Grubee Skayhawk II gearbox. the final drive with an 11 tooth drive and 56 tooth wheel sprocket is listed as 9.29:1. Since just the sprockets get you 5.09:1, the gearbox is good for about 1.83:1.
A 3:1 on the low end would greatly increase low end torque and take-off speed (and further stress the spokes...so that will need to be addressed)- and at the same time it would give more top end assuming there was torque to get it done, but I would assume there would be. There may not be enough torque to run flat out at a full 1:1- that would be equivalent to running a 32 tooth sprocket (actually slightly smaller), but I don't know the Honda GXH50 is pretty torquey.
To sum up- if a CVT that ran 3:1 to 1:1 could be made to work, on the bottom end, you would have equivalent to a 93 tooth sprocket on the low end and a 32 tooth on the top end- all with a 56 tooth sprocket.
$200 doesn't sound to expensive after all...then there's the 11ga spokes the Workman bikes use and the HD hub...would it still clear the crankset?...do i need more brakes....
Like I said- never "done"
08-28-2007 #10mppardee Guest
I was unaware of the vast array of grubee parts including a gear box:
but where do you buy them from and are there specifications anywhere and a price list?