Final Drive Solutions for CVTech - 6.9:1 Reduction Needed!!!

[The-Original-T]

Hi Guys,

Got a bit of an issue plenty of you will have already overcome so looking for advice.
I have an Kubota Z482 engine and CVT package from an Aixam, the CVT has an overdrive of 1:1.7 meaning that when the engine is at full chat 3600rpm the driven side of the CVT will be at 6120rpm.
I am reckoning that my bike will be able to do in the region of 65-70mph flat out based on other builds with the Kubota Z482.
My rear tyre circumference is 2081mm (160/80-16) meaning that if I ran an 11T sprocket from the jackshaft to a 76T sprocket on the rear I would be able to achieve 68.7mph with the wheel spinning at 886rpm.

This is OK (ish) and I have found a source to purchase the sprockets albeit from 2 separate places but there both 520 pitch so all is good.

My concern is that this gearing seems a little drastic and with such a small front sprocket I don't think it will last long and I'm worried about shredding chains as its a pretty tight radius to wrap its self around especially at 6120rpm!!
I have seen on multiple web sources that the for a 520 pitch chain you shouldn't go smaller than 13T although for certain makes and models of bikes they go down as small as 10T?!?!?!

If I used a 13T sprocket I would never get to the final ratio of the CVT as the top speed would then be 81.2mph!!!
This would also increase my ratio at a standing start slowing my acceleration down so its not really ideal.
I can't viably go any larger on the rear sprockets as then I'm into the custom manufacture territory then and I don't want to pay through the nose and wait weeks every time I need to change my sprocket. Also this 76T sprocket works out to be about 385mm diameter as it is which is only 20mm smaller than my rim!!!
The 76T 520 pitch sprocket is for speedway bikes and made by Talon http://finder.talon-eng.co.uk/products/tr169520/
The 11T sprocket I can get in multiple styles designed for a range of bikes from eBay or wherever.

I don't want a manual box or shaft drive just before any of you suggest it
I want to be able to utilise the full CVT range for more even wear on the pulleys right through the range and better acceleration

Have any of you come up with more novel designs???
Reduction box after CVT ect... if so what did you use???
Am I just been a pussy and should I just crack on with the above sprockets???
Thoughts???

[pietenpol2002]

Would there be value in dialing up the RPMs a bit (perhaps 3800-4000)? Many have, and with relative ease. Or would doing so require a major re-tune of the CVT to accommodate the increased RPM in the interest of a more favorable final drive ratio. Would seem to allow for a tad more flexibility.

[The-Original-T]

Hi Ron,

Thanks for your input however I think you have misinterpreted what I need to do,
If I dialled up the max rpm to say 4000 I would be in an even worse situation as my jackshaft would be at 6800rpm meaning I would need an even bigger reduction ratio.

[gilburton]

I think you should just crack on and use sprockets. They are cheap to buy and relatively easy to swap.
I can't remember the front sprocket I used on my MZ but using the standard bike rear I used something around 20T or 19T on the front.
This was with a 10hp single. It performed well enough once moving but a bit sluggish on moving off and on hills. I had planned to reduce the front sprocket 1 tooth but I never got round to it.
With your larger engine it should easily pull and be faster. I was getting about 50mph.
Why not ask alexanderforti what ratio he is using to get a baseline and you can play with the ratios once it,s on the road

[sbrumby]

Just a suggestion, as I don't have a cvt and no desire to have one. But from the ones I have seen with massive rear sprockets, it would seem the ratios needed are not quite right for the diesel engine. So just using my mechanical brain, If you have no gearbox you are likely to have a fair bit of space behind the engine for two jackshafts, this would mean the original shaft would have to move so the second could be back in line with the swing arm. This would give big variations and could be done with chain and sprockets for trials and changed to toothed belts later.

[gilburton]

The beauty of the cvt is that the rear pulley shaft is usually just held in place using 2 pillow block bearings and the pulley/shaft held in place with shaft collars.
All simple stuff and it only takes minutes to remove to change anything. Another point is that,if like me, you choose a bike that has the drive chain on the right the drive sprocket is easily removable. If,like most bikes, the drive sprocket is on the left behind, the rear pulley, it's still easy to pull it all off for a sprocket change.
Alexanderforti has the Aixam setup so ask what his sprocket sizes are?
Just tried to pm Alex but he doesn't seem to be on the site anymore? Contact Tim as he built Alex's bike and indeed has built a few based on the Aixam himself.

[alexanderfoti]

Hello, just got the PM.

Yes, I had the same issues as you, and to compound things, any calculations I did to predict the rear wheel speed where wrong.I measured rear wheel RPM using an IR Tachometer.

Not sure where it came about, but the Axiam CVtech CVT has limited ranges for tuning its operating characteristics. I have a very small front sprocket (15t) and a very large rear sprocket (76T). I use taperlock sprockets, so you may struggle to find smaller for the front than 15th. I have 428 chain.

Even with this combination, my rear wheel speed at top rpm (3600 for me) was 128mph. I had to limit both the driven and drive pulley opening by making shims. This effectively limited the CVT's maximum speed and allowed the engine to rev out (which was imperative for maximum speed achievment). Without these, with it geared to 126mph, the bike could still do 60mph but would struggle up hills.

See here for more info:
https://www.dieselbikeforum.com/view ... =41&t=3408

My rear tyre size is 100/90-18 btw so affects the calculations slightly

Out of interest, where did you get the overdrive ratio information for the Aixam CVT?

[The-Original-T]

Hi Alex,

I have read your post previously regarding the bushings and it seems a very novel solution similar to the restrictor rings used on mopeds/scooters CVT's.
Out of curiosity why put a restrictor ring in the driven pulley too? As the Driver is the one that controls the transition of the rear pulley that should be the only one needing the restriction ring.

I got the ratios straight from the owners manual that cam with the donor car, its the correct manual and has all its service stamps in ect...
see bellow photo of the page;

Aixam Manual - Ratio's

I have checked out my calculations to see if I got the same wheel speed as they quote for the Aixam car in the manual @ 1000rpm with the different wheel sizes and my math checks out within a 1ish km/h of the book figures. Based on this I'm confident with my calculations.

Seems odd that the back wheel would be going so fast on your bike??
Your rolling circumference is actually smaller than what mine will be @ 2008.1mm per rotation.
By my calculations considering your wheel size and gear ratio and assuming the ratio of 1:1.7 from the Aixam manual (assuming this is correct) at 3600rpm engine speed at full ratio your wheel should theoretically be doing 1,208rpm giving a top speed of 90/91mph.
Did your calculations look similar before getting that result of 110/126/128mph??
Are you certain of the accuracy of your IR gun? Do you have something that has a known RPM you could check it against? An electric motor or something perhaps?

I'm not disputing that the bushing didn't make any difference as it would of made a huge one and a positive performance gain, its a great solution.
I'm just wondering If the IR gun was reading correctly when you did your test?? Its always good to know your tools are functioning correctly

Luckily I have access to as many different machines I could wish for at work so I plan on making my Jackshaft and Sprocket carrier to accept off the shelf motorbike components so I don't have to machine my sprockets when I need to replace them or use the industrial taper lock stuff, not that there's anything wrong with using that stuff I just think it will make my life easier in the long run (hopefully)

[alexanderfoti]

You are correct, I had measured rpm at the rear wheel of 1280rpm roughly.

These measurements where before I installed the shims rather than after.

Also I think my circumference is a bit bigger than 2000mm, as I measured the tyre, I think it was closer to 2125 from measurements.

The gun was accurate, I had the same thoughts as you and cross referenced it to the engine tachometer and they read the same.

Through testing, I realised its important to limit both the pulleys. If I limited the driven pulley only then the driving pulley would burn rubber on the belt trying to force it up and visibly burn rubber! It would also pull the secondary shafts out of li e.

If I limited the driving pulley only the same would happen in reverse, the rear pulley would slip a little on the belt as the tension was lost.

Seems OK like this but the shims are showing a bit of dust, especially the driving pulley so I may have to make it from metal tube instead of plastic.

[The-Original-T]

To calculate the circumference of our wheels I just did:
width x aspect ratio % x 2 + rim diameter x Pi

I really need to buy my donor bike and measure the actual rolling circumference with some chalk lines and tape measure or use a tailors tape and wrap it round the wheel to give me a true measurement.
I'm just trying to do as much work up front while I save up for the donor bike so I can start the build immediately with all info to hand

If my rolling circumference is larger than what I anticipate I may look into doing what you have done with spacers/bushes in the CVT and just sacrifice some of my ratio range

we'll see..... time will tell,
Thanks for your input