Psuedo auto tranny idea?

coachgeo · Post by **coachgeo** » Fri Mar 13, 2009 8:18 pm

idea hit me.... but im a stupid idea guy. Maybe someday I will try this.

thinking of this in reference to issue folk have brought up about using CVT's. Issue being if you get it set right to have reasonable take off speeds you end up with unreasonable cruise speeds.... and visa versa. Forcing a compromise between the two.

Well this way you have enssentially.... a low gear(s) for take off and city driving.... once need cruising speeds... change gear for that. Anotherwods a cvt drive with under and over drive available.

grrrrr.. I drew this up but no place to host it. This board SERIOUSLY needs pic posting ability.

Sphere · Post by **Sphere** » Fri Mar 13, 2009 10:48 pm

Just use imageshack. It's not too bad once you get the hang of it.

Post by **Diesel Dave** » Fri Mar 13, 2009 10:49 pm

I think we have chatted about this before.

I wanted to use a grasstracker 2 speed box with a comet for town & offroad and a hi ratio for highway cruising.

Someone responded with the fact that comet had made and marketed one of these systems some time ago in the go kart market.

All good ideas though.

Dave

coachgeo · Post by **coachgeo** » Fri Mar 13, 2009 11:44 pm

had this discussion with another Dave that builds diesel bikes and his responce was you have to select a gear box that can handle the torque of a 19+hp diesel and rpms in the ranges a descent road bike needs. Most 2-4speed gear boxes out there can't handle it, least not with longevity.

Made since to me. Im not an expert so I have to go with advice from those with experience.

Don't know if the one made for gocarts by comet could handle it either much less a 2speed gear box made for a lawn tractor.

dieselbikin · Post by **dieselbikin** » Sat Mar 14, 2009 5:27 pm

Although an additional gearbox would add weight, complexity, and friction; here is some data from my experimentation that shows what you might expect from such a setup.

Both runs have the same clutch setup optimized for the 4.62 gear. However an optimal 4.62 setup outperforms an optimal 5.45 setup.
Ratio 5.45 Avg speed 45 mph/Top speed 48 mph
Ratio 4.62 Avg speed 50 mph/Top speed 56 mph

The 5.56 did have better acceleration up to 40mph but dropped off at 45 mph. The 4.62 gear gave up some acceleration to 40ish, but leaves the 5.56 in the dust afterwards. Remember that the Comet 94C already has the ratio spread of a good 5-speed transmission (3.45-0.78) every little bit of friction and weight counts behind a 9hp engine.

1. I predict that the small difference in acceleration would be onsumedthe extra weight and friction of an additional transmission.
2. The clutch’s puck/spring combo would have to be optimized for the 4.62 gear or its top speed advantage would be lost with no real net gain anywhere.
Conclusion: Don’t bother :v)

Finally, there seems to be some confusion concerning the relationship between torque and horsepower. Any two engines that make the same horsepower at the same RPM create the same amount of torque.
Power=force x distance/time
The only fundamental difference between gas and diesel engines from a power standpoint is the much higher compression ratio in diesels. Raising the compression ratio in an engine makes it more efficient and likewise increases its torque production at a given RPM.
Transmissions are almost always rated by horsepower capacity, not torque capacity. Therefore if a transmission horsepower rating matches the horsepower rating of your engine your good to go. Hope this helps.

Sphere · Post by **Sphere** » Sat Mar 14, 2009 7:32 pm

Do you experience beltslip? What about the longevity of this setup? Have you tried overreving the engine?

dieselbikin · Post by **dieselbikin** » Sat Mar 14, 2009 8:42 pm

1. There is very little belt slip. Not enough to worry about.
2. Longevity of the belt should be pretty good since I'm running 10hp through a 30hp clutch. The belt is actually rated for 40hp.
3. Yes the engine has seen 3800 RPM on a downhill run. I'm not sure if it still would since the throttle is finally adjusted properly.

I'm running a Comet 94C/90D clutch, so this may not apply to anything else.
Bike/me 600lbs gross
94C/90D clutch (540 grams=9x60 gram pucks)
pink spring
13/60 sprockets/4.62 final drive ratio
Carroll Stream Yanclone, ported intake&head, k&n air filter, header with GSXR 1000 muffler
Various aerodynamic mods.
Top speed flat 57mph Avg 50mph/downhill 59mph (Gained 1mph with new rack adj bolt)

Sphere · Post by **Sphere** » Sat Mar 14, 2009 9:27 pm

Thanks for the info. I stopped by ebay to check, but these parts carry a smart pricetag. Like, where you could also buy an ultima 6-speeder instead.

coachgeo · Post by **coachgeo** » Sat Mar 14, 2009 9:40 pm

dieselbikin wrote:Although an additional gearbox would add weight, complexity, and friction; here is some data from my experimentation that shows what you might expect from such a setup.....

I'm not getting how your numbers give a potential expectation from the proposal. That's probably due to my lack of knowledge though.

What "gear" would the 4.62 and the 5.45 be comparable in a gear box; a close ratio 1st and 2nd gear?

Using a setup as I proposed I would think the gear changes would be first, maybe second around town and then your top gear for open highway.

First gear ratio compared to top gear ratio I would think to be a much larger ratio change than the two you posted here but..... I could be way off since Im sorta clueless.

Sphere · Post by **Sphere** » Sat Mar 14, 2009 9:45 pm

Eh? No, he means neither. His combined, final drive ratio is 4.62. He has experimented with another ratio, 5.45. Depending on what you need - fuel efficiency vs. top speed - you may pick one over the other.

coachgeo · Post by **coachgeo** » Sat Mar 14, 2009 10:01 pm

when you change to the next gear in a tranny your final drive changes right?

If so is his final drive data between the two he listed simular to stepping from first gear to second..... or more like going from the next to last gear to the highest gear on a typical bike orrrrrr??

If its like changing gears from say fifth to sixth, or third to fourth..... then thats not a comparision that fits the proposed idea.

Proposed idea is more like selecting a low range gear (say first) and a high range gear (say highest availabe). Would think that much change in gear would be a much wider change in final drive.

Sphere · Post by **Sphere** » Sat Mar 14, 2009 10:05 pm

He doesn't change gears. Are you familiar with the concept of variomatic gearing?

dieselbikin · Post by **dieselbikin** » Mon Mar 16, 2009 2:02 am

Sorry for the confusion. If we were talking about a Ford F-150 the CVT would be the automatic transmission and the “final drive” would be the rear differential. First we tested performance with a 3.55 rear differential gear; then we tested performance with a 4.11 rear differential gear. We wanted the best of both worlds so we put an under-drive unit between the transmission and the rear differential.

The Comet CVT transmission I’m using starts out 3.45/1 ratio (lowest gear) and can shift all the way to 0.78/1 (overdrive). The Comet CVT transmission is connected to the final drive through a “jackshaft”. The final drive consists of a sprocket on the other end of the jackshaft, a sprocket on the wheel, and a chain connecting the two sprockets. The 4.62/1 & 5.45/1 were two different final drive ratios I tested at different times.

I was postulating that if I had an additional two speed transmission between the Comet CVT and final drive that you might get “these” results.

I can tell you that a number of my assumptions have been disproven through trial and error.

dieselbikin · Post by **dieselbikin** » Mon Mar 16, 2009 10:13 am

Here's a picture of my jackshaft. Notice the chain and sprocket for the final drive on one side and the driven from the CVT on the other side of the shaft.

coachgeo · Post by **coachgeo** » Tue Mar 17, 2009 3:41 am

I understand where your coming from. Im just thinkng your example is not relevant enough to extrapolate out that the proposal would not work.

I'm thinking, the over/under drive of the added bike tranny would be a wider split in final ratios compared to the small change in ratios you described in your example.

butttttt... I may be wrong in thinking the two ratios exampls you described would create incrimentally small changes compared to how much change you would get say if you used a 4speed harley bike tranny as under/over drive before the CVT.

Guess we need a math wiz to crunch the numbers.

again... just trying to achieve respectfull take off speeds AND cruise speeds..... instead of the trade off you get with a CVT ONLY of slow take off with nice highway.... or opposit ...respectable take off but slow highway

coachgeo · Post by **coachgeo** » Tue Mar 17, 2009 3:46 am

Sphere wrote:He doesn't change gears . Are you familiar with the concept of variomatic gearing?

Yes he does and does not change gears; well, he manually does not..... but the CVT does it for him..... another words.... yes..... I undertand the concept of a Constant Velocity Transmission (CVT- something like that)

my proposal puts a bike gear tranny driving the CVT. gear tranny acts like an over/under drive to try to avoid the compromises you are faced with when using a CVT

dieselbikin · Post by **dieselbikin** » Tue Mar 17, 2009 6:31 pm

It could be done. Using my bike for an example, a motorcycle transmission could be placed where the jackshaft currently is. There would not be any need for a clutch since the CVT would not engage at idle.

Let’s define the parameters. I am only experienced with a 10hp Yanmar and Comet 94C. The characteristics of this clutch are tuned by its clutch driver’s pucks and spring. To achieve maximum performance the clutch must be specifically tuned to the bike’s final drive ratio. If I fine tune the clutch to operate with a 4.62 gear ratio and then swap in a 5.45 gear without re-tuning the clutch; performance will be mediocre. Therefore I must compromise my clutch set-up to operate with two different gear sets. Now I don’t have the top speed of my 4.62 gear or the acceleration of my 5.45 gear. The whole point was to avoid a compromise. Additionally we have added extra weight, complexity, cost, and friction.

My CVT is load sensing. The load is the rear wheel. This load increases going uphill (harder to rotate wheel) and decreases going downhill (easier to rotate wheel). If I change my ratio from 4.62 to 5.45 I have decreased the load working against the clutch’s driven. The CVT compensates by gearing up higher. The overall gear ratio with a 5.45 final drive ratio is effectively the same as with the 4.62 final drive ratio. To change the shift rate and change the performance of the bike I must increase/decrease puck weight and/or change spring rate in the clutch’s driven side.

As far as my bike and test data go, I don’t show a benefit from a gear lower than 5.45 or higher than 4.62. The CVT already has a very broad ratio spread and I’m only working with 10hp at best.

These problems may be less pronounced on a larger engine. I know everyone would love to see it done.

tappy · Post by **tappy** » Sun Jul 05, 2009 9:17 pm

I was thinking about doing this at one point - largely so that I could achieve useful low and high ratios, with smaller diameter cones that were more in keeping with the bike's aesthetics.
Problem as I see it is during changing from low to high:
As you accelerate from standing, the speed of the driven cone gradually increases, and at some point you go to change gear.
There's no conventional clutch so you'd need to briefly chop the throttle to unload the (2 speed?) gearbox. But given the flywheel momentum on these engines that might not be hugely effective.
As you slide out of the low gear, the driven cone becomes unloaded so the CVT shifts into the highest ratio it can.
This is awkard because as the "high" gear engages, the driven cone is going to need to slow down quite a lot. This might be manageable by just letting the engine speed drop right off, but seems like there's going to be quite a lot of belt whip and shock loading going on.
Downshifting should be fairly good by comparison - shut the throttle and slow down. Engine revs drop, driven cone speed drops. As you slide out of "high" the unloaded driven cone speeds up, and will better match the "low" gear as you engage it.

I did wonder if using a planetary gearbox with friction clutches on the two ratios could be used - rather like in a conventional automatic car transmission - to lessen the shock loads etc. Or use a constant mesh 2-speed gearbox with clutches swapping between the two speeds.

In the interests of stopping thinking up strange ideas and actually making something reliable, I've ended up heading down the conventional clutch / enfield gearbox route...

Sphere · Post by **Sphere** » Mon Jul 06, 2009 7:36 am

Haha, he used enfield and reliable in one sentence! Hopefully your gearbox wasn't made on Monday morning. Mine is alledgedly 5000km old and already requires bearing revision.

tappy wrote: In the interests of stopping thinking up strange ideas and actually making something reliable, I've ended up heading down the conventional clutch / enfield gearbox route...

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