CVT and final gear ratio

[Sphere]

I'm wondering about CVT and final gear ratios.

If I read the messages on this forum right, Fiddler en jeremy take different approaches. Fiddler just leaves the final drive on his sportster to what it was. jeremy on the other hand employs 1:7 because to compensate for the 1:1 (or even 1:0.9 overdrive) CVT ratio.

The Adelaide uni BioBike doesn't seem to have a enormous rear sprocket, although the Husaberg does come with a 52 tooth rear sprocket I think. Then there's balboa_71 who says 1:7 is overkill and 1:4.5 should be fine with a CVT.

Can anyone comment on what is "the right path"?

[oldbmw]

I would suggest you have abike with easily changed ratios. You can theorise forever without finding out what is best. When you ride it you will know what you like. For me, ( and this is just theory) I would suggest if the unit runs happily without undue wear when not fully engaged it does not matter much, but if when the thing is slipping it is wearing a lot best be a little undergeared.

[Fiddler]

Sphere,

We are all on the 'right path'.
Balboa has set his final drive ratio correctly for the CVT unit he is using. Any higher ratio and he has found that his belt will slip on take off and going up hills.
Jeremy is setting his final drive ratio on the advice of his supplier and it should be hopefully ( give or take a tooth ) good for his 500 series unit.
Myself and others, who have used the Comet Duster CVT, can get away with much taller gearing. This is because this unit and belt is much more substantial and less prone to slipping under load.

So you are now probably wondering why we dont all use the more substantial product. Well you dont get nothing for free, the downside being extra initial cost and also this unit is very bulky, so its both butt ugly and can be difficult making the ergonomics fit, works ok on a cruiser style machine but would cause headaches on a trailie.

So there you have it, different spec CVT's can handle different loads, simple as that.

[Sphere]

Alright, I understand all this, but for a final drive, isn't the topend of your motor most important? Likely the max rpm?

To make it more easy to understand my confusion:

jeremy uses a 6horse 500 comet 1:7 ratio
biobike uses a 10horse 500 comet 1:4 ratio (ish)

Just seems strange to me...

[dieselbikin]

I am currently in the process of changing my final drive ratio from 3.85 to 4.60. I am satisfied with my top speed, but need more acceleration on the bottom end. Currently, at 60 MPH, my CVT ratio is 1.1 . With a 4.60 final drive my CVT ratio would be 0.92 at 60 MPH. I would still be able to get as high as 72 MPH on a steep downhill. Don’t laugh, I’ve done 69 MPH on a steep decline. Granted these figures are based on the bike’s speedometer, they should be in the ballpark.
I also have a tachometer, so I can verify engine RPM and use this in conjunction with the speedo to determine CVT ratio at a given speed. Additionally, I’m about to install a bicycle computer to keep track of top speed, average speed, ect. The computer has an accuracy of .5 MPH. Here is a mixture of my personal experiences and what I’ve learned from others.

My bike:
1980 Suzuki GS550/Yanclone L100
10HP@3600 RPM
7HP@3000 RPM
17.8 ft/lbs@1500 RPM
16 ft/lbs avg 1500-3600 RPM
CVT ratio spread 3.49-0.78 (Comet 94C)
Final drive 3.85
Tire 130/70/17 (24.17” Diameter)

General rules of thumb to the best of my knowledge.

•To double speed requires quadrupling horsepower
•Reduce weight to improve acceleration/Improve aerodynamics to increase top speed/Both effect fuel economy
•Rule of thumb 10 HP = 60 MPH/7 HP = 45 MPH
•Clutch and gearing optimized for 45 MPH may not allow 60 MPH/ Clutch optimized for 60 MPH may be more sluggish to 45 MPH.
•Leave some gearing to pick up speed on a downhill
•Use a good spreadsheet t o try out different ratios
•Dynamometers measure torque and RPM, not horsepower
•hp = (torque * RPM) / 5250
•Horsepower and torque are not interdependent
•Maximum MPH will be reached with engine RPM at peak horsepower and final drive gearing that maximizes torque multiplication.
•Maximum fuel economy will likely be reached at 45 MPH.
•Yanclones were designed to be most efficient from 3000-3600 RPM
•3000 RPM=7HP=45MPH
•Too low idle can oil starve engine

First determine your requirements and then what need to get there. I wanted:
•60 MPH cruise speed
•Option to use biofuels that was readily available to me
•Ability to make 70 mile round trip to work

I highly recommend using the lightest bike you can get your hands on. I have a 1974 Honda CB200 for the next round. Be weight conscious and do not underestimate the importance of aerodynamics. Clutch tuning is critical as is an intimate understanding of how your CVT works and responds to different inputs. Keep detailed logs using a bicycle computer or GPS data logger. Engines and bearing will loosen up over time and improve results. You must choose between economy and speed. Have a thorough understanding of the relationship between horsepower and torque. Understand the relationship between engine speed and gearing. Hope this helps.

[dieselbikin]

Sphere,
I reread your question and realized I danced all around it. You are correct that the peak horsepower RPM is important to top speed. Most of the industrial engines we use make peak horsepower at 3600 RPM. This assumes the correct gearing to provide maximum torque multiplication.

It’s really this simple; It takes a X number of horsepower to push vehicle Y to speed Z.

Using my bike with a 7 HP and 10 HP engines and our previous HP/MPH rule of thumb.

325cc = 7 HP @ 3600 RPM = 45 MPH (3600 RPM, 0.78 CVT ratio, 7.38 final drive, 45 MPH)

406cc = 10 HP @ 3600 RPM = 60 MPH (3600 RPM, 0.78 CVT ratio, 5.50 final drive, 60 MPH) Power

406cc = 7 HP @ 2250 RPM = 45 MPH (2250 RPM, 0.78 CVT ratio, 4.65 final drive, 45 MPH) Economy

[Sphere]

Alright. Hrm. In your experience, does do CVTs hog horsepower, or is that only the case when used incorrectly?

[dieselbikin]

The simple answer is that an application correct and properly tuned CVT may consume a little more power than an equally well suited manual transmission, but the CVT more than makes up for this by allowing the engine to stay at its optimum RPM at all times. Also, no human can shift a manual transmission as efficiently as a CVT shifts itself.

Now that’s apples compared to apples. In reality most of the stand alone manual motorcycle transmissions are made for 40+HP engines. The friction and rotating mass of a transmission tend to increase with its torque capacity. This especially holds true with most of the low tech HD and machinery transmissions diesel bikers use. Additionally, our engines have a narrow power band and require many gears and much shifting to be efficient. Finally, if you’re starting out with a bike that didn’t come with a standalone transmission, a CVT is easier to find and install. I paid a total of $60 for my 94C system and $150 for my 500 system buying used parts on Ebay. On the other hand, if your bike originally came with a standalone transmission then why wouldn’t you use it?

In conclusion, it really comes down to which transmission will make riding most enjoyable for you. Both the manual and CVT guys are getting great mileage.

Advantages of the CVT
Engines do not develop constant power at all speeds; they have specific speeds where torque (pulling power), horsepower (speed power) or fuel efficiency are at their highest levels. Because there are no gears to tie a given road speed directly to a given engine speed, the CVT can vary the engine speed as needed to access maximum power as well as maximum fuel efficiency. This allows the CVT to provide quicker acceleration than a conventional automatic or manual transmission while delivering superior fuel economy.

http://cvt.com.sapo.pt/performances/performances.htm
http://cvt.com.sapo.pt/performances/CVT_vs_MT_calc.htm
http://craig.backfire.ca/pages/autos/main
http://auto.howstuffworks.com/cvt5.htm

[dieselbikin]

I have finally found the sweet spot for the final drive/clutch combo. I am using a 13/60=4.62 final drive ratio. This is giving me a 56MPH top speed on a flat, 63MPH steep downhill, and 50MPH average over my relatively flat test course. The gross weight of bike/me is 600lbs. My old combo had good top end and decent average speed, but poor take off due to belt slippage. Take off is now greatly improved without sacrificing top end or average speed. I can now move on to fuel economy testing.

New bike stats:
1980 Suzuki GS550/Yanclone L100
10HP@3600 RPM
CVT ratio spread 3.49-0.78 (Comet 94C/90D)
Pink spring/ 9X59.9g pucks
Final drive 13/60=4.62
Tire 130/70/17 (24.17” Diameter)
Gross weight 600 lbs
Top speed:
Flat 56 MPH
Downhill 63 MPH
Avg "Flat" test leg 50 MPH
Fuel economy unknown
Idle speed 1400 RPM
Clutch engagement speed about 1700 RPM


Hope this is helpful.

[jeremy]

Sphere, this thread may be so old it's moldy and no longer of interest to anybody, but I just re-read the posts and realized that my particulars had been misrepresented.

My final drive ratio is: 14-tooth jackshaft gear tied to a 70-tooth rear sprocket, amounting to 5:1--not 7:1. Perhaps that takes some of the mystery out of the debate... I wish I had the detail to report but I am struggling to get to the "beta-test sequence" part of my project.

[Sphere]

Hi jeremy, that sure demystifies it. Thanks for clearing it up. I'm probably going to shoot for 1:4 - 1:5 with a Hatz 1B40.

[jeremy]

Sphere--I checked my notes and the 7:1 ratio suggestion actually came from "Don" at Comet Mfg. I guess I chose optimistically when I elected to go with 5:1 which I think may give me a top speed of around 67 mph. downhill at least. The speed at 1450 rpm should be 6 mph plus-or-minus, and around 10.5 mph at 2400 rpm. These speeds seemed reasonable to me in terms of whether my primary belt slips a lot or not, so I went with it. We'll see...

[jeremy]

The speed at 1450 rpm should be 6 mph plus-or-minus, and around 10.5 mph at 2400 rpm.

--Engagement or lowest-ratio speeds, that is...

[Sphere]

I will be very interested to read about your results. For now I will stick what comes with the chassis (Enfield gearbox) but the 4-speeder has a gap between 3rd and 4th that may cause problems climbing slopes.

CVT would certainly make such parts of a journey more enjoyable.

[Diesel Dave]

Sphere,

If the gap between 3 & 4 on the Enfield box is bothering you, I did post up instructions of how to swap a close ratio gearset.

It's an expensive swap but much longer lasting than the CVT, belt's will wear out and the clutch sliding parts are non-replaceable too.
Luckily the Comet types don't rely on the crankshaft splines like little scooters, When (not if) they shear off it's time to replace the crank.

Dave

[Sphere]

Hi Dave, I agree with you observations, but it is important to know when this would happen. Having to replace your comet CVT after 50k miles is much less of a problem, than having to replace it every 10k miles.

Thanks for pointing me to the close ratio gear set topic. For now, there's too much jargon in there for me, but I will cope if I decide that the standard gearing doesn't suffice. As you know Holland is mostly flat, so I hope I can manage.

[Diesel Dave]

Andy has an original diesel MZ Robin.

From his experience the belts last about 3000 miles then need adjusting (distance between pulleys).

His advice was to run 3 belts then adjust the distance and run the same 3 belts again, then repeat for the last time then buy new belts.

He says it's easy to know when they need adjusting - the bike loses top speed!

I suppose it really depends on how many stop/starts you do as the belts act as the take up clutch, scooters have a seperate centrifugal clutch to do this task. Even so my 180cc Piaggio needed new belts every 8000 miles but it did make 20Hp

[Sphere]

Hrm, comet also has a centrifugal clutch range in their shop, maybe that would be better.

[roodesign]

Hi there, I was just wondering, with all this talk of final drive ratios and all, don't we need to know the rear tire circumference to make sense of these numbers (or perhaps everyone is using the same rear wheel/tire?)...I ask because on my trike project, I am using a tire that is 19.1" actual diamenter and motorcycle tires are a lot bigger...any info would be great...thanks...Roo

[Sphere]

Yeah, that's important, but I have entered it in my calculation.

[Crazymanneil]

You may find this useful -

http://www.rbracing-rsr.com/gearspeed.html

Neil