CVT The CVT FAQ

[loquin]

The CVT, or Continuously Variable Transmission is a device to change the gear ratio between the engine and the rear wheel in a single, continuously variable range. This is as opposed to a gearbox type transmission, where the gearing is changed in discrete steps, or to the standard motorized bicycle single, fixed ratio approach.

There are currently three types of CVTs commonly in use with Motorized Bicycles.

1. The first (and the oldest) type is the Hoffman company (HOFCO) Comet Torque-A-Verter (tm) (Hereafter referred to as 'Comet'.) The Comet is a belt driven torque converter, with adjustable sheaves (sides of a pulley.) The belt sheaves move in/out, based upon the RPM of the pulley. At low RPM, the belt drive ratio is high (about 3:1) and when the RPM increases, the input to output ratio decreases to approximately 1:1. In some models, the ratio actually drops to about 0.9:1, meaning that the output shaft is spinning faster than the input shaft. The total ratio range varies over about a 3 to 1 range.
   
   The Comet CVT is also unique in that the input pulley sheaves are far enough apart at very low RPM so as to act as a clutch. (The belt is completely disengaged.) As engine RPM is increased above an idle, the Belt will become engaged, and the driven device will begin to move. Comet CVTs have long been used in mini-bike and go-kart applications. Larger versions are also used with snowmobiles, and in industrial applications up to 200 horsepower.
   
   While the Various Comet CVTs offer the most flexibility, they are not particularly well suited for most Motored Bicycle applications. There are several reasons for this. First, they are rated to handle up to 8 Horsepower. However, since almost all bicycle applications use small engines, rated at less than 3 HP, this means that Comets are over-sized for our bicycle application, and, their (up to) 95 percent quoted power transfer efficiency (of 8 HP) is actually down to around 80% when used with a 2 HP motor, and even less for smaller engines. In addition, Comet CVTs are somewhat expensive. Finally, most Comet CVTs are sold as several pieces, rather than as a self-contained unit. This allows the greatest flexibility in the application, but it requires more in the way of application design effort. Comet CVTs could be very useful in a trailer-based pusher system, used with a larger engine, though.
   
   In October, 2009, HofcoComet went bankrupt. Per their website, the new owners (Certified Parts Corp) have plans of continuing manufacturing.
   
   Another manufacturer of small engine torque converters is MaxTorque. Their torque converters are 2-piece units, with a separate drive and driven unit. They also sell a jackshaft which bolts to the engine, where the driven unit is the input pulley of the jackshaft.
   
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2. The next alternative are the Pocket-Bike CVTs. (Referred to as CVT in this FAQ) These torque converters are VERY similar to the Comet types of torque converters, but are downsized to match-up to the small motors used with pocket bikes and scooters. These CVTs are one-piece, self contained units. (Note that pocket bike motors are also of the same size and power as those commonly used with Motored Bicycles.) These CVTs do NOT adjust enough to completely disengage the belt from the pulley sheaves, and therefore the CVTs also include a standard 76mm centrifugal clutch bell. The CVT will conveniently bolt directly to the motors. In addition, these CVTs also include a small gearbox on the output of the CVT, to further reduce the operating RPM of the output shaft. The total belt adjustment ratio of these inexpensive CVTs is in the neighborhood of 2 to 1.
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   There are currently two variations of these CVTs.
   • The older CVT version was used with X2 type pocket bikes, and has a 4:1 gearbox on the output. At low RPM, the total reduction ratio, including the output gearbox, is about 8.75:1; at high RPM, the total ratio, including the transmission is about 4:1. This type of CVT (CVT-A) can be recognized by the fact that the transmission output shaft is 'outboard' of the driven pulley shaft. (the transmission output shaft is farther away from the drive pulley than is the shaft of the driven pulley.) Ref the picture, below. As a result, this CVT is a little less compact than the second CVT flavor.
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   • The second variation of Pocket Bike CVT (CVT-B) is used on newer X2s and X7 pocket bikes. The output gearbox has an approximate 3.2:1 reduction ratio. At low RPM, the total reduction ratio is about 7:1; at high RPM, the reduction ratio is approximately 3.2 to 1. This CVT can be distinguished by the fact that the output transmission is rotated 180 degrees from the CVT-A, so that the transmission output shaft is NEARER the drive pulley shaft than the driven pulley shaft. This makes for a more compact application, albeit with a lower gear reduction ratio.
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   Refer to the Belt-CVT diagram, below, for an overview of how belt CVT's operate. It is possible to adjust the RPM at which the shifting takes place by adjusting the weights in the drive pulley actuator. This actuator is also called a variator. Heavier weights will shift outwards (and push the drive pulley sheave in) at lower rpm, while lighter weights need to be driven faster (at higher RPM) to have the same effect. This 'tuning' of the CVT may, or may not be required. An external discussion forum called gopednation.com has had a good amount of discussion regarding pocket bike CVTs. For example, refer to this discussion of pocket bike CVT overhaul, which includes a labeled picture of all the component parts of a CVT-A spread out for view. In addition, a very good description of how these pocket-bike CVTs work is located here.
   
   Note that there is a third 'CVT' that is often sold; BEWARE - It is NOT a CVT - it's really just a chain drive speed reducer. A photo of this falsely labeled product is shown below.
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3. The final CVT we will be discussing is the NuVinci CVT. This CVT is completely different from the two variable pulley CVTs we discussed earlier. It uses ball bearings to transfer power from the drive to the driven surface, and offers a 350 percent adjustment range. (Diagram below) It is built within a bicycle rear hub. Staton offers this hub as a part of a kit, which includes a motor, 18:1 gearbox, chain drive and freewheel sprocket. The gear ratio is changed manually by the user, over the entire range. While this approach is very nice, and the kit well engineered, it is a rather expensive alternative, and in some states, may not be legal to operate as a motorized bicycle, as the operator must manually change the gear ratios while driving.

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Keep in mind that, given the wheel diameters involved with bicycles, in almost all cases involving belt-driven CVTs, we will need to reduce the RPM between the output of the CVT and the rear axle. For instance, a 26 inch bike tire at 30 MPH will need to be spun at approximately 388 RPM. Therefore, for a 26 inch wheel, a 7000 RPM maximum engine speed, and 30 miles per hour desired top end speed, a TOTAL speed reduction ratio between engine and rear wheel needs to be about 18:1. If you are using a CVT-B torque converter, with an approximate 3.2:1 reduction in its gearbox, this means an additional 5.6:1 reduction is needed. (Remember, you do NOT use the 7:1 reduction quoted by various sources in the top end calculation, as this ONLY applies to low-RPM/high torque calculations.) Since Belt-driven CVTs can introduce a fair amount of drag when coasting, a freewheel is recommended after the CVT. A convenient spot to put a freewheel (AND to have convenient sprocket sizing) is by using a jack-shaft arrangement between the CVT and the rear hub. Let's assume a 48 tooth rear sprocket, an 18 tooth freewheel sprocket on the output of the jackshaft, and a 17 tooth sprocket on the CVT. The final remaining sprocket, on the input side of the jack-shaft would therefore be about 36 teeth, resulting in a total system reduction ratio of:

48/18 * 36/17 * 3.2 = 18.07​

which would result in almost exactly 30 MPH at 7000 RPM.

To get an idea of the max-torque speed (when hill climbing), let's assume that we're using a Honda GSX35 engine, with a maximum torque output at about 5500 RPM. Remember that the CVT-B has a maximum reduction ratio of about 7 to 1 (including the approximate 3.2 to 1 gearbox,) which means that this CVT has an overall high to low speed ratio of about 2.2 to 1.

The resulting calculation would then be (based on ratios of RPM and additional reduction ratio):

30 MPH * 5500RPM/7000RPM * 1/2.2 = 10.7 MPH​

or, to perform a separate calculation:

Ratio = 48/18 * 36/17 * 7 = 39.5
Speed = 5500 RPM / 39.5 * 26 * Pi * 60 / (12 * 5280) = 10.7 MPH​

The actual maximum slope that this hypothetical drivetrain could climb, (unassisted by peddling) would depend on the amount of weight which needs to be pushed up the hill.

If using the NuVinci CVT, you have to add a gear train to drop engine RPM by about 18:1 before the CVT. Staton's NuVinci kits include a motor, the NuVinci hub, and his 18.75:1 gearbox.

If you have comments, or suggestions for improvement on this FAQ, please shoot me a Private Message with your suggestions, and we can look at implementing them.

 

[loquin]

In the CVT design process, my approach has been to

1. Calculate the high end speed/gearing first (Using the CVT gearbox reduction only,) knowing that approximately 10-15% of the available power may be lost in the CVT. (In other words, knowing that you won't be able to achieve the same max top end that you could without the CVT)
2. Then, back off to the 5500 RPM range, to determine the minimum cruise speed, when the CVT pulleys get to a 1:1 state. (this RPM value depends upon the motor used, but with the small Honda/RS 4-stroke engines, is in the 5000-6000 RPM range
3. Finally, calculate the max torque situation (at Max Torque RPM,) as if you're climbing a steep hill. (You divide the 'cruise' speed, from step 2, by the pulley ratio of 2.2)

With this approach, if you wanted to hit 30 as a top end, then you wound need 18:1 total reduction, cruise would be at 23.6+ MPH, and max torque at 10.7 mph.

Now, if there are lots of hills, and your main objective would be to have great hill climbing ability, then you could work 'backwards.' Suppose that you needed to have max torque at 9 MPH. because of steep hills. Multiply by 2.2 to get the minimum 'cruise' speed of 19.8 at 5500 RPM. The gear ratio would be calculated at 21.5, meaning that you would need an additional 6.7 gear reduction (after the gearbox) on the CVT. And, with a 12T sprocket on the CVT gearbox, you would need a 81T hub sprocket(!) or, a jack-shaft. The top end would then be about 25 MPH.

I think the key thing to remember is that the CVT is NOT the cure-all. Yes, it can increase acceleration (and hill climbing torque,) but it can't do this AND maintain the same top end that you could have without a CVT. There are some losses in the CVT (on average, over the life of the CVT belt, about 10%) that will reduce the top end that can be achieved.

And, you need to get the system gearing right, before you start playing with the variator weights. Otherwise, you just introduce a new set of variables into the equation.

Possibly, the chart below will help in understanding what the CVT does for you, when the bike is geared for climbing, per the second approach, above.

Assuming that you peddle up to 5 MPH, then gun the engine. Assuming the clutch pulls in at 2000 RPM, engine speed will quickly rise to the 5500 RPM level, and will then level off, until the bike's speed reaches the minimum 'cruise' speed breakpoint, and then, since the CVT has no further pulley ratio changes to make, the engine RPM starts rising as the speed increases.

Adjusting the pulley weights has the effect of lowering or raising the 'flat' portion of this rpm-speed curve. Adjusting the system gear ratio has the effect of shifting the curve to the right or left (shifting the the first two 'breakpoints and the top speed to the right or left.)

Note that the actual curve of a CVT does not have the sharp corners as shown in the sketch - the corners are actually quite rounded, and the 'flat' portion of the curve is somewhat sloped. That being said, the concepts are still the same.

 

[loquin]

Pocket bike CVTs, once so readily available (and, as a result, at a decent price,) are getting hard to find.

Recently, I've come across two suppliers who have them listed for sale:

• the super kids
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• pocket bikes unlimited
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Both of these CVTs appear to be the CVT-B type (above.)

NOTE: I haven't dealt with either site, and so have no direct knowledge the sellers - whether the parts are actually in stock, or if they'll be back ordered. Or for that matter, whether or not the businesses are legitimate. As with all potential internet transactions, do your homework:

1. Read the website sales/return policies.
2. Search the Better Business Bureau database, using the company's website address.
3. Search the internet, using the website address and the word "Review"
4. And, last but NOT least, only pay via Paypal or other escrow service, or at least, with a credit card, so you will have some recourse (if the seller happens to be bogus!)

 

[ratdoggg]

do you know how dependable the ctv-b type is ??
have you had any experience with that small 5/1 gearbox... they seem just tooo small to work for long!!

 

[zden]

Do you have any experience with this cvt ? How many km work good and how often is necesseary change some parts.