CVT What is my calculated top speed?

Discussion in 'Transmission / Drivetrain' started by Mike St, Jul 21, 2011.

  1. Mike St

    Mike St Member

    Any experts out there, to help me choose a cvt gear system? I'm using an 11T on the cvt and a 54T on the rear sprocket for a 24 inch rear wheel. This is all 8mm. What would be my top speed, assuming a 170 lb rider, using a 54T sprocket or 63T sprocket. The 54T has a diameter of 5.5 inches, and the 63T has a diameter of 6 and 3/8 inches. Any comments appreciated. MIke S
     

  2. srdavo

    srdavo Active Member

  3. Mike St

    Mike St Member

    I don't want to become a cvt gear expert right now. I'd rather hear from an expert who is familiar with these calculations and can make them with some degree of confidence.
    My motor is the EHO 35.
     
  4. Chalo

    Chalo Member

    The EHO 35 is rated at 1.6 hp. With a 170 pound rider and a 65 pound bike (fat tire cruiser or similar), that should be enough power to achieve about 30mph on level ground with no wind-- IF the power is delivered to the rear wheel. So you should not gear much higher than that, or the motor will not be able to wind out enough to make its rated power. If your CVT will eat up a significant portion of the power as friction losses (and they all do), then you need to lower the top speed gearing accordingly.

    Disregarding the ratio changing within the CVT, 11-54 gearing with 7000 rpm on the input yields 1426 rpm on the output. With a 24" wheel diameter, that's 102mph.

    Do you have a reduction stage between the motor and CVT? What's the range of reduction and/or overdrive within the CVT? You need to come up with more than 3:1 of additional speed reduction or your system simply won't work.

    Chalo
     
  5. Mike St

    Mike St Member

    I'm using the cvt everyone uses for motorbikes and gopeds; it's the pocketbike X2 version B which has a 3.2:1 additional gear reduction inside. I could install a 54T or 63T on the rear wheel sprocket. Which one is the question.
     
  6. loquin

    loquin Active Member

    The drive efficiency of your setup should be in the 75% to 80% range.
    (.85 for the CVT belt, .95 for the 3.2:1 gearbox, .95 for the chain drive, results in a 77% calculated efficiency.) When the CVT belt is new, the belt efficiency is could exceed 90%, as it wears, it gets as low as 80%. If the chain is perfectly aligned, and well maintained, chain efficiency could be as high as 98 percent. If misaligned, and not maintained, it could be less than 90%.

    Ref the attached image. As the gear ratio calculator shows, your top end could be as high as 32 MPH, based purely on the gearing you've mentioned. And, the power calculator indicates that 1.6 HP could push you along at up to 36 MPH on the level, and no headwinds. Since you want a little extra power to help with headwinds and slopes, I wouldn't decrease the total reduction between engine and axle.

    I haven't shown the calculation below, but with the CVT, your maximum torque will be at about 11.6 mph, and 5600 RPM. This means that, assuming the same efficiencies, and the 54 tooth sprocket with a 50 pound bike/drive weight (and 170 pounds of rider) you could maintain that 11.6 MPH at nearly a 19 percent (apx. 11 degree) slope...which is a very steep mountain road.

    The 63 tooth rear sprocket would limit you to a maximum of about 27 MPH, but would increase your acceleration and hill climbing ability. But, unless you're doing a lot of mountain riding, I don't think you would need to reduce the speed.
     

    Attached Files:

    Last edited: Jul 23, 2011
  7. Chalo

    Chalo Member

    That would be a sound number if you assume drop bars, high pressure road bike tires, and an aggressive sporting rider position. I have seen no bike here that conforms to this plan. What I see here mostly is chunky (or even knobby) tires that people have slowed further with puncture preventives, mounted on bikes with upright riding positions.

    I use the power calculator at kreuzotter.de. When I calculate power requirements for a motorized bike or e-bike, I set the parameters for "roadster" because that corresponds most closely to the MBs and e-bikes I have seen.

    As speeds rise above about 20mph, aerodynamic qualities become the driving factor, and rider position makes an increasingly large difference in top speed.

    Chalo
     
  8. loquin

    loquin Active Member

    I based the calculations for the power calculator I wrote on the underlying calcs here. At speed, the bulk of the drag is caused by aerodynamic effects, with drag proportional to the square (and the cube) of the velocity. Rolling friction is by far the smaller proportion of potential losses. Now, the 'chunky' tires you mention add about 26 more square inches to the frontal area than narrow, road bike tires, and possibly a small amount of additional rolling friction. (that 26 square inches is about 2.5% of the area of an 'upright' rider, and about 3.5% of the area of a 'typical' road biker. In any event, an approximation of cross sectional area is calculated, based upon rider weight. Oddly enough, tubular tires on a time trial bike actually had about 10 percent higher measured rolling resistance than street tires on a commuter bike.

    Any engineering tool contains assumptions; I've removed (or reduced the effect of fixed assumptions) by allowing the users to enter air temperature (which affects air density, and therefore drag) as well as an adjustment for rider position (upright, forward, and reclining) and the resulting effect on cross sectional area and resulting aerodynamic drag.

    Also remember that any of these tools provide estimates of maximum possible performance; many, if not most of the bikes here are compromises, with trade-offs between top speed and acceleration. (This means that the motors are often capable of driving the bike faster than they are geared to run. This is certainly the case of my bike; it accelerates right up to the RPM limit of the motor.) The CVT helps to overcome these tradeoffs; by sacrificing some top end speed, they allow better acceleration than a fixed gear system possibly could.
     
    Last edited: Jul 23, 2011
  9. Mike St

    Mike St Member

    Well, I found a nice gear ratio calculator, which gives me the top speed, when I divide the results by 3.2. Here it is: http://www.diygokarts.com/speed-calculator.html
    With a 54T, I get 31.8 mph, and with a 63, I get 27.1 mph. I realize these are rough estimates, but they give me an excellent start. Thanks all. Mike
     
  10. loquin

    loquin Active Member

    The key thing to remember about the ratio calculators (mine included) is that they provide you with a theoretical maximum speed. They do not consider the power requirements to overcome friction and aerodynamic losses. The various power calculators do consider this, so it is useful to use both tools.

    Note that the go-kart calculator only allows 1 reduction, whereas both the ratio calculators you can download from this site allow multiple reductions.
     
  11. Chalo

    Chalo Member

    That's it.

    I find it's most useful to use a power calculator first, to figure out about how fast I can go with the net power available. Then I figure our what ratio I need to get there.

    Chalo
     
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