CVT Honda GXH50 & Comet torque converter?

Discussion in 'Transmission / Drivetrain' started by hlombard, Apr 1, 2008.

  1. hlombard

    hlombard New Member

    I'm planning to install a Honda GXH50 on a Delta trike (2 x 26" wheels at the rear).

    1. Will the Comet TAV2 Torque converter fit on the Honda ? I'll need a bushing to go from 5/8" to 3/4" shaft, but is there space to mount the TAV?

    2. Can someone confirm my speed calculations?

    - Tav have reduction ratios of 2.7:1 at slow end and 0.9:1 at fast end.
    - Tav has 12 tooth sprocket and I have 48 tooth for axle
    - TAV kicks in at 2,200 rpm
    - Honda has max rpm of 7,000

    What is my minimum and max speed whith 26" wheels?

    I tried this but get crazy numbers.

  2. HoughMade

    HoughMade Guest

    You will get crazy numbers because you would need to gear this down more.

    A jackshaft that reduced it another 3:1 would result in an approximate top speed of 50 mph at 7000rpm, assuming no wind resistance or friction losses- in reality, a top speed in the 40s.
  3. loquin

    loquin Active Member

    The gearing suggests 15.7 MPH at the low end... 150 MPH at the high end. Obviously, you'll run out of oomph from the engine LONG before you get anywhere close to that. As Houghmade suggests, add more reduction. Really, 4:1 or 5:1 additional reduction wouldn't hurt. Especially with a trike. At high speeds, rolling a trike in the event of an emergency maneuver becomes a real possibility. At 5:1, you could take off from a standstill, and have outstanding acceleration up the top end of about 30 MPH. 4:1 would give you good acceleration and a top end near 40 MPH.
    Last edited: Apr 1, 2008
  4. reclaimer

    reclaimer Guest

    I have read (over in wild in the streets) the tav2 is only good up to a 20" wheel. I however have zero experince with them. hope to find out different.
  5. loquin

    loquin Active Member

    If it's hooked up directly, that would be true. But, we're talking about adding a jackshaft between the output sprocket of the CVT and the rear sprocket, to further step down the rear wheel RPM.
  6. reclaimer

    reclaimer Guest

    I always assumed reducing the rear wheel rpms to a suitable level would make it horribly slow. Top speed in the 40s isn't slow.

    Is the goal to get the jack shaft to turn near the same rpms a correct sized wheel would?(edit, or the rear wheel maybe)

    Good luck with your project, an please keep us posted on how it comes out.
    Last edited by a moderator: Apr 1, 2008
  7. HoughMade

    HoughMade Guest

    An engine has to operate in a certain rpm range to make enough power. In the case of the Honda GXH50, that would be between 3500 and 7000 rpm. Due to wind resistence and friction, you cannot simply reduce the gear ratios and reap higher and higher top speeds. The goal of a jack shaft is to reduce or multiply the gear ratio at whatever rate is needed to keep the engine in the optimum rpm range at the speed you want.
    Last edited by a moderator: Mar 27, 2009
  8. loquin

    loquin Active Member

    Suitable wheel RPM for a small wheel (pocket bike or scooter) is MUCH higher than suitable RPM for a 26 inch diameter tire. Think if it this way. When a 10 inch diameter tire makes one complete rotation, the wheel moves forward 31.4 inches (by the circumference.) When a 26 inch tire makes one complete revolution, it will move forward by it's circumference, which is 81.6 inches! Both tires have made one revolution, but, the 26 inch tire has gone 2.6 times further... This means that a 10 inch (outer) diameter tire must spin 2.6 times faster than a 26 inch tire, just to travel at the same speed.

    for a 26 inch wheel to be traveling at 40 mph, it needs to be spinning at approximately 517 RPM. If an engine is running at 7000 RPM, then, between the engine and the wheel, you need to reduce the RPM by a factor of about 13.5.

    Since the Comet CVT actually INCREASES the output RPM at the high end, (by apx. 11 percent,) the output shaft is actually turning at about 7780 RPM at the high end.

    So, to get the final wheel RPM, the total reduction between the Comet and the wheel, you need a total additional reduction of a bit over 15 (15.04)

    Adding a jackshaft with an input sprocket with 48 teeth, (assuming an 11 tooth sprocket on the Comet,) an output sprocket with 14 teeth, and a 48 tooth sprocket on the wheel would result in a 14.96 additional reduction. That 14.96 reduction, coupled with the .9 reduction of the comet at high rpms, results in a total reduction, engine to rear wheel of about 13.46, which, at 7000 RPM, would put your speed at a bit over 40 MPH.

    Let me reiterate, though. On a bike, 40+ MPH is fast, but manageable. On a three-wheel bike, it verges on the dangerous. You can lean into a turn on a bicycle. You can't lean a trike into a turn. It is much more susceptible to flipping when you turn, especially an emergency turn.
    Last edited: Apr 2, 2008
  9. loquin

    loquin Active Member

    FYI. ref the excel spreadsheet here (within the Zip file)
  10. reclaimer

    reclaimer Guest

    Wow I figured a 26" would go a bit faster than a 20", didn't realize it was such a dramatic difference.

    Thank you for explaining that so well. I think I'm starting to get it.

    What is it that causes damage to a centrifugal clutch when going up wheel size? Is it that the gearing changes so much it cant make enough low end power to engage the clutch proper?(previously I had thought it was the added mass of the wheel, learning is fun)

    I was playing with that spreadsheet last night(found it in another of your posts) pretty neat and I'm sure it will be of great value to me. Once I get my head wrapped around it anyway.
    Last edited by a moderator: Apr 2, 2008
  11. loquin

    loquin Active Member

    A centrifugal clutch can only transfer so much torque before it starts to slip. The slippage is the clutch shoes moving past the clutch drum. This wears the clutch pads, and to a lesser extent, the clutch drum. You rev the engine to speed up, and the motor is putting out more torque. If the sprocket/gearing ratios are too high, the pads on the clutch cannot transfer enough of the torque through to the axle and the clutch slips.

    A larger wheel, this reduces the overall engine to road speed ratio. Because the circumference is larger, the tire doesn't need to rotate as fast to maintain the same ground speed as a smaller tire. The engine is therefore running at slower RPMs to maintain the same ground speed.

    The higher the gear ratio, the greater the acceleration, as there will be more torque available at the rear tire. (Discounting frictional losses, the result of RPM time Torque is the same at the engine, and at the rear axle, and at any point (gear or sprocket) in between. Low RPM equals High Torque, and vice versa.) Since a higher acceleration means that the bike gets up to speed faster, there will be less time that the clutch slips before getting up to speed. Less time spent slipping means less time grinding away at the clutch shoe pads and subsequently, less wear...

    On the other hand, lowering the ratio, (increasing rear wheel diameter lowers the ratio) also lowers acceleration, which increases the amount of time spent slipping the clutch...

    You have to make a conscious decision as to as to which factor has a higher priority for you - better acceleration and hill-climbing ability, or a greater top end speed. That way, you can come to the right compromise for your situation.
    Last edited: Apr 3, 2008
  12. hlombard

    hlombard New Member

    More questions

    Thanks for all the help.

    I've made my own spreadsheet that does the same thing - see attached.

    But what speed should I shoot for on the slow end? I like the "takeoff from standstill" one. The CVT kicks in at 2200 rpm. With the suggested jackshaft that means 4 mph. Is this slow enough for takeoff from standstill?

    If the calculation results in 40 mph - what real speed can I expect on a flat road & 200 pound rider? 90% ? or more?

    I probably need sprockets that can free wheel - so I don't have the pedals spinning or don't have to swing the dead engine when I want (have to) pedal?

    Where do I get them?

    Attached is a picture of the Delta Wolf I plan to build. This guy has an electric on it. Thats to easy!

    Attached Files:

  13. sparky

    sparky Active Member

    Nice tip! I would have figured a 3-wheeler would have more stability, but I didn't think about cornering. Thanks for the advice!
  14. reclaimer

    reclaimer Guest

    Thanks for another thorough explanation.

    Might I ask how high is considered too high?
  15. loquin

    loquin Active Member

    according to Comet, you need to add gearing in the final drive so that the CVT can get out of the 'slip' mode (where the in and out pulley shives are not at the high end.) If you don't, the belt slips more, generating friction/heat, which degrades the belt and shortens its life. Also, as the belt wears, the power transfer efficiency goes down.

    Peak torque on the GX50 is at about 5500 RPM. Determine your cruise speed (not the top speed) and add gearing so that at 5500 RPM engine speed (divided by .9) the rear wheel is spinning at the RPM needed to roll at a little less than your cruise speed.

    Assuming a cruise of 25, a 26" rear wheel needs to spin at about 323 RPM. So, the total reduction between CVT and rear wheel is 18.9:1

    at max HP, (7000 RPM,) the speed then would be approximately 32 MPH, and at max rpm (7500) the speed would be about 34 MPH.

    at max torque, going up hill, the bike would be running at about 9 mph. It would just about climb the wall!
  16. chrustie

    chrustie New Member

    Thank ye, oh great, illustrious Gullooom. Chrustie