CVT re Calculating Power Requirements

Discussion in 'Transmission / Drivetrain' started by professor, Mar 8, 2010.

  1. professor

    professor Active Member

    Last edited by a moderator: Mar 23, 2010

  2. ibdennyak

    ibdennyak Guest

    Phew. You got me going there Lou. Considering that I am known as the nerdy scientist/mathematician with dirty fingernails in my circles, I was surprised to find myself reaching for my calculator after reading your post.....at least until you got to the kaboodle part. Kaboodles I understand. :grin5:

    Anyway, I found your numbers to be fairly accurate, but inefficiencies are greater than what I thught they would be. I plugged a couple of my builds into your formula using *Home Depot Hill* as my test track. My 25cc Honda pusher running through a worm drive reduction (high losses) and into a chain derailleur reduction could do 11 mph. The 49cc Honda clone, using a single belt reduction system,could maintain about 21 mph, while a 49cc two stroke could only maintain about 18 mph, mostly due to being out of its torque curve I think. Also, the clone would drop to as low as 19 mph as the belt wore. The 500 watt hubmotor pusher.....I don't know, I was too busy pedaling furiously. The hill is about 1 1/2 blocks long and is almost 11% as measured by a construction laser with angle abilities. (Yeah, you had me out there setting up my laser in snowy weather with 40 mph winds. :dunce:)

    I guess the point is that numbers don't lie, but a drive systems inefficiencies are quite a bit higher than one would think. Oh, and thanks for giving me enough mental stimulation to actually get out there and measuring the hill. I can only imagine what sort of rumors will be floating around concerning Home Depot for the next couple weeks. :grin5:
     
  3. Fabian

    Fabian Well-Known Member

    Now i know why i go so creepingly slow up hills with a fully loaded trailer.

    By the maths, i need between 6000 and 9000 watts to maintain speed of 20 - 30 miles an hour up a 30 degree slope.

    Crikey, that's somewhere between 8 and 12 horsepower.
    Considering i've only got 2 horsepower at best, it explains such woeful performance.

    Fabian
     
  4. loquin

    loquin Active Member

    Exactly. In order to get the maximum performance while climbing hills, you need to get your gearing just right, so that the engine is running at maximum torque while you're at the speed indicated.

    The efficiencies I posted above were, more or less, maximum efficiencies (except for Belt CVTs. I had an email from a COMET engineer a couple of years ago that described the efficiency of a belt CVT and how it changes over time.) As gears wear, as chains stretch and bearings wear, and things 'settle' in to place, misalignments can creep into play. All of these things tend to reduce the total system efficiency.

    Fabian, a 30 degree slope is tremendous - for every hundred feet horizontally, you rise almost 58 feet! It is a 58% slope... Most ranch-style homes have no more than a 4/10 pitch (40%), which is 21.8 degrees.

    As far as the Nuvinci at 1:1 having increased efficiency - Probably, I would think. I believe the hemispherical surfaces where the ball bearings ride are not quite perfectly spherical, so that as the gear ratio increases, they are slightly closer together, which increases pressure in order to reduce slip. (Fallbrook mentions 'geometry changes' which lead to increased pressure to support increased torque transfer.) Increases in surface pressure to support higher torque transfer would also increase 'rolling friction' in the hub; conversely, decreases in surface pressure should also decrease rolling friction, and increase efficiency.

    Also, there IS a thin film of fluid between the torque transfer balls and their mating surfaces. The fluid forms what the manufacturer calls 'liquid gears,' because it increases the shear forces (torque) needed to make the drive slip. Under pressure, the fluid acts to transfer shear forcees similar to elastic solids. There is also a very small amount of 'creep' or 'slip' inherent in the design. Ref Fallbrook's 'Traction Fluid FAQ'

    It sure would be nice if someone with a Nuvinci was able to get it on a dynamometer, then, put a standard hub on the same bike/engine on the dynamometer, so we would have some real data to look at!

    It wouldn't surprise me to find that the Nuvinci hub DOES lose 10% of the available power (or even more.) It makes up for it by allowing the 'motor' (whether that motor be electric, gas, or human) to run at max efficiency over a wide range of speeds.
     
    Last edited: Mar 9, 2010
  5. Fabian

    Fabian Well-Known Member

    Hi Lou

    The is a road in my extended local area that would have to be the steepest road i'e ever encountered in my life.
    I have no idea how the road making machinery ever laid bitumen without uncontrollably sliding down the hill.

    I'll give you some quick background info.
    It's so steep, that with an unloaded trailer and the following gearing: 9T jackshaft sprocket to 44T chainwheel sprocket and 30T dished chainwheel sprocket to 36T cassette sprocket, the bike wont even climb the second steepest section of the gradient, and i'm 74 kilos (163 lbs), the bike weighs 30 kilos (66lbs) and the unloaded trailer weighs 25 kilos (55lbs), for a total weight of 284 lbs

    Jumping off the bike (running alongside), with the engine at 4,000 rpm and me pushing my guts out, the rear wheel starts spinning as i get to the steepest section.
    From there the rear wheel is still spinning and i'm lucky to be pushing the bike and trailer up the hill at a slow walking pace, leaning forward on about a 45 degree angle.
    It's a vomit inducing push and heave.

    That's how steep the gradient is.
    I've tried to find information on the measured gradient but have found no such info.

    Check it out for yourself on google maps, Australia
    The name of the road is called - Mast Gully Road in the state of Victoria.
    The steepest section is close to where it meets the Mt Dandenong Tourist Road.

    Cheers Fabian
     
    Last edited: Mar 9, 2010
  6. loquin

    loquin Active Member

    the topo map shows a section (just east of the intersection of Mast Gully and Dealbata roads) with a 20 meter rise in about 60 meters horizontal distance... for a calculated 33% slope! (18.4 degrees!) That's an awfully steep rise.

    Per the map, if you travel west on Dealbata (from the same intersection,) you have 60 meters rise in 200 meters. This is also a 33 percent slope.
     
    Last edited: Mar 9, 2010
  7. Fabian

    Fabian Well-Known Member

    Hi Lou

    That area you've listed doesn't seem to be that steep from my memory.

    The area which is insanely steep is the location between Webbs Road and Victoria Grove (just before the entrance to the Mt Dandenong Tourist Road).
    Basically it's a short 100 meter climb that's a gut wrenching, vomit inducing angle of inclination, even for cars.

    I'll attach pics with a red coloured highlighted area marking the spot.
     

    Attached Files:

    Last edited: Mar 9, 2010
  8. ibdennyak

    ibdennyak Guest

    Hmmmm, being that you're from *down under* I naturally assumed that gravity would pull you UP the hill. :eek: (I know, I know....it's late). :jester:
     
  9. Fabian

    Fabian Well-Known Member

    I wished gravity worked for me rather than against me on Mast Gully Road when travelling uphill
    I'll be going back to that location again with a spirit level in one hand and a tape measure in the other, to provide photo documentation of the severity of gradient.
    Someone who's good with trigonometry can work out the maths.

    I somehow think it's a lot steeper than a 33% slope at that specific location.

    To put another angle (sorry the pun) on the concept of trying to visualise what a pushbike must cope with on that road, look at it from the perspective of braking.
    I've got a Shimano 8" (203mm) front rotor being clamped by an Avid BB7 caliper and a 160mm disc and Tektro caliper on the rear.
    Half way down the hill (from Mt Dandenong Tourist Road) the brakes are starting to fade, and i'm trying to keep the bike speed below 30 miles an hour.
    By the time i've reached the first flat section, the brakes are smoking hot and they've completely faded and i'm doing 37 miles an hour squeezing as hard as i can on the brake lever.

    I need either twin 8" discs or a 12" (305mm) front disc rotor if there was to be any hope of maintaining full braking power descending on Mast Gully Road.

    There's no other downhill run that i've come across which has had a 203mm rotor and BB7 caliper give out on me, not to mention almost going incendiary trying to absorb heat, albeit having a trailer pushing from behind doesn't help things.

    Fabian
     
    Last edited: Mar 10, 2010
  10. loquin

    loquin Active Member

    While topo maps are a great resource, they can't show the small variations accurately. The elevation lines are 'smoothed', depending on the scale being used. The more the elevation lines are 'smoothed' (averaged,) the less information is actually contained in them. And, since the amount of information being displayed directly affects the map delivery time over the internet, it's very possible that Google Maps provides a map that has been 'dumbed down.' (A map displaying a complex terrain has more information contained within it, and needs more bandwidth to supply that info.)

    When trying to estimate the slope from a topo map, it's going to be a better estimate when you are calculating the slope over multiple elevation lines. This has the effect of averaging the results. When you do so with adjacent lines, though, you are at the mercy of whoever supplies the maps. If the elevation lines are 'smoothed,' the terrain isn't accurately represented, and there's a good chance that the calculation will be 'off.'

    In the final analysis, the 'eyeballs on the ground' are the real judge of the terrain.
     
    Last edited: Mar 10, 2010
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