Your opinion on top speed/RPMs

Discussion in '4-Stroke Engines' started by conradcliff, Nov 4, 2008.

  1. conradcliff

    conradcliff Member

    Ok, so I think I have finally figured out exactly what I want to do for my build.
    My question to you is... With the GHX50 having this power curve:


    ...and a setup basically exactly like the SBP setup what will be my maximum possible speed at what RPM.

    I'm trying to figure this out so that I can choose the correct gear ratios before I purchase the gears.

    With the help of Jim Donovan at I have been able to figure a lot of things out..he is a real stand up individual; I would recommend doing business with him to anyone. He pointed me to a speed calculator on his website which I pretty much totally revamped. My best guestimate is that after the 6000 RPM range the torque loss in the curve is going to be the major limiting factor preventing me from going any farther.

    Here is a picture of my calculations with what I am guessing will be the bikes maximum speed(about 225lbs total is my guess of the weight):


    (I could be way off though as I really don't know what the heck I'm talking about..also, I don't intend to ride at these speeds, I'm just making sure that if I can and I want to then I'll be able to so please let's keep the yelling about me killing myself to a minimum :lol: )

    Remember, this is the absolute maximum speed I think I might be able to get close to, if you think I might be able to get to a higher speed at 6K RPMs then I will reduce the ratio to reach that speed, but if you don't think I will be able to get any where near that speed then I will increase the ratio to reflect the median speed that everyone thinks I could reach. Additionally, if you think I might be able to reach that speed but that it will be closer to 7K RPMs I will change the ratios accordingly and so on and so forth.

    Here is a link to the speed calculator just in case anyone wants to use it.

    I think that's about it for now, thanks a ton for any help and happy ridin! :cool:

    P.s. I fixed the typo in the calculator...
    Last edited: Nov 4, 2008

  2. HoughMade

    HoughMade Guest

    I doubt that you can reach 53.4 mph with a stock GXH50 no matter what the gearing if you are an average sized individual. What your calculations leave out a couple of factors. First, wind resistance increases as the square of the velocity. Second, the frictional forces of all the moving parts increases as speed increases as well.

    What all this means is that assuming the optimum gearing, the horsepower required for a given speed will be about the following (depending on weight of bike and rider, tire type, etc.- I used conservative figures):

    1 hp = 29 mph
    2 hp = 36 mph
    3 hp = 42 mph
    4 hp = 46 mph
    5 hp = 50 mph
    6 hp = 53 mph

    No matter how you gear it, I really doubt 2.5hp will get you over 50 mph.

    With higher gearing that would mathematically allow for over 50 mph at 6000 rpm, the engine will simply not have enough power to allow the engine to get to 6000 rpm.
    Last edited by a moderator: Nov 5, 2008
  3. conradcliff

    conradcliff Member

    Excellent! This is exactly what I needed. I have a friend that I was going to call and see if he could do a calculation including an average amount of loss in the system, the weight of the setup with me on it and any other factor that would require calculation.

    By your numbers I'm looking at about 40mph.

    Excellent post, thank you very much!

    Edit: I went to the speed calculator website and it gave me a speed of about 45mph for a unsuspended mtb. 40mph is probably going to be pretty close.
    Last edited: Nov 5, 2008
  4. Unsubstantiated speed rumors

    Hi Hough, well said Amigo! To the OP Conrad I can say this. The gx50 Honda is an amzing little performer. I ran mine about 45 miles with the little "box" exhaust, and the amount of carbon on my frame is absolutely minimal.

    I have operated a "Silent Power" manual clutch bike now for only about 55 miles. I now have an "EZ-Pipe" exhaust system on it, and not only do I have the expected performance, BUT a great sound also.

    My experiance is this, Start with no pedalling if desired, cruise 15, 25, high cruise 30, so far best speed in 1/3 mile is 32. Rider in the 300 lb class, I geared the bike "High" intentionally, as I wanted to reduce vibrations, and limit the speed (yes ulterior motives)

    Now for the "rumor". Somebody, someplace is operating a HS Silent-Power, with the EZ-Pipe, 24" balloon tires instead of the 26 I'm using, rider in the 170 lb class, and he reports back with a MEASURED (Whizzer Speedo) speed of 37mph, same gearing (almost) but substituting the smaller 24" wheels, bike weight would seem to be about the same, that is Basic Cruiser, springer front, large saddle.

    Maybe no-one has mentioned this to you yet, BUT, If you want motorcycle speeds, you should have a motorcycle.

    It is important to do a good job on motorized bicycle, ride and enjoy it, BUT equally important not to screw up everyone elses rights by building intentionally just to be speeding, and potentially injure yourself, and other peoples freedoms.

    Remember also, your tires, wheels, and brakes are not rated over 30mph anywhere that I have found, and I don't know how good of a frame you intend to use, but, therein lies another potential problem. Consider this,bicycles are designed and built for specific speed, and NO vibrations. We come along and do VIBRATIONS and some SPEED.

    Build carefully,

  5. conradcliff

    conradcliff Member

    I've been looking for an exhaust solution, where did you get yours?
  6. EZ-Pipe

    Hi Conrad, we make them. I'll check, and see if we are ready to release any yet.

  7. conradcliff

    conradcliff Member

    Excellent, thank you.
  8. Irish John

    Irish John Guest

    Somewhere on a thread in 4-strokes I reported what I got with a HS engine & grubee gearbox using an 11T drive sprocket wth 3 different rear sprockets.
    With the 44T I hit 65 km\hr and with the 56T I got 43 km\hr and with 48T I got 50 km\hr on the flat. The 44T was useless on hills and into a wind and I had to change it. The 56T revs the guts out of the motor and reaching a comfortable cruising speed created unpleasant vibrations. The 48T was the best but a 50T would be preferable if it was easily available.
    The Honda is different and the 48T suits it much better. Tonight I was cruising comfortably at 50 km\hr and reaching 55 - 58km\hr quite often without vibrations. I was pulled over tonight by the police for taking up too much road and not riding on the edge although other police have told me that a cycle has the right to the same side of the carriageway as a car. I didn't debate the point and just apologised. I was returning from hospital where I'd had my toe sewn up after ripping it to shreds when my hub gears failed and my foot slipped off the pedal onto the road and my shoe came off.
    The 48T sprocket is the best for the Honda but a 10T drive sprocket changes everything. Don Grube told me he will make a 50T sprocket standard on new kits because the 56T is just not right at all unless you are over 70 and live half way up Everest. These rigs are a trade off between various factors and if you want to pull off from a standing start you can't really do it comfortably even with the 56T. You really need to pedal a bit to help clutch out. When my pedal chain comes off as it is doing a lot right now because I have a problem with my Nexus 7 spd hub, I can still drive 30 miles without it but I have to avoid very steep hills. It's not good for the motor but the Honda can go up pretty steep inclines with no pedals at all but it's only to be done in emergencies and the engine will protest big time.. A 50cc single geared Honda has it's limitations and speeds of 54 mph are out of the question unless you have a streamlined fairing, weigh 5 stone, have thin profile tyres and have a hurricane blowing behind you and are using a 36T sprocket. Then you'll need to change your sprocket to a 48T to get home. The higher the gearing the higher the speed at which the engine falls below its power band and any hill will become a real nightmare. I've tried it and it's fun for about 30 minutes but the juddering soon takes the joy out of the experience.
    A well aligned motor and dive chain and really good wheel bearings will make a big difference on any bike.
    That's my 2 bobs worth.
  9. conradcliff

    conradcliff Member

    Thanks for taking the time to write that long post, it's some good info but for me I don't think I'll have to worry too much about any single drive gear seeing as how I'm creating a setup very much like the SBP setup and will be able to shift gears to give me a top speed running at 6K rpm's of 16mph to 45mph (if the engine will take me that fast). From the info I've acquired thus far I'm thinking that my setup will be able to propel me just past 40mph and I decided to gear just above that so that I'll have usable power on a downhill just in case I wanted it. At my highest gear I should have far more than enough power to pull me up any hill against just about any wind. I'm pretty excited to see what happens when I get this thing put together!
  10. Irish John

    Irish John Guest

    I see now - you're using a jackshaft to transmit the drive to the right side. I don't know how you will get an extra sprocket on hub gears but the shimano 7 speed hub would be a good one to try. Perhaps you won't have a pedal chain at all but that would make your bike into a motorcycle. I would think hub gears would baulk at the force from Honda on a Shimano hub or any geared hub at all. The Sturmey Archers can be fixed when they break much more easily than the Shimanos. A derailleur & freewheel wouldn't last long I reckon if you drove onto that and the chance of an accident would be more likely. Why not try the NV hub? Also get a double walled rim and 12g spokes so the wheel is really strong - thornproof tubes and good tyres - the Specialised Armadillo tyres are very strong and give good wear.
    I am intrigued to see what path you take and to see what results you achieve.
    Good luck and keep us informed.
  11. conradcliff

    conradcliff Member

    I will be using the same freewheel crank system that the SBP kit uses which will create yet another gear reduction. My rear gears will be staying intact just like the SBP kit. If you've found a flaw in my numbers somewhere please let me know. Or if you understand the SBP kit to be a flawed kit I would like to know that as well.

    I've looked at the NV hub but it's a bit rich for my blood and also I do have plans to upgrade my tubes and tires. I was going to stay the upgrading process of my rims and spokes to see how the stock ones hold up. Seeing as how I will not be attaching a drive gear to the spokes I felt that they would fair much better than other kits.

    Thanks again for your interests, I will definitely have detailed pictures and explanations of my build once I get it off the ground. :cool: