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 GXH50 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 MaxTorque.com 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 ) 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! P.s. I fixed the typo in the calculator...

Sorry, I had to edit that...they wont let you upload an excel file. PM me your email and I'll send it to you.

With a shift kit, 11 tooth jackshaft sprocket and 5 speed rear 26" bike I top out at 32 mph at about 6000rpm. Engine vibs just won't let it rev any faster. Good luck

Fantastic info, thank you! I'm assuming that's with the same engine. Is the total weight of you and the setup about the same?

Works great on paper, Have you talked to Walt Disney about it? Bike is going two fast for the frame. No, No, No. The Carb/Firb Frame is not the same. Steel/ wiil hold up.

Thanks for the post but honestly that doesn't help me out too much...I believe the frame is aluminum. The speed I'm thinking is far too high, as it is right now I'm thinking about adjusting the ratios down around 40mph unless someone chimes in with some solid info to the contrary.

You have not figured in how much horsepower it takes to move this combination at speed. If you do not include that, you can gear the up far enough that, theoretically, 1 hp would result in 300 mph- but we all know that will not happen. There is more to it that gearing. Why don't bicycles have 80 tooth front sprockets with 12 tooth rear so we can all pedal at highway speeds? To defeat all the forces, wind, friction, gravity, etc.- you will need closer to 5 hp to get to 50 mph. With 2.5hp, you can get around 40 mph with proper gearing, but not much more no matter what ratios you plug in. There is just not enough power to get the engine to spin fast enough to get close to 50 mph.

This I know and that was the point of this post..to see just how fast a 2.5HP GXH50 would push me. Now that I have a good idea of that, thanks to your post on the other website I will be able to choose a gearing that at my lowest rear ratio I am only traveling at or around 40MPH at the given RPM range which will give me optimum power throughout the entire gear range. Thank you so much for your help HoughMade, I think I have the info I needed

I made a model on excel using very conservative numbers calculating the coeficient of drag, rolling resistance of a road bike tire, and assuming 100 percent drivetrain effecientcy with a gear ratio set to run at rpm of maximum power for the motor. you would still need over 7hp to reach that speed on flat ground. That is also assuming you are riding with a road bike, tires inflated to 100 psi with your hands on the drop bars. If you weigh around 220 lbs as I do, this would mean you would need to be riding down a hill. By my calculations to gain the 4.8 hp to reach your 54 mph speed you would need to be riding down a 17 percent grade at least. Not to mention it would need enough distance to allow you to accelerate and decelerate, I would think by using the breaks to the tires limits you would experience some fade in breaks as your pads melted. Remember these numbers are all theoretical with ideal conditions. Also, I have gone 40-45 mph on an unmotorized road bike down hills, and it is scary the handlebars and frame shake violently and just touching the breaks is intimidating. I wouldn't recommend it. Chance of death, approximately 1 percent.

I recently experimented with my shift kit. I found that I could fit 2 of my old 3 speed front sprockets to the shift kit, which both are more teeth than the one supplied with the kit. My intentions were just to slow the engine to cruise. The big sprocket, I tried first would let me go over 30 mph in first gear with 4 more to go. I couldn't go over 35 mph in any gear. 5th gear would only maintain about 33-35, but wouldn't accelerate. I could only just maintain what speed I reached with the first 4 gears. Just not enough hp to go much more than 35 mph. It's all flat ground here. I used the smaller sprocket (don't know how many teeth) but it's bigger than the one with the shift kit. It works perfect. It allows me to use the torque zone of the engine (maybe 2000-4000 rpm) to cruise at 25-35 mph. With the 5 speed, it may work good on hills also. Any one know where I can get a tach for this engine?

Tiny-Tach is the company most of the scooter guys use but its a digital display and it updates every 2.5 seconds. Id rather have a guage but they are hard to find cheap.

2.5 seconds is a pretty long refresh rate, in low gears you wouldent even be able to read it acurately. Also they are around 40 bucks which could easily be spent on cooler and or more nessisary upgrades.