Bigger Tire

Can I get a few MPH out of a bigger tire and will it fit?

Stock Tire, a spare rear rim and tire.

Trying a 2.5 inch tire:

Stock tire has a circumference of 58.25 inches. This tire has a circumference of 61.25 inches.

At 10,000 RPM the stock tire would give us:

38.79 MPH = 10,000 RPM * 58.25 / (14.220 overall engine gear ratio * 12 in/ft * 5280 ft/mile) * 60 minutes/hour

The bigger tire would give us

40.79 MPH = 10,000 RPM * 61.25 / (14.220 * 12 in/ft * 5280 ft/mile) * 60 minutes/hour

This tire would be good for roughly 2 MPH gain, assuming the motor has the power.

Tire is from Treatland.tv. 

Motion Pro spoons to put the tire on. These Moped tires are very easy to put on compared to changing dirt bike tires. I have even done them with my plastic bicycle spoons.

Mocking it up on a spare engine, it's really close.

Really have to think about where the magneto wires are going to run.

Looking at the scooter, I don't think it will work with the back fender on.

Back fender is already tight on the stock tire.

I did have to modify the rear fender a little bit. The back bracket was easy, I just bent it a little bit and bolted it down at a different angle. The front, I re-drilled the location of the bolt for the front bracket, to move the fender up and over one of the ridges on the motor. It clears the back tire, not by much, but it does clear.

Not a lot of room to spare, but after weeks of running this, it hasn't ripped the back fender off.

There was a big benefit. The Honda Express was really sketchy at 50 MPH (Yes, I have the poor little motor spinning over 13K, it's not making much power up there, but enough.) Putting the bigger back tire on it helped. The 2.5-14 tire only raises the back by about an inch, but I think that must put a little bit of pressure on the front wheel, since the stability at high speed improved significantly. This makes me want to find a shock with adjustable sag, that I can experiment with.

Also, some later Honda Express's came with a 40 MPH speedometer rather than a 30 MPH speedometer. This is a direct drop in replacement. With the bigger tire, the GPS and speedometer are pretty close. Even at speeds beyond 40 MPH, the speedometer seems behaved and you can tell whether if you are running 45 or 50.

Faster

I knew this motor felt good.

I didn’t want to GPS it until I got a different spark plug. It was detonating with the protruded plug. Still stock gearing, getting all the miles per hour out of turning higher RPMs. It’s going flat on top, not fading away, which likely means I need to drive more heat into the pipe. So I will retard the ignition to drive some heat into the pipe. I have a bigger rear tire which should give me a couple MPH.

I am running a B8ES plug with an Airsal head, premium gas, I can still hear some occasional detonation, but with the switch to premium gas and the B8ES, I am not seeing evidence of detonation on the plug electrode. I have B9ES and B10ES plugs to try. Temperatures on the head are peaking at about 386F, which I know is high. I am using a Trailtech head temperature gauge from treatland.tv to measure the head temperature. I think I am going to experiment with E85 anyhow, so I will leave this combination.