# What size drive roller?

Discussion in 'Friction Drive' started by DamoMotorized, Sep 26, 2009.

1. ### DamoMotorizedNew Member

What size drive roller should I use for my friction drive bike?
I have a 31cc Ryobi weed wacker, 24 inch tyres, I only weigh about 50kg (110lbs) and my area has some hills.

Thanks.

3. ### DamoMotorizedNew Member

Thanks for the link, but i don think that calculator is accurate because i have a remote control car and the engine spins at around 18000rpm and it would have no where near enough power to drive me. Do you know what type of engine that calculator is for?

4. ### fetor56Guest

Since u don't weigh much i'de try a 1.25" roller;wheel size is unimportant.If necessary go to a more torquey 1" roller but in doing that you'll loose some top end speed.
It's all experimentation & compromise. :grin5:

5. ### DamoMotorizedNew Member

Yeah i reckon that i will use a 1.25'' roller. I had a look around my garage and found heavy steel pipe about 1.25''. If thats not good then i have a few smaller pipe and bigger pipes to experiment with.

Thanks

6. ### loquinActive Member

That site doesn't tell you what factor they use for 'losses'

(which varies by tire pressure, the amount of roller deflection in the tire, the tire manufacturer, the tire type, the tread pattern, etc.)

Nor does it account for the power of the engine, or for the weight of the rider. It assumes that the engine will have enough power to at least eventually get you up to speed. As if you are on the flat.

Really, IMO, you should calculate the top end speed, taking NO deductions for efficiency losses, as each and every situation is different. You would KNOW that the speed calculated is the maximum possible top end, and realize that many factors can cause the actual top end speed to be less than that maximum.

For a friction drive, top end speed can be calculated as follows:
Code:
```Speed = RPM * Roller Diameter * 60 * pi / (12 * 5280)
or, simplified

[B]Speed = RPM * Roller Diameter * 0.002975[/B]
or
[B]Speed = RPM * Roller Diameter / 336.1[/B]

where the roller diameter is in inches and Speed is in MPH

For metric calculations (roller diameter in cm, speed in KPH)
[B]Speed = RPM * Roller Diameter / 530.5[/B]

```

Last edited: Sep 27, 2009
7. ### DamoMotorizedNew Member

Its been a long time and i still havn't finished this project. But now that i have long school holidays i will be able to finish it. Would a 3.5 cm (1.38 inch) roller make a big difference over a 1.25 inch roller. Remembering that I have only put on about 6 kg since the first post making me around 56 kg (123.5 lbs) now and not that many hills.

You will probably have to pedal assist the motor up hills. Even 1.25" may be too large for your weed whacker.

9. ### 5-7HEAVENGuest

Twins!

Put two weedwhacker engines on the bike, one for front and rear. :idea:

I did that with two 2.2hp Mitsubishi engines. It was awesome.:devilish:

10. ### loquinActive Member

Plug in your desired max speed, the bike weight, your weight, and set it for flat conditions with zero wind speed. That will give you the approximate power you need to run under those conditions. In your case, since your engine is already spec'ed, adjust the max speed up or down until the power required matches what you have available. For a friction drive, you should be able to use 90% as a rough efficiency.

Then, download one of the gear ratio calculators and calculate the gearing to achieve the max speed above, for your friction drive.

When you introduce slopes into the calcs, things get a bit more complicated. Measure the angle of the slopes on your hills, plug that angle and you/your bike's weight into the power calculator, and adjust the max speed until the power doesn't exceed your engine's power. Then, plug that speed and your engines max power RPM into the gear calculator to get the roller diameter size.

11. ### DamoMotorizedNew Member

Haha

I could, i got 5 working weed wackers that people threw out (they just needed new fuel lines) but its too much work.

12. ### DamoMotorizedNew Member

Thanks for showing me these. I've used the "gear rat" one before. It says that i will be able to go 38.46 kph (23.84 mph) at 6000 rpm with a 3.4cm (1.34" spindle) which i'm definitely happy with.

The bike power calculator tells me I will need 1 hp (which approx is how much my engine has) to get to 38.46 kph (23.84 mph) with no hills/wind.

Thanks :grin5:

13. ### BiMoPedBanned

1.25" is probably too big
1.125" / 1 1/8" is the right size
Yes, 1/8" makes a difference

14. ### DamoMotorizedNew Member

Just a quick update. I ended up buying a 32mm (1.26") pipe. I am just waiting on these bearings to come from US: http://www.vxb.com/page/bearings/PROD/Kit7345. I will show you guys in picks how it will all look when i finish. Once those bearing come from US, pretty much all I have to do is just put everything together.

15. ### DamoMotorizedNew Member

1.25" is going to be ok. Have a look at this bike: http://www.youtube.com/watch?v=WY-4p4sa03k he is using the same engine and drive roller diameter as me and it runs great. Plus he probably weighs about double my weight.