Speedo in 21" wheel ??

[Gerwin Brommer]

Guys, need your help.

I've got a xs360 (xs400) hub, laced into a 21" wheel.
The original wheel is an 18" .
Speedo drive gear in hub is the standard version for the 18" wheel.

Which speedo do I need for an accurate reading ?

I've read somewhere that I need a 2240:60 ratio speedo , because of the 21" wheel ?

 

[pamcopete]

Gerwin,

The 2240 RPM @ 60 MPH is a Japanese motorcycle standard for the speedometer regardless of the size tire. The important thing is the circumference of the tire, not the wheel size, so mark the tire and the floor and move the bike forward one revolution of the tire and mark the floor again. Measure the distance traveled. Do the same with a 19" rim and tire. If the rolling distance is the same, then your speedo will be accurate. For greater accuracy, the wheel and tire should be installed on a bike and inflated to its recommended pressure. If you do not have a bike with a 19" front wheel anymore, perhaps someone here will volunteer to do that test for you.

 

[Gerwin Brommer]

I only have the current 21" wheel with the Avon Speedmaster tire, so I don't have
anything to compare with/to.

 

[gggGary]

It'll be close enough for government work! They aren't all that accurate anyways, what's a few percent one way or the other.

 

[pamcopete]

I only have the current 21" wheel with the Avon Speedmaster tire, so I don't have
anything to compare with/to.

Well, do you still have a 19" tire? If so, then just measure the diameter of the 19" tire and compare it to the 21". You will probably find that they are very close. The 21" tire has a lower profile but I think the diameter is the same.

Or, just go ahead and mount the tire and tape your GPS to the handle bars, set it to read speed and take a short run to see if the speedo is accurate or not. Even in the best of circumstances, the accuracy is not that great.

If you are sure that your tach is reading correctly, then you could use this Excel spreadsheet to calculate your speed based on the RPM's of the engine.

http://www.amckayltd.com/xs650revs.xls

Ultimately, even if you find that the speedo is not reading right with the 21" tire, there is nothing that you can do about it anyway.

Meanwhile, it's raining and cold here today, but I will go down to my garage and measure my rolling distance on my 19" front wheel.

OK. I'm back from my freezing cold garage. I did a rolling distance check on the front wheel of my '81/H with a 100/90 - 19" tire inflated to 28 PSI and came up with 80". I have done the GPS check before with this tire and it is within 1 MPH at 60 MPH. Using the above Excel spreadsheet, a 1 inch difference in rolling distance would equate to about .75 MPH.

 

[BigJimmyW13]

Dude, these speedos are not that accurate to begin with. Don't worry about it. Mine vibrates soo much that I barely can tell how fast I'm going. I primarily have it for gas tracking and mileage.

 

[Gerwin Brommer]

Thanks all !

I wouldn't be too concerned about my speedo, but I wanna integrate it in my tank.
That means some welding has to be done to the tank.
That's why i wanna have the right speedo in it the first time.

 

[BigJimmyW13]

You could always just get one from :

http://www.motogadget.com and have it set up for yours.

 

[alfredo]

Jimmy: what's your mpg looking like these days anyhow?

 

[BigJimmyW13]

Before I tore everything down for rephase and 750. It was roughly 40mpg

Posted via Mobile

 

[alfredo]

Gerwin: If you really wanted to make this work, then maybe your fix will be a smartphone or gps mount to view and guage your mph. There's a recent forum showing an iPhone mount and a trick looking app that shows the mph.

 

[Gerwin Brommer]

Fong-bros styled hardtail with an Iphone ???????????????????????
No way !