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18" front and rear wheels on my xs650

Discussion in 'The Garage' started by Deadhead, Mar 23, 2020.

  1. Deadhead

    Deadhead XS650 Member

    I want to run a speedo and tach for my build but I realize the ratio may not be the same due to wheel change. does anyone know about this. thanks ahead
     
  2. Machine

    Machine Race the wind Top Contributor

    Mornin Deadhead, welcome here.
    The difference from 19" to 18" in diameter will slightly affect the Speedo read of course.
    Can you better describe your goals as to wheel style ?
    An example of an 18" spoked aluminum front wheel would be an SR400 front which may swap? Are you interested in lacing up a rim yourself?
    Maybe you'd prefer a cast mag style front wheel such as a Yamaha Vision 18" which could also swap in.
    Get is more to go on and pic?
    -R
     
  3. Deadhead

    Deadhead XS650 Member

    I'm using 1976 RD400 cast wheels they look similar to the wheels on my XS400
     
  4. Machine

    Machine Race the wind Top Contributor

    Aah got it. You have the wheels.
    :thumbsup:
     
  5. fredintoon

    fredintoon Fred Hill, S'toon. Top Contributor

    Hi Deadhead and welcome,
    running an 18" front wheel instead of a 19" means it'll turn 1.0556% faster and if you keep the stock speedo drive
    your speedo & odometer readings will be optimistic by the same %age.
    The tach is driven from the engine so it's readings won't be affected by a change in wheel size.
    The photo shows a hardtail build and that's between you and your spinal column but what it don't show is brakes. Brakes is important and the life they save may be your own.
     
    Last edited: Mar 23, 2020
    GLJ likes this.
  6. Deadhead

    Deadhead XS650 Member

    there will be brakes front and rear disc all the way
     
    gggGary likes this.
  7. Deadhead

    Deadhead XS650 Member

    oh there will be brakes front and rear disc for sure
     
    gggGary likes this.
  8. grizld1

    grizld1 Grumpy old man Top Contributor

    Difference in circumference is what affects speedmeter and odometer readings. Numbers will vary a little with the tire you use, but using 3.14 for appproximate pi yields circumference figures of 59.66" for a 19" rim and 56.52" for an 18" rim. The difference is the same 5.26% you'd get working from diameter alone. Experience matches theory; I've run an 18" laced front rim on my XS650D for years.

    Unless that amount of error doesn't concern you or you decide to make speed corrections in your head, you have a choice between replacing the speedo with an old one with the right gearing or an electronic speedometer that can be programmed to the circumference of your front tire. I've used an Acewell tach/speedo unit for around 14 years, and it's performed flawlessly. IIRC Dime City Cycle has them.
     
    Last edited: Mar 24, 2020
    gggGary likes this.
  9. Deadhead

    Deadhead XS650 Member

    thanks very helpful
     
    gggGary likes this.
  10. arcticXS

    arcticXS XS650 Guru Top Contributor

    Use an RD350/ RD400 tach drive? If the wheel fits your forks and axle, that is the straightforward fix. I assume your front tire will be slightly bigger than the 3.00 or 90/90 most commonly used on the RD400, but that will most likely correct the often "overly optimistic" speedo reading from oem setups.

    Otherwise, grizld recommended an electronic speedo, and I agree. They can be set to be very accurate, down to 0.05% error, as wheel circumference is often entered in mm. I normally calibrate electronic speedos by distance. I reset the trip meter at a mile marker/milestone and ride for 10 km/ 10 miles, then check the trip meter reading. And correct the wheel circumference value as required, using simple math. The electronic speedometers actually measure distance (number of wheel rotations), and use their internal clock to determine speed. So using a known, longish distance for calibration is very precise.
     
    gggGary likes this.
  11. Deadhead

    Deadhead XS650 Member

    Did you mean speedo drive from an RD400?
     
  12. arcticXS

    arcticXS XS650 Guru Top Contributor

    YES
    (Using the tach drive would definitely require some serious engineering)
    Just started on my second cup of coffee here, came on night shift an hour ago. So not firing on all cylinders yet ;)
     
    gggGary likes this.
  13. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

  14. Bushyeyed1

    Bushyeyed1 Cafe Racer XS650.com Supporter Top Contributor

    Last edited: Mar 24, 2020
  15. Deadhead

    Deadhead XS650 Member

    cool that's what I thought
     
  16. Deadhead

    Deadhead XS650 Member

    gggGary likes this.

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