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Tire Diameter / SpeedoDRD calibration on converted 250L

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So I have made some modifications to my 250L over the last few years, and I have never felt like I can get the speedo adjusted properly. In my experience everyone here is familiar with the 250L stock specs so I will leave them out. Below are the modifications that will affect the speedo, please let me know if I have left anything out that could be part of my issue.

  • Front/Rear wheel are stock 17's for a  CRF250M
  • Rear tire size is 140/80-17 (Shinko 705)
  • Sprocket setup is 14/42 (Still using a 106 link chain)

I have used several tire size calculators to determine the tire diameter (see attached images) and I cannot explain why the tire circumference is LARGER with a smaller rim.

Thoughts? As always, thank you in advance for the help everyone! :thumbsup:

ratios.PNG

tires.PNG

Edited by Aavani

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AFAIK front wheel size has no bearing on top speed or speedometer calibration. it's just the rear tire, and that's only because the speed is gauged in the transmission and changing the rear tire changes the gearing ratio. the front tire doesn't effect the gearing at all.

 

tire circumference is larger because the tire has a larger side wall.

 

the 120 is the tire width in MM, the /80 is the percentage of the width that the side wall is.

80% of 120 is 96mm

80% of 140 is 112MM

so the side wall on the 17" rimmed wheel is 16 mm taller, and you've got two sidewalls so the tire is effectively 32 mm taller

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4 hours ago, Crapflinger said:

AFAIK front wheel size has no bearing on top speed or speedometer calibration. it's just the rear tire, and that's only because the speed is gauged in the transmission and changing the rear tire changes the gearing ratio. the front tire doesn't effect the gearing at all.

 

tire circumference is larger because the tire has a larger side wall.

 

the 120 is the tire width in MM, the /80 is the percentage of the width that the side wall is.

80% of 120 is 96mm

80% of 140 is 112MM

so the side wall on the 17" rimmed wheel is 16 mm taller, and you've got two sidewalls so the tire is effectively 32 mm taller

Excellent explanation, this is math I can wrap my head around! You are correct there is a countershaft speed sensor in the transmission. I only mentioned it to be sure readers understood I was not working with a stock rims. You sir are a gentleman and a scholar, many thanks.

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For calibrating the DRD, I reset it first, take a run atleast 60MPH, compare the speedo on my phone(in a Ram mount on the bars) with the speedo in the dash, and just use the DRD calculator to determine what correction it needs.  It's always a couple percent different than calculating by numbers only, my speedo was a little off from the factory, and tire diameter can vary a small amount between different brands in the same size.  

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29 minutes ago, alucard0822 said:

For calibrating the DRD, I reset it first, take a run atleast 60MPH, compare the speedo on my phone(in a Ram mount on the bars) with the speedo in the dash, and just use the DRD calculator to determine what correction it needs.  It's always a couple percent different than calculating by numbers only, my speedo was a little off from the factory, and tire diameter can vary a small amount between different brands in the same size.  

I would totally use this method, but I can't justify the cost. If I am gonna nav I pop in headphones and use voice commands. I realize having directions displayed could be useful, I just don't think I would use it often enough. This would be the most accurate way to do it though.... I may bite the bullet and pick up a mount.

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1 hour ago, Aavani said:

I would totally use this method, but I can't justify the cost. If I am gonna nav I pop in headphones and use voice commands. I realize having directions displayed could be useful, I just don't think I would use it often enough. This would be the most accurate way to do it though.... I may bite the bullet and pick up a mount.

If you have a GPS enabled camera that encodes the data in the video, it can work too.  I have a Hero5 Black, can overlay a GPS speedometer in the footage, just point the camera at the speedometer, and when you get home, add the gauges to the video with the software, and you can compare both speeds.  Pacing a buddy in a car, figuring it out mathematically, or if your state puts up those police speed warning signs, could also work, but probably won't be as accurate.  Given the info in the OP, I might just set it to increase 4%, and go with that.  

First thing I did to the bike was to add a mount. Waze helps a great deal around here, showing hazards, police, and detours help even when going to places I know.  I get audible turn by turn through my Sena, but the map is easier for me to follow, especially with PA's love of complicated intersections and the fun network of backroads around here.

Edited by alucard0822

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38 minutes ago, alucard0822 said:

If you have a GPS enabled camera that encodes the data in the video, it can work too.  I have a Hero5 Black, can overlay a GPS speedometer in the footage, just point the camera at the speedometer, and when you get home, add the gauges to the video with the software, and you can compare both speeds.  Pacing a buddy in a car, figuring it out mathematically, or if your state puts up those police speed warning signs, could also work, but probably won't be as accurate.  Given the info in the OP, I might just set it to increase 4%, and go with that.  

First thing I did to the bike was to add a mount. Waze helps a great deal around here, showing hazards, police, and detours help even when going to places I know.  I get audible turn by turn through my Sena, but the map is easier for me to follow, especially with PA's love of complicated intersections and the fun network of backroads around here.

That is fucking genius! I do have a Hero5 black also, totally gonna do try this tomorrow when my bike comes back from the shop.

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you don't even need a mount for the phone to do the GPS speed test (also....like...a $25-$50 bar mount is super expensive? ¯\_(ツ)_/¯). if you just want to temporarily hold the phone in a place where you can see it, just use some rubber bands to strap it to the bars. or find a GPS speedo app that has top speed tracking, take several runs, get to a preselected top speed on the bike's speedo (50 or 60 or whatever) then pull over. see what the GPS registered as the top speed and do your calculations from that

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Ram sux. Get one that doesn't need rubber bands to hold securely. 

 

This is $12 and will flex instead of breaking

https://www.amazon.com/Silicone-Cellphone-Universal-Smartphones-Handlebars/dp/B075T71KN7

Or leave the phone on your pocket...

Start recording a track, then ride 50-60/ mph on a highway according to your stock speedo. Then look at the track and see the speed of each point. 

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1 hour ago, ohgood said:

Ram sux. Get one that doesn't need rubber bands to hold securely. 

 

This is $12 and will flex instead of breaking

https://www.amazon.com/Silicone-Cellphone-Universal-Smartphones-Handlebars/dp/B075T71KN7

Or leave the phone on your pocket...

Start recording a track, then ride 50-60/ mph on a highway according to your stock speedo. Then look at the track and see the speed of each point. 

says "get one that doesn't need rubber bands to hold securely"... immediately links to LITERALLY a rubber band.

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I just did this for my Speedo Healer install..  I just zip tied my phone to the handlebars cross pad, opened up waze and checked Waze at 60mph versus my speedo.  Go back home, do the calculation and set your speedo healer.  Done!

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17 hours ago, Crapflinger said:

says "get one that doesn't need rubber bands to hold securely"... immediately links to LITERALLY a rubber band.

 

I think you've missed the point.... For years ram sold their x grip mount and ignored people complaining about the fingers falling off, ejecting phones. Then they finally started shipping with rubber bands (actually silicon) to hold the phones in place when, not if, the fingers ejected it.

What we're doing here is skipping the bad design (x grip/fingers) and going right for the better design, which doesn't eject phones on washboard dirt roads, highways, or anywhere, even after hitting trees. It also absorbs vibration instead of transferring to the back of the phone. 

And yes, the silicon mount is faster/easier/more secure than the x grip with a rubber band bandaid  ;-)

 

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