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I'm about to get a set of Race-Sport tires for my motorcycle. The RS tires offer more lean angle. Notice the visual difference, the tire on the left has a much steeper drop off at the sides.

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When the motorcycle is leaned over it seems the part of the tire touching the road on the RS tire is closer to the axle than on the standard sport touring tire. Basically the tire diameter seems smaller at higher lean angles. Does that mean the speedo will show exaggerated values when the bike is leaned over on those tires? And is there anyway to correct those numbers? I use a wheel speed sensor from a bicycle, GPS is ridiculously inaccurate.

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    Dunno ... the GPS I've used in the past is spot on (within a 1/2mph) ... might depend on where you're at in the world, though. – Pᴀᴜʟsᴛᴇʀ2 Jun 6 at 22:03
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The speedo will read incorrectly by the difference in the rolling radius.

However, if you are that far over in a curve you would be well advised to be watching where you are going and for road imperfections than the, usually small, difference in speed of the speedo...

  • I will not be looking at the speedo while riding. However I do transfer the data from my speedometer to my computer where I can check how fast I went through which corner. – AzulShiva Jun 6 at 19:18
  • If you can measure the corner distance and time then compare to the speedo reading you might get something interesting... – Solar Mike Jun 6 at 19:22
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    Any speedometer inaccuracy in the corners will drown in the definition error: are you measuring the speed of the point where the wheels touch, the center of gravity or the speed of your head. On straight lines, GPS is very accurate, but will inevitably measure less when you're going through curves. – Lenne Jun 7 at 15:50
  • @Lenne - why don't you make that an Answer? – Mike M Jun 12 at 0:00
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Short answer is yes and no. Not sure what you mean by exaggerated, but it shouldn't be by that much. On the other hand, speed reported by the wheels hardly ever are correct on racetrack; if you are serious about telemetry you must use GPS data.

All that said, mathematically speaking, you should be fine for comparing individual corners on different laps because the source of the speed is the same even if the values are exaggerated, meaning the graphs of say corner 1 will show you the difference you are interested in as long as you are using the same source data.

Important thing is if the speedo is working correctly, you have a source of data you can compare your laps with. That said, again, just because it shows higher doesn't mean you are going faster, because of slip, rotation and radians, so, if you are serious you have to use GPS.

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A problem in your quest for measuring the speed is where do you measure the speed. Is it the bottom of the wheel, the contact point, the centre of mass or the top of your head? All have the same angular speed around the centre of the turn, but different linear speed; because of the different radii. Comparing speeds in the corner isn't important, the time for taking the corner is. To get accurate measurements you'd need sensors on the road or time the cornering by using fixed points and measure the crossing times from camera footage.

GPS measurements are very accurate on straight lines but suffer in corners due to the averaging done. Just imagine driving around in a small roundabout: the speedometer may show 60km/h with good tires, but your average speed measured over say a second is much lower. From the GPS's view, you aren't going anywhere.

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