15

I have been thinking about this, from quite long ago.

Why is speed always calculated from the front wheel?

Let's just consider the following situation:-

You are in a race, and you have given full throttle in 1st gear and you have ended up in a wheelie and for 2nd gear too. Let's assume your front wheel is in the air.

In this situation, if you want to measure speed, then the rear wheel should be the correct one, right?

I would like to know, is there a specific reason why vehicle manufacturers measure speed from the front wheel?

  • 7
    If you are in a race, and your car can lift off some of its wheels, you won't be caring what your exact speed is, and you will be using more precise and external methods of speed measuring. – Иво Недев Jun 17 '16 at 8:57
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    All the race bikes I've seen had no instruments. Any cognitive bandwidth you might spend looking at a speedometer is much better devoted to going faster and not crashing. If you're on a street bike at a track day, instructors advise you to put tape over the speedo. You don't need a tach either. You can feel the power falling off past the peak, and once you know the track you know where you're shifting anyway. – Ed Plunkett Jun 17 '16 at 13:11
  • Aside from profit is there not more variability on load of the rear wheel which makes variation in rotations versus speed? I'd expect the front to proved a more stable consistent measurement. – chux - Reinstate Monica Jun 18 '16 at 19:23
12

Its not a rule of thumb that the Speedometer is connected to the front wheel.

There are many Suzukies and hondas in my country that have the speedometer connected to the rear wheel.

As Rory stated, the only reason I guess is due to the convenience factor since the front wheel is closer to the cockpit.

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  • That's trippy, I have never seen a motorcycle with a speedometer cable at the rear wheel unless it's an electronic sensor for traction control which would place a sensor on both wheels to measure the delta. – DucatiKiller Jun 17 '16 at 9:26
  • @DucatiKiller en.wikipedia.org/wiki/TVS_Flame – Shobin P Jun 17 '16 at 9:47
  • @DucatiKiller en.wikipedia.org/wiki/Suzuki_GS150R This one has the sensor connected to the chain sprocket – Shobin P Jun 17 '16 at 9:53
  • So strange. Although this isn't a cable, it's an electronic sensor, correct? – DucatiKiller Jun 17 '16 at 9:54
  • @DucatiKiller Yeah! – Shobin P Jun 17 '16 at 10:53
22

There are pros and cons of each, remember you can easily spin or lock up the rear wheel, and in fact the rear wheel may be far less often at the correct speed.

So the decision on this stems from physical connectivity

  • it's much easier to connect from the front wheel to the odometer which is on the handlebars than to route from the rear wheel, up under the fuel tank, coping with handlebar movement etc.

It is really that simple.

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12

The front wheel is used in most cases with mechanical speedometers (though there are exceptions) because it's just easier (and probably cheaper) to couple the front wheel to the mechanism. Additionally, a mechanical speedometer uses a cable. The longer it is, the less reliable and accurate it becomes, thus the speedometer takes the speed from the nearest wheel.

In more modern motorcycles with electronic speedometers the speed is taken from either front wheel, rear wheel, or the gearbox (which causes a bit of a problem when you use aftermarket parts to change ratios).

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6

All of my competition cars have had speedos, some of which didn't work at all. I have never once been in a race and had time to take my eyes off what was happening around me to look at the dials. This is why competition cars tend to equip their dashboards with bright lights which come on when attention is required.

The times a speedo is useful is during road use when speed limits must be observed. If you are racing on the public road you are probably committing an offence.

I have to say, when I've got the wheels off the ground, I'm usually looking for where to land.

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2

The speedometer is placed in the location that performs adequately well and that costs the manufacturer the least amount of money, and therefore generates the most profit. Period.

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2

Here's a thought: Most cars have rear-wheel drive. Therefore, in most cars, measuring speed from rotation of the front wheel(s) is more reliable because the front wheels should only be able to spin if the car is in forward motion. In contrast, the rotation of the rear wheels is driven by the engine and need not reflect the actual motion of the car - they could be 'just spinning' [examples: stuck in mud; skidding on ice].

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