Wheel Spin at Constant Speed Question

[Sean Malone]

The question was brought up on the motorcycle forum as to why our make of bike has a speedo that is over ‘actual’ by 10% consistently. One of the theories was that the wheel slips when under power even at a constant speed. If I am going 70mph indicated and going 60mph actual due to wheel slip, that means I’ll arrive at my destination with four apples and two oranges? Seriously though, some of the posters claim their bikes have a linear discrepancy…

Linear:
10mph = 1mph off.
30mph = 3mph off
55mph = 5.5mph off
100mph = 10mph off

…where as others claim a constant discrepancy from 10mph and up. They claim to have tested with GPS.

Motorcycles, even sleek looking sport bikes, have a drag coefficient of around .4 to .6 which is about the same as a full size pick up truck. So the argument is that due to the light weight of the bike (353lbs dry and sans rider) and the high aero drag the rear wheel spinning to push the bike is what is causing the speedo being over by 10mph. Since I follow science by ‘gut instinct’ (:gomer: ) my gut tells me there might be some truth to it, but not the whole story.

I simply asked the question in the thread as to whether anyone who had other make bikes had the same overage? Seems like a quick way to ascertain whether it’s the wheel or just the speedo. No one has replied yet.

[Methanolandbrats]

Bike makers probably put short speedo gears in the bikes so you guys "think" you're going faster than you are........kind of a safety margin ;)

[Sean Malone]

two guys just posted that their other make bikes are 3% to 10% off as well.

[dando]

I've not noticed this on my V*, but I'll hafta check mine with my GPS. It'll be a couple of mos, tho. Brrrrr..... :(

-Kevin

[eiregosod]

One needs to do the discrpency test over the full range of speeds, get your buddies to test at 10, 20,30,40,50,60, mph and see what the discrepency is over the full range of speeds.

doubt if there's any slip, except under acceleration while starting up. probably its a discrepancy in the gearing of the speedomoeter.

[Sean Malone]

One needs to do the discrpency test over the full range of speeds, get your buddies to test at 10, 20,30,40,50,60, mph and see what the discrepency is over the full range of speeds.

doubt if there's any slip, except under acceleration while starting up. probably its a discrepancy in the gearing of the speedomoeter.

That's what I think too. The 'world is flat' side of me has a hard time accepting that my drive wheel is slipping even at a constant highway speed of 70mph in dry conditions.

[eiregosod]

That's what I think too. The 'world is flat' side of me has a hard time accepting that my drive wheel is slipping even at a constant highway speed of 70mph in dry conditions.

when you're going at a constant speed, there is no splippage. Splippage only occurs when there is insufficient friction between the tyre & the road to tame the acceleration of the wheel, only matters at startup.

[Ankf00]

when you're going at a constant speed, there is no splippage. Splippage only occurs when there is insufficient friction between the tyre & the road to tame the acceleration of the wheel, only matters at startup.

[JLMannin]

The speedometer on my minivan is about 7% slow - I checked it with my GPS. The other car was right on.

[Sean Malone]

The speedometer on my minivan is about 7% slow - I checked it with my GPS. The other car was right on.

One of the posters said they contacted Suzuki and Yamaha and that they acknowledged that the speedometers are off but 10% is within industry acceptable standards.

One guy said his BMW M3 has an electronic speed limiter that cuts off at 128 (135 indicated). So I guess speedos being off is fairly common. The wheel slipping thing is what I had a problem with.

[Dirk Diggler]

Oh, thank Christ! I thought this was going to be another airplane on a conveyor belt thing!

[Sean Malone]

Oh, thank Christ! I thought this was going to be another airplane on a conveyor belt thing!

I was tryin'. They weren't bitin'.

[SteveH]

What if you rode your bike on a conveyor belt? If you had 10% wheel slippage where you were only being propelled at 45mph even though the speedometer was reading 50 mph. And the conveyor belt was at 50 mph, you'd actually be riding backwards! :gomer:

[G.]

Clueless, here. Really. (I prolly will prove it as follows...)

I thought that motorsickle's speedos measured front wheel rotation. Not the whole, RPM, Gear selection, transmission thing.

Also, if you were cruising along with the tire getting jiggie wit it, a gentle cross-wind would cause the bike to flop, no? I mean, if the back tire is spinning, you do NOT have traction.

[Indy]

What if you rode your bike on a conveyor belt? If you had 10% wheel slippage where you were only being propelled at 45mph even though the speedometer was reading 50 mph. And the conveyor belt was at 50 mph, you'd actually be riding backwards! :gomer:

[/Keanu]Whoooaaaaaahhh...[/Keanu off]

[RacinM3]

One guy said his BMW M3 has an electronic speed limiter that cuts off at 128 (135 indicated). So I guess speedos being off is fairly common. The wheel slipping thing is what I had a problem with.

BMW speedos are notoriously off across the board - 5 to 7 MPH. Theory is this keeps drivers on the right side of the law (?).....maybe the bike makers have the same lawyers!

[Cam]

BMW speedos are notoriously off across the board - 5 to 7 MPH. Theory is this keeps drivers on the right side of the law (?).....maybe the bike makers have the same lawyers!

w00t! You are confirming my findings! :thumbup:

[KLang]

My '07 328 is off by about 5mph. I try to forget that so I stay out of trouble. :cool:

[JoeBob]

If people were ticketed for speeding, but the speedo said they were within the law, there would be a huge class action lawsuit. (Honda had to settle one because their odometers were showing higher numbers than the cars were actually driven.)

Better to tell people they're going 70 when they're really doing 65 than to tell them they're going 70 when they're really going 75.

[Stu]

maybe the speedometer guages are just designed to read a speed higher than you're really going. :eek:

[indyfan31]

Clueless, here. Really. (I prolly will prove it as follows...)
I thought that motorsickle's speedos measured front wheel rotation. Not the whole, RPM, Gear selection, transmission thing.
Also, if you were cruising along with the tire getting jiggie wit it, a gentle cross-wind would cause the bike to flop, no? I mean, if the back tire is spinning, you do NOT have traction.
Finally, the guy that thinks he's clueless is the one that gets it.

[RichK]

Most modern motorbikes have the speedo on the countershaft sprocket, so they measure the rear wheel speed.

[WickerBill]

What if you rode your bike on a conveyor belt? If you had 10% wheel slippage where you were only being propelled at 45mph even though the speedometer was reading 50 mph. And the conveyor belt was at 50 mph, you'd actually be riding backwards! :gomer:


Not for long

[Sean Malone]

Most modern motorbikes have the speedo on the countershaft sprocket, so they measure the rear wheel speed.

Hey, you gettin' any track days in?

[jcollins28]

I have only owned sport bikes and everyone of them I have had the speedo ran off the front tire.

[RichK]

Hey, you gettin' any track days in?

I'm averaging about one every 2 months or so, still on the ZX-6RR. Are you doing trackdays?

I just got a 2006 ZX-10R to replace the VFR. It's scary-fast!