General Question - Speedometer Function: Cable vs GPS

[sharpestjim]

This will probably seem like a stupid question, but I need to ask:

How do speedometers/odometers operate these days? Do bikes (4-wheeled vehicles, too) have a physical linkage - either mechanical or electronic - from the front wheel to the speedometer? Gee, Mister Spiccoli, I don't know.

I was riding my R1250RS last weekend and noticed that the bike's speedometer seemed to synch almost perfectly with the speed indicated on my Radenso detector which, of course, uses GPS. Do today's vehicles use GPS or do they still have something linking the front wheel to the speedometer? The RS has a wire running up from the left side of the hub, but it is not a cable like bikes once used. I suppose that it could be transmitting front wheel rotational speed to the instruments. I thought that this could be some sort of sensor for the ABS since the wire is tied to the left caliper hydraulic line. Back in the old days, my buddies and I would remove the speedometer cables from our bikes and put a goober of silicone in the sockets on the front hub and the bottom of the speedo to keep nasties out. I don't know if I can do this on my RS, even if I wanted to (I don't).

I was at the Mazda dealership yesterday with the wife's 2020 CX-5. I asked the service manager this same question and he said that autos (at least Mazdas) still use a physical linkage. I did not ask if it was mechanical or electronic.

 

[MCMXCIVRS]

Most modern vehicles and motorcycles use electronics to operate the speedometer. I find it hard to believe Mazda would be still using old style cable driven speedometers. The gauge, if analogue, may be mechanical, but it would be driven by an electrical mechanism. The system uses a speed sensor, usually one of the existing wheel sensors for the ABS, to determine the vehicle speed. Your RS uses one of the ABS wheel sensors for the speedometer.

There are systems that use GPS for speedometers, but I'm not aware of them being used as factory in auto or motorcycle applications. Typically used for things like boats or planes where other means of indicating are less accurate. I've also seen them used in hot rods where the vehicles drive train has been significantly modified as a easy way to get an accurate speed reading.

 

[sharpestjim]

Most modern vehicles and motorcycles use electronics to operate the speedometer. I find it hard to believe Mazda would be still using old style cable driven speedometers. The gauge, if analogue, may be mechanical, but it would be driven by an electrical mechanism. The system uses a speed sensor, usually one of the existing wheel sensors for the ABS, to determine the vehicle speed. Your RS uses one of the ABS wheel sensors for the speedometer.

There are systems that use GPS for speedometers, but I'm not aware of them being used as factory in auto or motorcycle applications. Typically used for things like boats or planes where other means of indicating are less accurate. I've also seen them used in hot rods where the vehicles drive train has been significantly modified as a easy way to get an accurate speed reading.

Thanks very much! As I mentioned, I did not ask the gentleman at Mazda exactly how it worked. I figured that they had come a long way from the days of the square shaft rotating inside the cable sheath.

 

[bkkjack]

I would imagine that very few modern motorcycles use the old cable attached to the front wheel method. My 2013 Triumph Street Triple R reads vehicle speed off the countershaft. I would doubt that any would use GPS, though. There are still places in the world you can ride a motorcycle where your bike cannot see the satellites - underground parking garages and tunnels, for example.

This is an interesting "general knowledge" question I hadn't thought of, though. You can very easily find out if our bikes (I also have a 1250RS) use rear wheel speed by putting it on the center stand, starting it up, putting it in gear and seeing if the speedo reads. I'd be willing to bet it does. All the sensors on modern bikes read just about everything happening, and there almost has to be one measuring the rotational speed of the driveshaft. It would be too easy to include a basic calculation to turn that metric into vehicle speed. If no vehicle speed is shown, then you'd have to get your front wheel off the ground and see what happens when you spin it. If you don't register speed from either wheel, then I guess I'd be wrong about the GPS. Wouldn't be the first time.

I was also struck by how accurate the speedo on our bikes is. I haven't been riding as long as many on here - just since 1982 - but since then one of the motorcycling truisms has always been that motorcycle speedometers are intentionally optimistic in a futile attempt to try to slow down us hooligans. Using a GPS, I've determined the one on my Street Triple shows my speed to be about 8% faster than I am actually going.

 

[collingsbob]

Speedo cables have squared ends..the cables are all round.
The Smiths Chronometric speedo on my 1953 Vincent (recently sold) was more accurate than the one on my FJR1300.
Mechanical speedo drives are highly reliable, easy to manufacture and very low maintenance..I owned my Vincent for nearly 40 years - and I only cleaned and lubed the cable 2 or 3 times.

 

[pglaves]

BMW has used a variety of methods for speedometers. The Airheads of course had cable driven speedos driven off of a gear in the transmission. The classic K bikes had electronic speedometers fed from the rear wheel sensor. R1100 and R1150 Oilheads had cable driven speedos with the cable driven off the front wheel. The early F800S had electronic clusters where the speedometer was fed off the front wheel sensor and the odometer and trip meter were fed off the rear wheel sensor.

Beyond that I have lost track, if I ever knew at all.

 

[rhbike]

I would imagine that very few modern motorcycles use the old cable attached to the front wheel method. My 2013 Triumph Street Triple R reads vehicle speed off the countershaft. I would doubt that any would use GPS, though. There are still places in the world you can ride a motorcycle where your bike cannot see the satellites - underground parking garages and tunnels, for example.

This is an interesting "general knowledge" question I hadn't thought of, though. You can very easily find out if our bikes (I also have a 1250RS) use rear wheel speed by putting it on the center stand, starting it up, putting it in gear and seeing if the speedo reads. I'd be willing to bet it does. All the sensors on modern bikes read just about everything happening, and there almost has to be one measuring the rotational speed of the driveshaft. It would be too easy to include a basic calculation to turn that metric into vehicle speed. If no vehicle speed is shown, then you'd have to get your front wheel off the ground and see what happens when you spin it. If you don't register speed from either wheel, then I guess I'd be wrong about the GPS. Wouldn't be the first time.

I was also struck by how accurate the speedo on our bikes is. I haven't been riding as long as many on here - just since 1982 - but since then one of the motorcycling truisms has always been that motorcycle speedometers are intentionally optimistic in a futile attempt to try to slow down us hooligans. Using a GPS, I've determined the one on my Street Triple shows my speed to be about 8% faster than I am actually going.

This likely varies by jurisdiction, but generally I don't believe GPS based speedos/odometers can legally replace wheel or drive shaft sensor base speedos/odometer.

 

[Rainman48]

Newer BMWs (from at least 2010) have slotted rings attached to the disc brake rotors on both ends, along with electronic pickups about 1-2mm from the rings that pickup rotational speed electronically - thus commonly known as "speed sensors". No cables, just wires. They are not just used for ABS or the speedometer. Any bike that is also equipped with traction control also uses them. The ECU reads the difference in rotation between both wheels as well as what "modes" the rider may have the ability to set and acts accordingly. Rain mode vs. Road mode vs. Dynamic mode provide varying degrees of safety to risk, and some models (S series at least) prevent or allow wheely control based on wheel speed differences or "backing it in" - all tied to throttle control. Old fashioned cables are a thing of the past.

And here's another thing you might find interesting. This form of speedometers are highly accurate as long as specified tire sizes are used. In high performance models, using tires sizes not known or calibrated to the ECU complicate the traction control functions, and are known to disable functions.

 

[lkchris]

one of the motorcycling truisms has always been that motorcycle speedometers are intentionally optimistic in a futile attempt to try to slow down us hooligans. Using a GPS, I've determined the one on my Street Triple shows my speed to be about 8% faster than I am actually going.

Actually there are two things ... it's illegal many places for a speedo to read slower than the bike is actually traveling ... and since different brands or models of tires marked the same size may in fact not be, there needs to be room for this to avoid the former.

We in USA where there are no laws relatively speaking are sometimes perplexed by our German motorcycles.

BMW for one were always quick to explain that despite deliberate "speedometer advance," our odometers are accurate. Especially for warranty purposes.