BimmerFest BMW Forum banner
1 - 12 of 12 Posts

·
Read Only
Joined
·
752 Posts
Discussion Starter · #1 · (Edited)
This topic has been touched upon before, but not quite the way I'm asking the question today. The ultimate question happens to overlap with my educational, and former professional expertise, so I've given this serious thought.

1) Not long ago, the speedometer reading was set by a gear in the tail shaft of the transmission, counting teeth on the ABS wheels by either choosing one or averaging all, or on really old vehicles a cable turned by one of the wheels, usually the front.
Ergo, if the wheel/tire diameter changed, the speedometer reading would change from real ground speed.
2) All my vehicles purchased after 1999, have come pretty darn close to the real speed traveled, as measured by a stop watch and accurate road markers. But, I always stuck to stock tire sizes.
3) I measured the accuracy, on my 550, on the few occasions I could, in stock form, and the speedometer at 75, was within 1-2 mph of ground speed.
4) Just changed to real quality tires, changed to 1" larger diameter than showroom rear tires. Had my wife follow me in the M4, which also has been timed over measured distances, and came up with exact(!) speedometer & ground speed numbers, up through 90 mph.
5) Big item. GPS, today, has extreme accuracy and very low latency (lag between event and display of numbers/readings). A GPS calculation of ground speed, could if engineers were willing, be displayed through the speedometer, and the average driver would not notice any latency or unusual display.

Given the same exact ground speed, for a given speedometer number, is it possible/probable that BMW is now using the GPS unit to actually calculate and display the real ground speed of their vehicles? I have no other explanation for the lack of reading change, yet two completely different size rear tires.
 

·
Lost but making good time
3er + 3er + 4er = 10er, Bimmers?!
Joined
·
6,615 Posts
...is it possible/probable that BMW is now using the GPS unit to actually calculate and display the real ground speed of their vehicles?
No, for the simple reason that a GPS signal (i.e., satellite reception) is not always available.
 

·
Read Only
Joined
·
752 Posts
Discussion Starter · #3 · (Edited)
No, for the simple reason that a GPS signal (i.e., satellite reception) is not always available.
Not as far as I know. The Earth has a pretty extensive network. Even at the poles, one can capture the signal from 3 GPS satellites. Remember, they are 12500 miles up. Albeit, 1/2 the way below synchronous orbit, where satellite TV & weather Satellites reside.

https://www.gps.gov/systems/gps/space/

However, in a tunnel, such as under Mt. Blanc, one would not capture the signal from 3 different satellites. Which begs the question; "What happens to one's map display, inside that tunnel?".
 

·
Lost but making good time
3er + 3er + 4er = 10er, Bimmers?!
Joined
·
6,615 Posts
Not as far as I know. ... However, in a tunnel, such as under Mt. Blanc, one would not capture the signal from 3 different satellites. Which begs the question; "What happens to one's map display, inside that tunnel?".
You're over-thinking it (the speedometer question, that is). There are plenty of situations where a GPS lock can be lost even when a sufficient number of satellites are above the horizon. It doesn't take much to block a signal that has attenuated to femtowatt levels by the time it reaches the ground. Tall buildings. Parking garages. Dense forest. Driving through hilly/mountainous terrain, where the "horizon" changes moment to moment. It's not too difficult to catch this happening with a hand-held GPS unit (or even a smartphone with a GPS status-monitor app).

When a position fix is not available, the navigation system extrapolates your position from inertial references. It does that by integrating velocity data over time to compute current position and compass heading.
 

·
Read Only
Joined
·
752 Posts
Discussion Starter · #5 ·
Okay, most modern GPS project one's trajectory when a signal is lost. This may happen more often then we first thought.
Back to my initial question, rephrased; "How did my 550i know that I was traveling exactly 70mph, on 1" smaller diameter rear tires, and then on 1" larger diameter rear tires?".
I am conceding that my data collection of both the stock 550i & the M4, may have been off by fractions of a second, due to my reaction time with the stop watch. However, 1" change in diameter of the 550's rear tire should have thrown my speedometer off by just short of 3%. It doesn't appear to. It doesn't appear to be off at all.
Is BMW projecting my speed, during moments of lost signal, and displaying that on my speedometer, if they are in fact, using GPS to measure ground speed?
 

·
Registered
Joined
·
17,180 Posts
GPS can be jammed (rendered inoperable) and spoofed (caused to give false readings). There are military systems that do this. I ran into some GPS spoofing by accident. Imagine if your cruise control was receiving the vehicle speed being calculated by a spoofed GPS receiver.


There's also some lag in the GPS calculation of speed. Some dash-cam systems have GPS speed that is displayed on the video, and it's often a second or so behind the actual speed. That could really cause some craziness with cruise control.
 

·
Read Only
Joined
·
752 Posts
Discussion Starter · #7 ·
Okay, two sufficient arguments why BMW would not rely on the GPS to display ground speed on the speedometer. I still have the paradox of identical displayed/real ground speeds from two different diameter rear tires.
While "over thinking" this further over dinner, I came up with a possible engineering alternative, that BMW may have used. All M's have differing diameter wheels front to back (specs on BMW USA's site). All BMWs have vehicle stability control; which uses the teeth on the ABS disks to compare rotational speeds to do its thing. Why do M's not throw temper tantrums every time someone drives down the road, since front and back are rotating at different speeds? Because...the engineers that designed the VSC system had to account for M's, and therefore must use only the front ABS disks to compute actual ground speed. To balance the car, they must keep 2 running average rotations; front/rear, to calculate the direction of the car.
When I changed tires on my 550, I stayed with the original front diameter. Now, who out there can confirm my new hypothesis one way or the other?
 

·
Registered
Joined
·
17,180 Posts
If you'd put larger diameter tires on the front, the computer would think that the rear tires are spinning and would throttle back. A co-worker bought a demonically possessed used Corvette that simply wouldn't run. The problem was the wrong size tires (larger diameter) in the back. She put on the correct size rear tires and the car ran like new.

The VSC, ABS, and DTC use all four wheel rotational speeds (and corrected for different diameters) to calculated translational speed of each wheel. But, yeah, it's very possible that the speedometer and odometer displays only use the signal from the front wheels.
 

·
Registered
Joined
·
19 Posts
In Chicago, there is quite an extensive road network under trains/bridges/whatnots. Every time I have to drive in Chicago I make sure I memorize the directions (if unknown), because once you go under - all GPS signal is non-existent: Phone, car, sat radio, handheld GPS, nothing works there. Going through mountain tunnels also shuts everything-GPS down. The speedo is fine, though :)
 

·
Registered
Joined
·
92 Posts
Speedometer is set in the Integrated Chassis Management module in the C_Dyn_Rollradius parameter and is the radius of your tire expressed in centimeters

Example: 225/45R17 has a diameter of 63, so your value would be 31 (no half sizes)

Special thanks to Shawn for teaching me that when I was swapping winter wheels from a 535ix to an X3
 

·
Read Only
Joined
·
752 Posts
Discussion Starter · #11 ·
Speedometer is set in the Integrated Chassis Management module in the C_Dyn_Rollradius parameter and is the radius of your tire expressed in centimeters

Example: 225/45R17 has a diameter of 63, so your value would be 31 (no half sizes)

Special thanks to Shawn for teaching me that when I was swapping winter wheels from a 535ix to an X3
This is what created the dilemma of my initial post. Above all presupposes all 4 tires/wheels being the same diameter and size. The 550 M-sport came equipped with 245/40-19 F & 275/35-19 R. Upon stopwatch clocking the car over precise mileage markers at various speeds up to 90mph, I determined my speedometer was dead on, within the tolerances of my reaction time on the stop watch.
Then, I switched to 245/40-19 F & 275/40-19 R, giving me ~ 1" greater diameter on the rear axle. If Shawn had given you the entire algorithm for calculating the display on the speedometer, why are my speeds still exactly dead on, up to 90 mph, after my last speedometer check?

From the thread, Shawn had 1/2 the picture. Except, the only explanation to fit the data is that BMW uses only the front wheels to perform the rotational average.
 
1 - 12 of 12 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top