Tom & Ray recently answered a question about a blow tires on a 2005 Vibe after spinning in snow. Part of the answer stated that the tire speed was double the speedometer indicated speed due to the average axle speed (If speed shows 60, but only one tire is spinning, then actual was 120).
I am not an Vibe expert, but I find this hard to believe! In the old days, speed was based of the transmission output (cable at rear of transmission led directly to speedometer). I suspect in newer cars the speed is either calculate from electronic data from the transmission, or a combination of the engine data and transmission. I have never seen sensors on axles for speeds (not counting ABS sensor, but I don’t think they are use to dispaly speeds)
Are you certain that the speed shown on speedometer is average of axle(s)??? I really doubt it.
If you lock one wheel so it doesn’t spin, then the other wheel goes double speed, that’s the way differential gears work. So, the wheel that’s free to spin will spin at double the mph that the speedometer indicates if the speedometer is driven off the transmission output shaft.
Next time your car is on a lift, turn one of the drive wheels, the other one will turn in the opposite direction if the transmission is in gear or in park not allowing the driveshaft to rotate. One wheel turning forward while the other is turning backwards equals a zero average.
Well put. I was getting ready to say something to this effect.
sorry, i still don’t buy it. see http://auto.howstuffworks.com/differential2.htm and http://auto.howstuffworks.com/differential4.htm
The ring gear in the differential turns at the same speed whether one wheel ‘slips’ as in making turns, or if slipping in snow or whether driving straight with both wheels are turning, at a set engine rpm and particular transmission gear. So the speed indicated with the speedometer is not averaged between the axles!
Also, what of the case of Limited Slip differentials aka posi traction? For a given rotation rate of the driveshaft, the rotation rate of the ring gear is the same. An open differential (one wheel drives) and a limited slip (both wheels drive) will show same miles per hour (assuming same gear ratios)
Looking at it another way, the driveshaft rotation (thus ring gear rotation) indicates the speed, not the averaging of the axles. Again, i know for certain older cars use the transmission/driveshaft rotation to control the speedometer (speedometer cable came directly from rear of transmission, straight into the speedometer instrument cluster). Were those speeds ‘double’ in older cars? No
The function of the tires rotating opposite when the car is jacked up is a product of the two pinion gear rotating against each other since the ring gear is locked (i.e. ring gear can’t move since in park, see http://auto.howstuffworks.com/differential2.htm note small red pinion gears). It does not prove ‘averaging’ of the axles for the car/wheel speed.
With a given drive shaft rotational speed (assuming a 1 to 1 ratio) both wheels will spin at the same rpm as the shaft, but if one wheel is immobilized, the other one will spin at twice that of the drive shaft.
Why?
Here is how a diff works: http://www.youtube.com/watch?v=yYAw79386WI
You can skip by the motorcyclists at the beginning, unless you want to watch them.
Still assuming a final drive ratio of 1 to 1:
Basically the ring gear has a direct connection to the pinion gears. When going around a corner the ring gear rotates at the same speed as before, but the pinion gears rotate inside their case to allow the outer wheel to rotate faster. So when one wheel cannot turn, or the other one can turn with no resistance the pinion gears act like a set of planetary gears. They ride against the stationary gear and rotate following the ring gear. The other axel? (I don’t know the name) gear is then turned at twice the speed of the drive shaft.
Think about it this way. If you have a rubber ball and you put your hand on top of it and move it in any direction your hand will always be moving at twice the speed of the ball.
The idea was the tire came apart because it was being spun faster than its speed rating? (this is a question) The idea that all a tire needs in order to come apart is a few seconds (or a minute) at a speed above its rating is a bit of a stretch for me. I would think the tire had to be damaged before all this spinning took place in order for it to come apart.
OP if this is not the theory why the tire came apart please elaborate (you write “about a blow tire”) I am thinking you mean to write “about a blown tire”.
Think of it this way: the ring gear rotates at the average between the right axle and the left axle. When the left and right axles are rotating at different speeds, the difference is taken up by the planetary gears rotating around the axles’ pinion gears.
In short, for the ring gear to rotate at a speed that would equate to 60 mph with one wheel not turning, the other wheel would need to be spinning at the equivalent of 120 mph. The planetary gears would be spinning as well as orbiting the axles’ pinion gears and taking up the difference in the axles’ speeds.
A tire’s maximum speed, its “speed rating” defines the maximum speed in mph that a tire has been tested without load to be safe before exploding from centrifugal force. Were a tire to be rated less than 120 mph, it is quote conceivable that a loaded tire when spun at the equivalent of 120 mph could blow, especially if subjected to the sudden stresses of suddenly hitting pavement.
The example is spinning in snow, I sense a strawman fallacy here (strawman being a "dishonest characterization) no I am not saying you are dishonest,just defining “strawman”
With one wheel spinning in snow, the problem at hand fits my description perfectly. One wheel is stopped, one wheel is spinning, and the difference in axle speeds is taken up by the axle pinion gears causing spinning and orbiting of the planetary gears. The orbiting of the planetary gears turns the housing which turns the ring gear (at the average of the axle speeds) which turns the driveshaft which is converted to mph by the speedo.
My point was to look at the problem from the wheels inward rather than from the driveshaft outward to better understand how the spinning tire presents itself on the speedo. Persuant to the blown tire, I also wanted to clarify that this manifestation is indeed viable.
You should buy it, what you are being told is true.
What does the final drive have to do with it??
It’s true. The tire was spinning at twice the speed as the speedometer. The proof is that the tire was blown. A tire usually will not blow at 60mph but will certainly blow at 120+ when it heats up. I can also assure you that most drivers will exceed 60mph when their vehicle is stuck in mud, snow or ice.
“With a given drive shaft rotational speed (assuming a 1 to 1 ratio) both wheels will spin at the same rpm as the shaft, but if one wheel is immobilized, the other one will spin at twice that of the drive shaft.”
I don’t know of any final drive that is 1 to 1. The drive shaft turns from 2.76 to 1 up to about 4.10 to 1 faster then the wheels. That’s 3 to 4 times faster than the wheels. But as has been said here, when one wheel stops, the other has to turn twice as fast.
It makes no difference where the speedometer gets it’s reading, or if it’s a Vibe or a Trans AM. The speedometer will read the average between the speed of the wheels.
I think the OP thinks we are trying to tell him that the speedo “computes” the average between the 2 wheels. NO, it DICTATES the speed of the wheels. They have to average the speed that the speedo (driveshaft) dictates. If the speedo reads 60 MPH and one wheel is spinning at, say 70 MPH then the other has to turn at 50 MPH. Clear??
So, are we clear now? Must be, no one has commented lately/
swojciechowski, all clear now??
Just because you don’t understand it doesn’t mean it isn’t true.
A tires speed rating is not the point it explodes without load. It is the maximum safe cruising speed , with a built in safety factor, with the car at maximum loaded weight in the heat of summer.
Hello, probably that’s off the topic, but may be smb will be interested.
Recently i went to check suspension for my 1999 Focus, in the process mechanic lifted one wheel off the ground and asked to spin it and turn off engine and put out of gear. At fastest moment speedo showed about 140km/h (87mph), so wheel spinned about 280 km/h (174mph). It was repeated 2 times, although, second time may be it was a little bit slower. Tyres are old (about 7 years) and are rated 160 km/h (100mph) top speed. Nothing happened (tyre didn’t blow). Of course, moment was brief, but still, i don’t think tyres blow unless usage is really way out of specs.
He spun the wheel that fast by hand? Wow. This guy’s exceptional!
Of course, that was an unloaded speed.
Let’s put it this way, yes, there are now speed sensors on each wheel these days. It’s a part of the Antilock brake system. It’s also a feature to help the vehicle better determine the actual speed.