Physics talk

[mossy1200]

I'm not sure how much truth there is in this.

Yes when you pick a wheel up (radio controlled car works good) and accelerate the tyre deforms I know how your so called "centrifugal" force works, but once you put it on a road, with a normal force, things start changing dramatically.

Centrifugal growth can cause tyre diameter to increase 2% on a steel belt and more on older tyre.
At slow speeds inflation pressures can change the distance travelled per rotation also.
A under inflated tyre will cover less distance at slower speeds but will grow taller than a correctly inflated tyre at high speeds.
They are only small amounts.

The same goes for a gearbox driven speedo. As the bike gets faster the percentage of wheel slip increases and the speedo reads faster than true speed travelled. Some bike racing on salt lakes travelling 300 have a rear wheel speed of 330+ losing 30kph or more in wheel spin.

These are all small factors when riding a bike and wouldnt be enough to make a difference at road legal speeds though.

[Oakie]

Some bike racing on slat lakes travelling 300 have a rear wheel speed of 330+ losing 30kph or more in wheel spin.

Is a slat lake like a slat bed?

[Ocean1]

Some bike racing on slat lakes travelling 300 have a rear wheel speed of 330+ losing 30kph or more in wheel spin.

Must remember that next time...


Top Fuel racing slicks can grow 10" in diameter at 300mph. That's a Meter per rev more.

But there's no speedo to worry about, eh.

[mossy1200]

Must remember that next time...


Top Fuel racing slicks can grow 10" in diameter at 300mph. That's a Meter per rev more.

But there's no speedo to worry about, eh.

Its my excuse for doing 286 at the last quarter mile. Im sure I had 14kph of wheel spin going on.

[mossy1200]

Is a slat lake like a slat bed?

Yes. Nobody races bikes on waterbeds.

[Hawk]

want if you are traveling in the opposite direction of the earths rotation does that mean you are still moving, going slower, or faster

[FJRider]

want if you are traveling in the opposite direction of the earths rotation does that mean you are still moving, going slower, or faster

That would depend on the settings of your Flux Capacitor ....

[Dave-]

Centrifugal growth can cause tyre diameter to increase 2% on a steel belt and more on older tyre.
At slow speeds inflation pressures can change the distance travelled per rotation also.
A under inflated tyre will cover less distance at slower speeds but will grow taller than a correctly inflated tyre at high speeds.
They are only small amounts.

The same goes for a gearbox driven speedo. As the bike gets faster the percentage of wheel slip increases and the speedo reads faster than true speed travelled. Some bike racing on salt lakes travelling 300 have a rear wheel speed of 330+ losing 30kph or more in wheel spin.

These are all small factors when riding a bike and wouldnt be enough to make a difference at road legal speeds though.

Centrifugal force doesn't exist, so I'm not sure what you're on about there.

But even if your tyre expands by 2% it's pressed on to the road by some mass which is going to be greater than the 2% increase, or so I think, and my very quick calcs would suggest.

I'm actually putting together the idea of the smart-bike which would take your velocity primarily from GPS and/or accelerometer it's only a matter of time before the bike becomes "smart"

Even if you are going down a very steep hill and your movement around the earths circumference is 100 but trig calcs would have you doing a true speed higher than the gps shows.

Does a GPS register distance calculated including verticle climb angles?

GPS reads in 3 dimensions from at least 4 satellites in geosynchronous orbit (which means they don't move relative to the earth) so even when you're going up or down hill you get the magnitude of velocity.

[scumdog]

But even if your tyre expands by 2% it's pressed on to the road by some mass which is going to be greater than the 2% increase, or so I think, and my very quick calcs would suggest.

.

Hmm, from memory those who race dragsters have to calculate tyre 'growth' into the calculations for the correct gearing...have a look at one at rest and then look at the tyre diameter change when the dragster is in 'full flight'.

[Dave-]

Hmm, from memory those who race dragsters have to calculate tyre 'growth' into the calculations for the correct gearing...have a look at one at rest and then look at the tyre diameter change when the dragster is in 'full flight'.

That's cause the ratio of the weight of the car vs the centripetal force of the tyre spinning is lower, so yes I would expect that.

Then again they have a great big wing pressing them on to the track, but that'll only work for ~120kph? at a guess? so before that while the wheel is spinning they may very well have to account for it.

Like I said, I may be wrong, but I'm fairly certain that the expansion of a road bike wheel wont make much of a difference on the force the body provides putting it there, my quick calcs suggested it would, but I had to pull some numbers out of my ass for an arbitrarily thin piece of tyre.

plus I'm electrical engineering, I'll run it buy the mechanical guys.

someone ought to tell me I'm a know it all scientist soon, that's the usual KB way.....

edit: the mechanical guys have just told me they think the tyre will get smaller cause it'll be mushed into the track.

there was a shot at bathurst showing this down conrod straight a few weeks ago, probably on youtube somewhere.

[Coolz]

Dave...your idea that centrifugal force dosn't exist and bikes are getting smarter all the time suggests to me that you have lost that race already!

[onearmedbandit]

Then again they have a great big wing pressing them on to the track, but that'll only work for ~120kph? at a guess? so before that while the wheel is spinning they may very well have to account for it.

Apparently the rear wing at speed generates 5000lb of downward force at speed, yet the diameter of the rear tyres grow a few inches right through the whole run, including terminal velocity.

[Dave-]

Dave...your idea that centrifugal force dosn't exist and bikes are getting smarter all the time suggests to me that you have lost that race already!

Centrifugal force is what they teach you in high school cause your dumb undeveloped teenage brain can't comprehend that the direction of the force is towards the center of rotation.

And it works well for most people. If you want to know more most universities cover it in 100 level physics.

To conclude I think there's other things that attribute to the uncertainty in speedo reading, mechanical slip of the tyre, electronic pulses, the impedance of the inductor on the speedo increases with frequency, temperature, etc, on modern digital tachometers (the thing that shows you revolutions per minute) to save on costs they don't filter the signal, and it can get really saw-tooth shaped if you don't, when you do however you get a much more accurate representation, I know this cause I designed and built one at uni this year.

If you're really worried about it get a GPS, they're fail proof....except for when you cant connect to enough satellites

[Gremlin]

edit: the mechanical guys have just told me they think the tyre will get smaller cause it'll be mushed into the track.

there was a shot at bathurst showing this down conrod straight a few weeks ago, probably on youtube somewhere.

The tyre is getting squashed into the track because of the rather large amount of downforce being generated by the car above the tyres. On the flip side, watch drag cars when they warm up their tyres in the staging. Square conventional looking tyres turn into donuts (no RC, leave alone), quite an impressive deformation. I also enjoy the wheel shots from V8 supercars as they're going around corners

Dave...your idea that centrifugal force dosn't exist and bikes are getting smarter all the time suggests to me that you have lost that race already!

As Dave said, Centrifugal force doesn't actually exist... hell, I didn't learn much in physics, but at least I remembered that...

[dangerous]

the concorde strechs two foot longer in full flight, hows that farking work brain box's

and ive walked faster than a 767 doing 500kph... honest