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-   SV Talk, Tuning & Tweaking (http://forums.sv650.org/forumdisplay.php?f=111)
-   -   sv650 top speed? (http://forums.sv650.org/showthread.php?t=113377)

flimbar 09-07-08 09:49 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by simesb (Post 1562964)
That's assuming that the error is the same each time, you could have
distance covered = real position B + error - real position A - error = real position B - real position A + 2xError
Yes, I was assuming they were talking about a systematic lag due to the round trip that is the same each time, like a systematic error due to being calibrated to the wrong wheel circumference or something.

There will of course be a random error that will be different each time but I reckon it is small for GPS compared to a speedo which is more likely to be consistently showing a speed that is wrong due to calibration.

TheStudent 09-07-08 10:44 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by flimbar (Post 1562910)
I agree, as they discuss on this thread:
http://www.physforum.com/index.php?showtopic=14909
if there was no friction then the top speed would be the speed of light no matter what size of bike or power of engine.
The physics forum provides a somewhat poor discussion to be honest, gears for one and this dude Einstein for two imply nothing is gonna reach the speed of light.

Quote:
Originally Posted by PeterM (Post 1562790)
No, it isn't. Reduced weight will allow you to reach a given speed quicker but the effect of drag at higher speeds beomes the largest impediment to achieving a high terminal speed.
Consider this, unbalanced forces design cause acceleration, balanced forces gives constant velocity. Forces against...
  • Drag from the rider and vehicle (Air resistance).
  • Resistance from tyres (Relative to tyre compound, surface, AND RIDER WEIGHT PUSHING DOWN ON TYRES).
With...
  • Acceleration from rear wheel.
Granted top speed will always be limited by gearing, you can't out do the bike, so top speed is always fixed in that sense. So... I'm riding my SV at 120mph and it won't go any faster, A is balanced with R & D. If i were 10Kg's lighter, then D lightly to be similar, but R "WOULD" be smaller, forces would unbalance, and i would accelerate until they re-balance at say 125mph.

Thus decreased weight will increase your top speed.

You could argue that drag increases with velocity and you would be right but it opens a can of worms in that drag isn't constant, it depends on air temperature, pressure, wind (and direction). But realistically it won't affect the forces as much as wieght of say, the bike or rider would on an SV anyway. Surely this is obvious? A bike and rider is maybe one fifth the size of a car, and much much quicker, you're not telling me bikes are so much faster than cars at speeds +80mph because of their drag coefficients, it's a weight game!

Baph 09-07-08 10:54 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by TheStudent (Post 1563065)
Consider this, unbalanced forces design cause acceleration, balanced forces gives constant velocity. Forces against...
  • Drag from the rider and vehicle (Air resistance).
  • Resistance from tyres (Relative to tyre compound, surface, AND RIDER WEIGHT PUSHING DOWN ON TYRES).
With...
  • Acceleration from rear wheel.
Granted top speed will always be limited by gearing, you can't out do the bike, so top speed is always fixed in that sense. So... I'm riding my SV at 120mph and it won't go any faster, A is balanced with R & D. If i were 10Kg's lighter, then D lightly to be similar, but R "WOULD" be smaller, forces would unbalance, and i would accelerate until they re-balance at say 125mph.

Thus decreased weight will increase your top speed.

You could argue that drag increases with velocity and you would be right but it opens a can of worms in that drag isn't constant, it depends on air temperature, pressure, wind (and direction). But realistically it won't affect the forces as much as wieght of say, the bike or rider would on an SV anyway. Surely this is obvious? A bike and rider is maybe one fifth the size of a car, and much much quicker, you're not telling me bikes are so much faster than cars at speeds +80mph because of their drag coefficients, it's a weight game!
By your argument, lets assume two riders testing the same bike, going down a 46degree incline.

First rider weighs 65kg, second rider weighs 140kg.

Which test would see the bike achieve a higher speed, assuming that neither test runs out of road, ever?

yorkie_chris 09-07-08 10:57 AM
Re: sv650 top speed?
 
All subjective to the particular situation anyway.

Rolling resistance can be reduced at higher speeds with more rigid (read inflated to higher pressure) tyres. So it's not really that much of a factor...

flimbar 09-07-08 11:02 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by TheStudent (Post 1563065)
The physics forum provides a somewhat poor discussion to be honest, gears for one and this dude Einstein for two imply nothing is gonna reach the speed of light.
When I say the top speed would be the speed of light I mean the limit is the speed of light, yes you'll never actually reach it.

flimbar 09-07-08 11:06 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by TheStudent (Post 1563065)
You could argue that drag increases with velocity and you would be right but it opens a can of worms in that drag isn't constant, it depends on air temperature, pressure, wind (and direction). But realistically it won't affect the forces as much as wieght of say, the bike or rider would on an SV anyway. Surely this is obvious? A bike and rider is maybe one fifth the size of a car, and much much quicker, you're not telling me bikes are so much faster than cars at speeds +80mph because of their drag coefficients, it's a weight game!
No, it's a drag game. It depends on the difference in the profile a bike and a car present to the wind not their relative weights, as said before, that only gives the bike a better acceleration.

Baph 09-07-08 11:20 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by flimbar (Post 1563111)
No, it's a drag game. It depends on the difference in the profile a bike and a car present to the wind not their relative weights, as said before, that only gives the bike a better acceleration.
I'd go as far as to say that it's drag vs power/weight ratio.

However, weight is a factor, just a negligable one compared to drag. For anyone that doubts this, a simple test is to ride an SVS as fast as possible, then take the screen off & repeat (whilst rider remains in the same position for both tests).

The second test there will have massively increased drag compared to the second, it will also be marginally lighter, but a lot slower.

TheStudent 09-07-08 11:38 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by Baph (Post 1563075)
By your argument, lets assume two riders testing the same bike, going down a 46degree incline.

First rider weighs 65kg, second rider weighs 140kg.

Which test would see the bike achieve a higher speed, assuming that neither test runs out of road, ever?
Can't be bothered to do the maths but at a glance... 46degrees implies the two components of their weight split, are in increased in quantity favouring motion rather than resistance to it implying the heavier rider would be faster.

However, the angular "tip" factor is only fractional at 46, rather than say 70! Personally I would guesstimate the difference is too great and the larger rider would still be slower, but without doing the maths it's only a guess.

Quote:
Originally Posted by flimbar (Post 1563111)
No, it's a drag game. It depends on the difference in the profile a bike and a car present to the wind not their relative weights, as said before, that only gives the bike a better acceleration.
](*,)

It was just an example to exagerate the effects of weight. I think you need a better understanding of forces IMO. Top speed and acceleration are both linked when talking about power to weight ratios and drag. They only vary differently with gearing.

Quote:
Originally Posted by Baph (Post 1563148)
I'd go as far as to say that it's drag vs power/weight ratio.

However, weight is a factor, just a negligable one compared to drag. For anyone that doubts this, a simple test is to ride an SVS as fast as possible, then take the screen off & repeat (whilst rider remains in the same position for both tests).

The second test there will have massively increased drag compared to the second, it will also be marginally lighter, but a lot slower.
WRONG....

The better test is that I've raced a naked many times, and the results are almost identical. These bikes have a similar PtoW yet massively different drag coefficients!

PtoW all the way!

flimbar 09-07-08 11:40 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by Baph (Post 1563148)
I'd go as far as to say that it's drag vs power/weight ratio.

However, weight is a factor, just a negligable one compared to drag. For anyone that doubts this, a simple test is to ride an SVS as fast as possible, then take the screen off & repeat (whilst rider remains in the same position for both tests).

The second test there will have massively increased drag compared to the second, it will also be marginally lighter, but a lot slower.
The power/weight ratio only affects the acceleration, not the top speed. The only way that weight affects the top speed is if it increases the friction of the tyres as student says. But I think this will be a negligible amount compared to, as you say, removing the screen.

When the engine is kicking out its maximum power the vehicle will continue to get faster until the friction equals the thrust provided by the engine regardless of the weight.

I think some people think that it is "obvious" that weight plays a part in that we are so used to thinking about how quick a bike takes off compared to a car based on power/weight ratios, but top speed is a different matter.

punyXpress 09-07-08 11:41 AM
Re: sv650 top speed?
 
Quote:
Originally Posted by simesb (Post 1562964)
That's assuming that the error is the same each time, you could have
distance covered = real position B + error - real position A - error = real position B - real position A + 2xError
Ever since my ( cheap & basic ) Garmin showed I had WALKED at 19mph, I have slightly doubted its accuracy regarding speed! :confused:


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