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"Ninety Seven Horsepower is nothing to sneeze at in a cruiser, but not really earth shattering. However, I can hardly say the same about a no-foolin One Hundred Twenty Three Foot Pounds at the rear tire. Yes indeed, that just might shatter the earth."



Hardly - it's not earth shattering either. Sure it sounds impressive, but the engine maxes out at ~5000 rpm. I wonder what the gearing ratios are?



I'd imagine that if you looked at the thrust curves for the Vulcan and compared them to an FZ1, ZRX, etc, that peak torque value would suddenly be less impressive than what was quoted above.



-- Michael



 

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Nope. I do have a B.Eng, though.



What can I say: magazines/people quoting peak torque values without context is one of my hot buttons. The simple fact is that peak torque isn't a very reliable indicator of performance.



-- Michael





 

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"Less intelligent" is a bit harsh: the fact is that many people don't really understand the relationship between torque and horsepower, and that magazines quite often confuse the issue.



The concepts aren't really that hard, and the math involved is pretty simple.



A quick calculation based on your post:



Assume we want to spin a rear wheel at 1000 rpm. What's the torque applied to that wheel for these two engines? ( Assuming full throttle, and that we gear for the torque peak. )



Geared down to 1000 rpm, the Vulcan will have 366 lbft of torque at the rear.

( 3000/1000 = 3 ; 3*122 = 366 )



Geared down to 1000 rpm, the FZ1 will have ~548 lbft of torque at the rear.



It's not a complete example by any means, but it does illustrate the point...



-- Michael



 

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I totally agree. Ultimate performance is a combination of many things, one of the biggest being the rider.



I'm also not saying that everyone should buy litrebikes, or that one type of bike is "better" than another.



My point is very basic: peak engine torque values alone aren't very useful for determining comparative performance. As was shown, a bike with lower absolute engine torque values may in fact put more torque to the wheel because of the ability to deliver that torque at high revs.







-- Michael



 

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Why?



Bikes have different gear reduction ratios, so at the same road speeds will being turning at different RPMs. What is the point of comparing at a given RPM value?



-- Michael



 

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WTF, over? Ya lost me in that second para.



Torque is force. Acceleration of a bike follows the torque curve.



Horsepower is the ability to do work over time.



If a bike is operating at the HP peak, this means that *for that speed* the bike is putting the most torque to the wheel/thrust to the road that it possibly can. In an ideal world, we'd have CVTs, and under hard acceleration the engine would always be reving at the power peak...



-- Michael
 

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It was a thought experiment - to illustrate why comparing just the absolute peak torque value is not all that useful. I had a disclaimer at the end of my post after all... :)

Let's look at a real world example.

Question - what puts the highest peak torque to the rear wheel in first gear: a Buell XB12R or a Yamaha R6?

Place your bets folks, before we continue!

From

http://www.motorcycle.com/mo/mccompare/03_Funny_Twins/index.motml

XB12R 72.6 lbft @5750 rpm

From

http://www.motorcycle.com/mo/mccompare/03_600s/dynos/600TQ03.jpg

it's not amazingly clear, but we'll say

R6 44.3 [email protected],000 rpm

Anybody change their bet yet? :) If you still focus on the absolute number, you'd say Buell, right?

Now, let's factor in gearing.

For the Buell

http://www.buell.com/en_us/products/motorcycles/firebolt/12r/12r_features.asp

we see that the first gear reduction is:

(1.5)*(2.407)*(2.688) = ~ 9.7

For the R6

http://www.yamaha-motor.com/products/unitspecs.asp?lid=2&lc=mcy&cid=5&mid=8

the first gear reduction is:

(1.955)*(3)*(2.846) = ~ 16.69

And to find the torque to the back wheel...

XB12R 72.6 * 9.70 = ~704 lbft

R6 44.3 * 16.69 = ~739 lbft

So it appears that the correct answer is: the R6! [1]

That's why comparing absolute engine torque numbers isn't all that useful. Horsepower is a bit better, but personally I like to actually see what the thrust curves[2] are, 'cause then you'll see the effects of gearing.[3]

-- Michael

[1] And before I get flamed I'd like to point out that the R6 is significantly faster than the XB12R in the 1/4. There's a reason for that...this isn't just theory!

[2] This involves an additional calculation, based on the tire size, and the "lever" that the torque acts on wrt the ground.

[3] And even this isn't perfect. Another discussion for another time... :)
 

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>You just failed physics 101.



Ok.



>A dyno measures torque at the back wheel, so the gearing is already accounted for.



A dyno measures torque at the back wheel, and then "normalizes" the values to get an accurate representation of what the engine is capable of, after frictional losses.



Manufacturers often quote torque/hp values based on readings taken from an engine dyno. That's why rear wheel dynos are important - they give a more realistic view of the engine's potential.



Think about it: if the dyno charts accurately refected what was supplied to the rear wheel, then you've basically said that the various gears actually don't have any effect on the torque delivered to the road.



So I guess your bike accelerates at the same rate from a standing start in 5th gear as it does in 1st?



Another point to consider: if the dyno shows what is happening at the back wheel, then bikes are insanely fast. For the Vulcan: top rpm is ~5200 rpm. Spinning the rear tire at 5200 rpm is roughly equivalent to a speed of 400 mph. A GSXR with a redline of ~12000 rpm? Good for 900mph.



Does that set off any alarm bells?



-- Michael

 

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You are confusing the speed of the bike (mph), with the speed of the engine.(rpm)



I said that if a bike is at the HP peak, then for that road speed, the bike is putting the most torque to the back wheel that it can.



An example:



I'm feeling Buellish, so we'll use an XB12R.



Data for an XB12R:



89.8Hp. @ 6,700rpm

72.6LbFt. @ 5,750rpm



Rear tire: 180/55ZR-17



This isn't entirely exact, but the conversion from rear wheel rpm to mph for such a tire is roughly mph = 0.0738 * rwrpm



Pick any road speed you want. I'm going to go with 40 mph.



In order to go 40 mph, the rear tire must be turning at 40/0.0738 = ~542 rpm



Assuming full throttle...



If you are going 40 mph, and are at the torque peak, the torque being applied to the rear wheel is



72.6 * ( 5750 / 542 ) = 770 lbft



At the horsepower peak, the Buell is putting out



( 89.8 * 5252 ) / 6700 = ~ 70.4 lbft from the engine



but the torque to the rear wheel for 40 mph at the HP peak is



70.4 * ( 6700 /542 ) = 870 lbft



Repeat the calcs for other speeds/bikes/whatever. It doesn't matter.



For a given road speed, you will put the most torque to the back wheel if you are at the HP peak.



-- Michael





 

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>You can't turn 44 inch boggers on your 4x4 without a lot more gear reduction.



Well, sure. You've increased the load on the engine, so the torque that the existing gearing delivers may not be sufficient. Big wheels means more rotational and linear inertia to overcome.



You may have to regear if you want decent performance.



-- Michael

 

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Re: THis thread is getting too long

Let's agree to disagree.

Nah. That's no fun.



Still, what I am saying, is that a motor like the V2K's, with a massive low-rpm torque peak, offers useable performance without wringing it out.



And all I've been saying is that "massive" torque peak isn't all that massive. That's the point - unless you are willing to calculate thrust, you may as well ignore the peak torque value if you want to compare bikes.

I'll give you another way to look at it: 123[email protected]000 rpm is ~70hp. While 70hp isn't "wussy", it's not awe-inspiring either.



Sure, you could have a six cylinder 250cc engine turning 24000 RPM which TECHNICALLY transfers more torque to the pavement but that torque isn't as ACCESSIBLE. I have



I never advocated 24000 rpm bikes - I said if you just compare bikes based on the absolute engine torque peak, you could easily wind up making an invalid comparison. Hence the example of the R6 and XB12R.

I'm not sure what you mean by "ACCESSIBLE"?



dragraced and beat 600cc supersport motorcycles with my heavier and less powerful VFR, but it takes a more skillfull rider to exploit the power that a peaky motor like a 600 offers.



What's the difference in thrust values between your VFR and a 600?



A bike like the V2k isn't at the cutting edge of technology, but I bet it would be a gas to ride.



I never said that it wouldn't be. Most bikes are fun.

-- Michael
 

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The RPM-gearing calculations do not help, as I can get from my model RC car's .16 engine more torque than a Sherman tank, if suitable gearing is found.



Gearing is a trade off between speed and torque. You'll get the torque, but at a speed of 0.0000001 mph. Not very useful, wouldn't you agree?



So the torque that is worth talking is solely engine torque. And a torquey motor is not a wheezy 7,400RPM thingie, but one where the stuff comes by $hitloads at 1,800 RPM.



Because, as we all know, bikes don't have gears or a transmission and the reduction ratio between the engine and rear tire is always 1:1. (cough, cough )



Either method will prove without doubt that those litre bikes do not compare to this Kawimonster.



Totally true. Litre bikes will do the 1/4 mile in the 10s, while the Vulcan will be somewhere in the 12s.

The Vulcan might be the right choice for you, but I'd rather have something with some gumption. I'll take the FZ-1.

-- Michael
 
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