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Discussion Starter #1
I know power to weight ratios are used to give an idea how quickly a bike will accelerate. But assuming you had two bikes of similar weight, power, and gearing, wouldn't the bike with the greater or broader torque accelerate quicker? Does anyone know of a formula that factors torque in as well to give a more accurate idea of a bike's acceleration? I
 

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I'll probably create a firestorm with this, but here goes.
Torque defines how hard (in our case) a rotating crankshaft twists. In other words, could a Bonneville engine develop enough torque to lift a 40 pound weight if it was attached to the crank with a one foot "weightless" lever? Torque simply defines whether an engine can do or not do a task, regardless of the time it takes it to perform the task.

HP on the other hand. defines how much work an engine can do within a specific period of time, or how much work can the engine perform within a second, a minute, etc.

The original steam engine formula defined hp as the ability to lift 33,000 pounds one foot in one minute, in other words, how much work could the engine perform over a measured amount of time.

In the US, where we use foot/pounds to represent torque, the formula for hp is:
HP= Torque (in ft/lbs) x RPM / 5255

So, given that hp determines how fast an engine can do work, what you need to do is calculate torque x rpm at various rpm ranges to get an idea about how much work (how fast can the bike accelerate) over a measured time period.

Let's say a motorcycle engine develops 100 ft/lbs of torque at 3000rpm, but that the torque drops off drastically after 3000rpm, to, say 60 ft/lb at 7000 rpm or redline.

hp at 3000rpm would be 100 x 3000/5255, or 57 hp.
hp at 7000rpm would be 60 x 7000 / 5255, or 79 hp

If that same engine could maintain 100 ft/lb torque at 7000, it would be a much quicker bike, since it would now be capable of developing 133hp at 7000rpm.

Soichero Honda figured this out long ago, spin it faster, and even f the torque wasn't all that high, you could get a lot of work done with an engine that could spin up to 12000rpm or higher. That explains why some of these small engines, like the Street Triple 675, with not a lot of torque, but able to rev into the 5 digit range, generates enough hp to blow the doors off a slower revving Bonnie that most likely generates a few more ft lb of torque (someplace) in its rpm range.
 

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What Ohio says is true.now another case say one bike makes alot of ft/lbs at lower rpms and the other bike doesnt make as many at low rpm but holds on to more ft/lbs at higher rpm. in that case the low rpm ft/lb bike will jump the other off the line but the high rpm bike will pull it on top end of the rpm range and if the rpm says up will out run the low rpm bike.So the low rpm bike makes more ft/lbs but the high rpm bike makes more hp.
 

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Discussion Starter #4
Thanks ohio, that's a great explanation.

By the way, the reason I'm curious about this is since I haven't had the opportunity to ride the bike I'm hoping to get - an '09 Scrambler - I'm trying to get a feel of how it's acceleration will compare with my last bike - an '83 Honda 750 Shadow. I did some power to weight calculations on both and the Shadow came up a little better - .142 to .131 for the Scram. The Shadow had 66 HP vs. 59 for the Scram, and weighed slightly more - 465 vs. 451 for the Shadow. Yet the Scram produces slightly more torque - 51 ft.\lbs. vs. 49 for the Shadow, so I was wondering if those few extra ft.\lbs. might offset the discrepancy in the power\weight ratio any. In other words, will higher torque, especially if it's produced over a wider rpm range, say something about a bike's acceleration that the power\weight ratio alone doesn't?

I do know that the Shadow was way quicker than my previous bike - an '81 Yamaha XS650 - even though it was just 100cc larger. It was easy to tell the difference because there was a period of almost a year when I owned both at the same time. The bikes weighed almost the same, the Shadow being like 7 lbs. heavier, but the Shadow produced about 11 hp more than the Yam, and about 7ft.\lb. more torque. I always felt torque had a lot to do with the difference as the Shadow just seemed quicker at low rpms like when coming out of a turn.

Even though it's been 18 years since I had the Shadow, I'm just hoping the Scram will at least feel pretty close to as quick as the Shadow acceleration-wise, though I don't expect it to come close to the top speed the Shadow's 6th gear gave it.
 

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Discussion Starter #5 (Edited)
thanks mike - just saw your reply and that helps answer my question as well. I think my Scram vs. '83 Shadow acceleration question may just come down to which has the broader torque band, something I may never know. Probably just have to wait until the seat of my pants tells me!
 

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Discussion Starter #7
Hated - yeah, the Scram's is measured at the crank and I'm almost certain that the Shadow's was too. I'm certain my old '81 Yam's was.
 

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It's a funny thing .... I owned a BMW K1200RS that was rated at 130hp at the crank, and approximately 110 at the rear wheel. This was at the same time I owned an R100GS rated at 58hp, and a Kawasaki W650 rated at 50 rwhp.

Funny thing was ..... the hp didn't seem to make one iota of difference about the fun factor.

I'd ride the K1200 and enjoy it, but could jump off it onto the W650 with less than half the power and have just as much fun riding it. Maybe even more. And there were times where I decided the old R100GS, with less hp, was the ideal bike for a long distance road trip.

For me, horsepower doesn't make the bike more or less fun. It's all in how the bike puts the power to the ground. Maybe more important, is how you perceive how 'genuine' the bike is, as it puts whatever power it has to the road.

It's all in how the bike feeds back to you. And how you respond to these feedbacks.

Bob
 

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Discussion Starter #9
Bob - real good point on not getting hung up on "power". There are dozens of subjective intangibles that make a bike fun or not fun. For me, one of those intangibles will be riding a bike you don't see every day and I think that's one of the most appealing things about the modern classics and Triumphs in general.
 

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I agree with the smiles per miles attitude, and the bonnie wins hands down over several other bikes I've owned. But for an example, a working buddy has a 750 Aero (Shadow) He was always complaining that it was a slug compared to his last Harley. At the time I had an 1100 V-Star. I always thought it lacked acceleration even though it ran fine. One day we had a go at it and the Yam blew away the Honda. I just chalked it up to C.C.'s. One thing I will tell you is that my 07 bonnie has much better accelaration than the V-Star. I know this because points on my ride home daly I crack it for fun, and the bonnie hits 85mph in the same distance the Yam only got to 70. Not scientific but this may rest your concerns about the Scramblers "Scram" ability.
 

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I know power to weight ratios are used to give an idea how quickly a bike will accelerate. But assuming you had two bikes of similar weight, power, and gearing, wouldn't the bike with the greater or broader torque accelerate quicker? Does anyone know of a formula that factors torque in as well to give a more accurate idea of a bike's acceleration? I
Well, when I do the math for engine tuning I use Driving Force (DF) in kiloNewtons (kN) which allows one to work out both velocity and acceleration for each gear throughout the rev range.

Out of interest if the bikes had similar weight, power and gearing then they would perform in a similar fashion.

To give an example a bike with 68bhp producing 2.4kN in first gear would probably wheelie like mad, in 6th gear producing 0.7kN would not wheelie (it would be gradual acceleration).
 

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Consider two bikes with identical hp but one has a much larger flywheel. From a dead stop but with the motor revved up, the flywheel mass will have STORED considerable power even though the bike doesn't make more power. It might be faster accelerating temporarily off the line and the rider might notice it. It would lose some responsiveness and make less measured power on a dynojet dyno because some of the power goes into accelerating the crankshaft instead of spinning the roller. It's the same concept as the bigger tires & wheels stealing hp on cars, the rotating inertia robs some power, except on the first example with two bikes drag racing revving the motor before the launch is permissable and the "robbed" power of the rotating mass becomes "stored" power available to move the bike.

Thinking about a different kind of race, if the same two bikes were going a steady 30mph and both riders opened the throttles at the same instant, the bike with the lighter flywheel would win (I think) because some of the power of the other bike would go into spinning up the heavier flywheel. The same total energy would be produced by both motors. The heavier flywheel bike would coast a little farther at the conclusion of the race.
 

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Discussion Starter #13
Derby Boy - I'm pretty sure the Scrambler will indeed scram fast enough for me, and there's gotta be something to all the "smiles per miles" stuff I've read concerning the modern classics that even if they don't scorch the tarmac, they bring a lot of "character" to the riding experience.

andy and dale - thanks for the interesting technical input. Guess there are a lot of factors to consider when judging a bike's true performance.
 

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I brought my bike purely for its character. I've always been a fan of both parallel twins and inline fours. I have found the latter getting too sanitised now so I opted for a new twin.
 

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A lot of torque can make bike slower.

My former big bore GSX1255R Suzuki with a lot of mods did 112lbft/164hp on rear wheel. Modern GSX1000R has 77,5lbft/165hp with only slip on and some finetuning of EFI. Despite less torque the new one is a lot faster on quarter mile because my bike was very difficult to handle. Enormous low end torque seemed to only lift the front end instead of moving the bike while new one is soft at low rpm and maintains good power on wide range at high rpm and is quite easy to launch.
 

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It's a funny thing .... I owned a BMW K1200RS that was rated at 130hp at the crank, and approximately 110 at the rear wheel. This was at the same time I owned an R100GS rated at 58hp, and a Kawasaki W650 rated at 50 rwhp.

Funny thing was ..... the hp didn't seem to make one iota of difference about the fun factor.

I'd ride the K1200 and enjoy it, but could jump off it onto the W650 with less than half the power and have just as much fun riding it. Maybe even more. And there were times where I decided the old R100GS, with less hp, was the ideal bike for a long distance road trip.

For me, horsepower doesn't make the bike more or less fun. It's all in how the bike puts the power to the ground. Maybe more important, is how you perceive how 'genuine' the bike is, as it puts whatever power it has to the road.

It's all in how the bike feeds back to you. And how you respond to these feedbacks.

Bob
Nicely said, and exactly what I was thinking.

I sold my '05 Suzuki Bandit 1200 and ended up (very happily) with the Bonneville as a daily rider. The Bandit was no lightweight, but no slouch in the Ooomph Dept. either.

Both bikes share relative simplicity, which I appreciate, particularly since I do my own wrenching. But the look, the character, the feel, and the way the Bonneville goes down the road, still has me grinning like an idiot. It feels special. Perhaps because it is special. :D
 

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Discussion Starter #17
MONTTU - that's interesting, but makes sense. Guess when it comes to torque there's such a thing as too much of a good thing if a bike's frame, suspension, geometry, or whatever can't transfer it into forward motion.
 

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Discussion Starter #18
rwantin - yeah it's not about getting from point A to point B as fast as possible, but standing out while doing it. That's why for me while it's fun to speculate on the Scram's performance while I wait to try one out, my gut tells me it'll give me all the thrills I'll need largely due to it's uniqueness. That's the fun of the modern classics - knowing that yours is apt to be the only one seen by any given person that day, that week - maybe even that year!
 

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My little BSA bantam would out drag the lot of ya :)

In top gear (3rd), it develops nearly a hole four of them horse power thingies.. And has plenty of them torques too, you just have to paddle the bike forward with your feet until it has enough momentum.
 
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