I have a fundamental question:

Which of the following statements are true?

1) peak horsepower will happen on peak rmp after which a failure is likely. (lets call this max_hp_rpm)

2) peak torque will happen sometime before the peak rpm. this is when the bike accelates the fastest. (lets call this max_torque_rpm).

3) the smaller the value of the ratio
max_hp_rpm/max_torque_rpm
the better the bike's acceleration is (at low speeds 0-70 mph). (lets call this peak torque ratio)

Assuming 2 bikes have same or similar hp and similar peak torque values, but very different peak torque ratios, then the one with lower peak torque ratio will out accelerate the other one (speeds of 0-70 mph).

Is this statement true?

What am I missing?

Quote:


Well, no. What everyone is actually measuring on a dyno is torque. Horsepower is very coarsely torque times rpm. If the torque curve falls away after peaking at well below max rpm the peak horsepower can't occur at or near peak rpm as they are related. Max power would be found at max rpm only if max torque rpm was quite close to max rpm. Theorethical max rpm is mostly limited by piston speed (which depends on the lenght of engine stroke) and there's an old adage about mean piston speed - if it's over 23 m/s at redline rpm the engine's either very good design or you are pushing your luck. So - peak torque rpm is limited by state of tune and engine design, peak power rpm is related to torque curve and both of them are in most cases well below max safe rpm.  

Quote:


No, the torque very basically tells you how much work the engine can do. Power tells you how quickly the engine can do the work. For performance standpoint it's better to have torque at high rpm because it will make more hp that way. The peak acceleration appears somewhere between the torque and power peaks. If the torque falls away fast after peaking then you will get maximum acceleration close to torque peak rpm. It's a question of torque curve shape, also aerodynamics, gearing.. and we can't make exact assumptions where the peak acceleration (maximum thrust) will appear.

Quote:


This is quite close to what I think as truth. If the peak torque rpm was very low and the max hp rpm high, the max hp would be at high rpm only because the engine was still making torque at those highish revs. You would have wide torque band which helps with acceleration compared to very peaky engines having only top or low end. You would have for example two 60 hp engines, both making max power at 8000rpm, both making 40ft/lbs of torque, the first one making the peak torque at 3500rpm and the other one at 7500rpm, yes, the first one would likely be faster. Do also remember that peak torque is just that - if the other engine made 39ft/lbs all the way from 1500rpms to 7500rpms, then it also had very wide torque band and would be just like the first one. The curve shape is more important than the absolute peak value.

In practice there are things like bike weight, gearing, aerodynamics which affect the true acceleration figures. Peak torque or power is not everything. I just recently found what I rate as the first reliable performance data for Triumph Rocket III. A true 119 hp and 137.6ft/lbs. In top gear roll ons the thing is absolutely useless. 40 to 90mph in top gear it's barely faster than the fastest 600 sport bikes and gets totally whumped by a good 1000 four. I thought the Triumph would do much better than this.

On my friend's racing sidecar outfit the change up point is after peak power to have the same BHP both before and after the change.

Clive W  

Tumi Laamanen wrote on 12/31/69 at 16:00:10:


Well, you gotta believe what ya gotta believe.  I listed the magazine title above.  It was that one and Cruising Rider that did the tests.  Both with high praise for the bike.  All that I could find this morning online was an intro article (the publicity hype stuff).  However there are even better articles on the bike that I cannot find right now.

The bike isn't intended to compare with 600 or 1000 sportbikes and it weighs heavily on the sidestand as well at nearly 800 ibs.  However the comparisons were made with the V-Max which still claims production bike 1/4 mile times after all these years....and the V-Max is a 1200 and much lighter.  Still the Rocket, in both mag tests, was about a 1/2 second faster than the V-Max (Have you seen the new anniversary version V-Max?)

The bike was also compared with the Vulcan 2000, a new Harley and a couple others.  Bikes that actually weigh more, so the Rocket isn't all dinosaur either.

Now, don't get me wrong, the Rocket scrapes footpegs before even getting close to a 600 sportbike, but that isn't because of the engine either....you know there is much more too it than that.

The Rocket was simply tested for handling for what it is designed to do, and at the same time it returned dyno and 1/4 mile results that blow all others away.  You can't sneeze at that torque and power, and I personally can't wait to pull up behind a bike with 3 mufflers 'cause I'll be looking....

It's all about weight and power.  If you need more weight to contain the power go for it.  The Rocket comes on it's peak torque at 2K, how fast you got to wind up a sportbike to do the same?

BTW - my last bike was a Yamaha 850 triple, so I'm partial to singles and triples  

Well, I guess we've come to a happy medium on this discussion.  All that I know is what I read (I wish it were the old days when test rides were encouraged by the dealers), but lacking my own experience, reading is all that I have.  I compare the numbers and I compare the articles with my own knowledge of bikes and I make judgements from there....

By the way, the only "modern" sportbike that I have really ridden was a 1200 Bandit.  That bike screams, I will admit.

With every bike I've owned, they have been different from the popular majority.  Just my way to be different.  Thumpers, triples, 2 strokes.....so I have a Savage.

IF and when I decide to get another bike it will be a Triumph Rocket III.  Whether it IS the fastest or not is never my priority.  What I like is that it is a real performer (maybe not the best), and it is unique.  It has a longitudinal inline 3 engine with 4 inch pistons.  Man, what a powerplant.  It also has the largest rear tire on a production bike at 240.  What a footprint that puppy makes.  It also "supposedly" wheelies any time you want it to...and that will be all that my senior citizen adrenaline levels can handle anyway  

Besides, it is a Triumph and I first rode a Cub as a teenager and never forgot.