barry68v10 wrote on 05/29/08 at 18:07:38:
What makes you think that you have to slip the clutch at higher revs when you change your gearing???  I  pull away from the line in road cars and bikes slipping the clutch in just above idle, wherever the torque peak may be.  If you always slip the clutch to pull away at peak torque then I’d agree – that’s very unhealthy for your vehicles.

barry68v10 wrote on 05/29/08 at 18:07:38:
Mostly true, though in some cases the twin can be more compact than a long stroke single, and mpg won’t necessarily drop if the smaller engine has a more efficient combustion chamber size.  I wonder though why cars aren’t all single cylinder vehicles, if they’d be lighter, cheaper, more compact and more efficient if they were all singles?

barry68v10 wrote on 05/29/08 at 18:07:38:
None, as well you know, because high revving powerful bikes end up having their gear ratios raised to prevent excessive lifting of the front wheel under acceleration – my sports bike is geared for over 90mph in first just to keep the front wheel on the ground if I open the throttle.
However, the fact remains that cars and bikes with higher-revving motors are geared to take advantage of producing torque higher.

This applies to a lot of ‘sport’ models of cars – they get a more aggressive camshaft to raise the torque curve, and get geared down to take advantage of it.  It even happened in the late 60s and seventies – I seem to recall that some of the Firebird Ram Air 4 and Trans Am models with higher lift cams produced exactly the same torque peak as the standard cam, but higher up the rev range – with the result that they could run a lower differential, and that made them go quicker.  In the real world.  On the highway.

barry68v10 wrote on 05/29/08 at 18:07:38:
Sorry, but that’s categorically wrong.  It is entirely dependent on gearing, and the combination of gearing and torque curve to determine tractive force at the wheels.  By using advantageous gearing, the higher revving motor can give greater tractive force at the wheels across the entire rev range, and therefore accelerate harder.

I don’t advocate using high revving engines for bikes like the Savage – it’s fine just the way it is – but rest assured you make any vehicle quicker if you raise the torque curve and gear it to suit.

This seems to be a good comparison.


http://photos.motorcycle-usa.com/bmw_vs_xr_dyno_5_23_03.JPG
http://www.biketestusa.com/Article_Page.aspx?ArticleID=71&Page=4

The two bikes produce similar ‘amounts’ of torque, in terms of spread of delivery above 25ftlb, but the BMW twin produces its slightly higher peak about 2000rpm further up the rev range than the Honda single.

The article seems an impartial comparison, but this quote is relevant:

“the 324-lb. XR pulls harder at lower speeds than the 412-lb. BMW. The XR's lower torque peak gives a rider immediate access to its fairly shallow well of power, but the BMW walks away from the XR during impromptu drag races, despite the Honda's better power-to-weight ratio”.

Paladin. wrote on 05/29/08 at 19:31:58:

Can't argue with that, and I've ridden a KLR650 so I can confirm the torque is great for lifting the front wheel.
But, as you'll see below, the spread of torque you get from the single is just lower down the rev range than from twins and fours - they have significantly wider spreads of more torque, they just start higher.

The only real advantage of the thumper is that the torque is very low and allows you to pootle round town in high gear without changing down (which is great in itself and makes them fun to chug round on) - but it certainly isn't going to make a single out-torque or out-accelerate a twin or multi.

KwakNut wrote on 05/29/08 at 16:43:17:

Technically I disagree with you.
Power is not a function of rpm's and torque, otherwise HP and torque curves would be a function of each other and there would be no need to map each curve.
Power is the ability to move X pounds a distance in an amount of time.
Torque is the ability to twist a shaft.
And traction is the ability to transmit power to the road.
And stating that traction equates acceleration assumes power to be greater than traction.  Acceleration equates to the lesser of the 2.
ie. spinning wheels at start, yet slow times at the trap.

Hmmm, 1000 HP lawnmower.... you related to Tim Allen?

verslagen1 wrote on 05/30/08 at 10:14:16:
Well, I guess since that post you’ll have seen Furious70 and Robertomoe’s posts quoting the formula which links HP to torque, so you may be reconsidering your technical disagreement on the power function matter.

You only need to have a read out of the torque curve for an engine, and you can plot power, or vice versa – you’re quite right that you actually don’t need to map each curve, when you’ve seen enough dyno printouts you can see both in your head with just one on the sheet!

Torque is indeed ‘twist’ on a shaft, force times distance hence lbs ft.

Power is a bit more of a leap of faith because it’s harder to visualise, but in automotive terms:

Power = tractive force x velocity

(That’s why you need 8 times the power to go twice as fast against wind resistance – power is tractive force times speed, and wind resistance (the tractive force required) is proportional to the square of speed, so power required is cube of speed increase – a 100hp car which can hit 100mph would need 800hp to hit 200mph).  Power is also voltage times current, and energy is the integral of power just to complicate matters!

I wrote an Excel spreadsheet a few years ago into which you can enter your power (or torque) curve, gearing details and vehicle weight, and it will work out your tractive force at the wheels and give you 0-60 and quarter times – it is very accurate too against real world tests and manufacturer’s figures.  I’ll dig it out and post a link to it on here somehow (it’s quite a big file, something like 300 lines of iterative calculation for each point on the curves).

I don’t see how there’s an assumption that power is greater than traction – though it often is.
Tractive force transmitted through the wheels is limited by available traction (as you state), but that tractive force is measured in pounds of thrust, or Newtons, and it is that which decides your acceleration back to one of our old high school formulae: F=ma  force = mass x acceleration.

Quote:

Very interested in playing around with this!

Quote:

The point I was trying to make was that you measure tq and you calculate horsepower.  The reverse is not true.  So while you can manipulate the formula to solve for TQ if you know HP and rpm, it's a moot point because you had to know the tq value to get HP in the first place.  That manipulation of the formula is only useful for determining what tq an engine would need to produce at a given rpm for a theorical HP goal in mind or something.