Hellfire
|
| posted on 5/8/10 at 05:09 PM |
|
|
It's also quite difficult to compare torque for BEC v CEC. There seems to be a common misconception that bike engines lack torque due to figures
being quoted at the crank. What you really need to consider, is that bike engines have a torque multiplier in the form of a primary reduction gear. In
the real world, BEC torque actually isn't as low as it would first appear............
Phil
|
|
|
|
|
Rocket_Rabbit
|
| posted on 5/8/10 at 09:30 PM |
|
|
quote:
Originally posted by Hellfire
It's also quite difficult to compare torque for BEC v CEC. There seems to be a common misconception that bike engines lack torque due to figures
being quoted at the crank. What you really need to consider, is that bike engines have a torque multiplier in the form of a primary reduction gear. In
the real world, BEC torque actually isn't as low as it would first appear............
Phil
Well said 
Too much of people watching and believing Top Gear and quoting 'Mid Range torque'
Here is an article I had published in a magazine after I got fed up one too many times with muppets quoting the clueless:
|
|
|
coyoteboy
|
| posted on 5/8/10 at 10:28 PM |
|
|
With respect, I'm far from clueless about power and torque, I've a fairly reasonable engineering background  While I'm still
researching the gearing behind bike engines but I would guesstimate that even taking bike gearing into account something like an R1 would struggle to
put the same values to the wheels as a much larger displacement turbocharged lump.
What are the primary and secondary reduction ratios of something like the R1? As you say, if the 70lbft is multiplied by something like 5:1 it could
be comparable. That said, what's the curve like? The 3S-GTE has a flatish curve from about 3000rpm to redline.
[Edited on 5/8/10 by coyoteboy]
|
|
|
coyoteboy
|
| posted on 5/8/10 at 11:26 PM |
|
|
Specs, found from 'tinternet;
Primary Reduction Ratio: 65/43 (1.512)
Secondary Reduction Ratio: 45/17 (2.647)
Gear Ratio - 1st Gear: 38/15 (2.533)
So looking at 1st, torque at hub, only to compare in a reasonable number of lines of text :
70lbft * 1.5 * 2.5 = 263lbft at the output sprocket. Then of course scaled further by final drive/chain-sprocket combination Typical values?
vs
300lbft * 3.583 (1st) * 3.93 (final) = 4,224lbft
I'd say my comment that the CEC posing better torque figures at 300/300 was a fair one, even after transmission unless the final drive on the
BEC is 16:1? Vmax of about the same (140ish) at redline 5th.
[Edited on 5/8/10 by coyoteboy]
|
|
|
Rocket_Rabbit
|
| posted on 6/8/10 at 01:11 AM |
|
|
quote:
Originally posted by coyoteboy
Specs, found from 'tinternet;
Primary Reduction Ratio: 65/43 (1.512)
Secondary Reduction Ratio: 45/17 (2.647)
Gear Ratio - 1st Gear: 38/15 (2.533)
So looking at 1st, torque at hub, only to compare in a reasonable number of lines of text :
70lbft * 1.5 * 2.5 = 263lbft at the output sprocket. Then of course scaled further by final drive/chain-sprocket combination Typical values?
vs
300lbft * 3.583 (1st) * 3.93 (final) = 4,224lbft
I'd say my comment that the CEC posing better torque figures at 300/300 was a fair one, even after transmission unless the final drive on the
BEC is 16:1? Vmax of about the same (140ish) at redline 5th.
[Edited on 5/8/10 by coyoteboy]
Despite your engineering background, you have completely failed to grasp the torque concept at hand.
Worse, you haven't read the article I posted, because if you did, you'd be going 'ahhhhhhh'.
Power is rate at which work is performed.
Work, that's what you have to do to accelerate something.
Torque is a product of force and perpendicular distance.
I can get more torque by increasing the distance (which is free) so effectively, my little finger could provide 1,000,000lbs/ft of torque.
It isn't rocket science, it's basic A Level Physics.
|
|
|
coyoteboy
|
| posted on 6/8/10 at 09:27 AM |
|
|
RR - while it was late when I was reading/writing it, I'm fairly sure I've not missed anything, in fact I have agreed with every
(technical) statement you made in that article, however you have simply ignored the fact that only certain gearings exist/are usable. You can't
simply abstract torque completely because it suits you. I don't just say "ahhh" because someone has written an article and bow
without question, TBH publishing an article in a jap performance mag might please you and offer forum willy-waving ability to those who are easily
impressed, but it means nothing. Especially as your quoted engines, in the article, are rated at the same power. The point I was making is that you
cannot gear the bike engine low enough to produce the same torque as the car engine, at the wheels. You could to out-do SOME car engines, but not the
one in question.
Well you could but you'd have a lot of sprockets to place. You may have twice the RPM to play with but that still only gives you twice the
torque at the wheels when put through appropriate gearing to bring it back to road wheel speeds, and that figure is still half that of the car when
put through appropriate gearing. In my road car ny first gear gives me approximately the same gearing as the above bike engined car with a 4:1 diff
attached, roughly 15:1. So while I get from 0-30 with 4200lb/ft at the hubs, a bke engine/box might get from 0-60 with 1050 at the hubs due to its
double RPM limits. But scale that back further so 1st is 0-30 as it is in the car (got another 2:1 sprocket somewhere?) and you have 2100lbft, still
half that of the car producing half the linear force to accelerate the same mass. Even looking at the fact that the CEC will be 50% heavier, F=ma =
At the end of the day the car engine still has the advantage due to higher torque (at the wheels)/weight ratio, regardless of engine RPM.
Your bike engine can scream away all it likes but ultimately its crank torque is limited by its low crank offset and small piston size. To compensate
it needs to be multiplied by the gearbox (as you say), but unless physics has changed significantly in the last few years, if you want to
approximately the same roadwheel speeds to keep it usable as a car, you're limited in your gearing choices and your bike engine will not compete
with a car engine of twice the torque achieved at half the revs? Your points rely on an engine of equivelent power, the ability to do work at a
certain rate. If you pitted a bike engine of 300hp/150lbft against my car engine of 300hp/300lbft then the fact that your bike can do 15000rpm would
of course mean you could scale it with a gearbox and achieve exactly the same performance (assuming area under the curve is the same and we dont have
one flat torque curve and one peaky one). But this isnt the case here, or anywhere that I'm aware of excepting maybe custom one-off turbocharged
monster bikes. You could, of course, gear it mentally so all 6 of your gears fall within my 1st gear speed ranges and you might beat the car engine
off the line, but then the CEC would just change to second and walk away as you bounce off the limiter
Gearing can get you so far, but it isn't the ultimate answer as the real world is limited. I totally agree (never even came close to making a
statement to the contrary) that some bike engine/gearing combinations can out-perform car engines with more torque, but seeing as we were discussing
two very specific examples, I can only assume you didn't read that properly or your own maths would have landed you with the same conclusion.
[Edited on 6/8/10 by coyoteboy]
|
|
|
adithorp
|
| posted on 6/8/10 at 09:37 AM |
|
|
If what you want is torque then yes go with a CEC. Thats not why you'd have a BEC....
You're comparing apples and oranges. You know what an apple tastes like but not an orange. Then you're asking if an orange (which
you've never tried) tastes as appley as an apple. Just taste an orange and see if you like it.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
|
|
|
coyoteboy
|
| posted on 6/8/10 at 10:01 AM |
|
|
I'm comparing apples and oranges because the two in question are vastly different powerplants. The car engine is a lot heavier and less fun to
throw around, I agree. Its less noisy too. But I suspect that outside that a large percentage of the difference is psychological. This is why plenty
of BEC owners seem to try to explain away their cars lack of drag strip performance etc, relying on the cornering/braking etc of the lighter vehicle.
Thats wholely understandable and acceptable, and a perfectly justifiable position. I hope it didn't come across as "one's better
than the other" as it wasn't intended to, until our rabbit friend above jumped in and tried to teach me a gearbox lesson I was quite happy
accepting that they are apples and oranges to some degree and that an orange in my case may suit my driving better than an apple. On closer thought,
at the end of the day it boils down to mass and torque at the hubs. The BEC has lower mass but lower torque at the hubs than can be very easily
achieved with a £500 car engine/box with a little more mass. Now that higher mass will make a fairly vast difference to handling too, not just
acceleration as we're not drag merchants.
My point, at the end of all that, is that if I were looking at a zetec or an R1 it would be a much closer call as it could be geared to get the same
torque at the hubs within the same usable speed range, and the only real difference would be weight. Ths would tip it in favour of the bike engine.
But when comparing to a car engine of notably higher power AND torque, im thinking the mass of the engine is cancelled well and truly apart from when
we think about stopping and cornering. I think a large percentage of the "fun" of driving a BEC is from the psychological grin factor of
stupid RPM and ultimate cornering. That's a valid point, and could well persuade me. But in the comparison above I still would vouch that the
CEC would be better performing in any real situation not involving stupendous cornering on a track. I compare it to some rally training I had, where
we were given different cars to time trial on the same track, a group n scoobie and a 1.4 pug 106. The 106 was nearly as fast (1 second a lap off,
mainly lost on the straights) and was far more fun to throw around. I can accept that. But I'd rather be driving a scoobie on the streets as
long straights and overtakes are far more prevelant than corners requiring stupendous grip and braking (if you drive reasonably legally and with some
thinking time in bends). I'd probably pick the 106 again for the full day if I went back as it was far more fun, cheaper to fix and easier on
fuel. But I'd then hand the keys back and get into something akin to the scoobie to go home.
I suspect this is because I've not tried driving a 106 like a nutcase around the public roads, but I still am aware I'd be overtaken by a
scoobie on anything more straight/wet
[Edited on 6/8/10 by coyoteboy]
|
|
|
