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Itchy
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Joined: 07 Apr 2005
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PostPosted: 19:49 - 07 Jul 2006    Post subject: bike design question Reply with quote
I know some stuff about bikes like engines and the reasons behind why say V2s are faster accelerating that IL4s etc and the physics behind it,

but I've never known why a stiffer frame helps in corners, having never owned a spine or a trelis or a engine as a stressed member bike and only ever ridden a great deal on twin spar designs.

makes me wonder why an old twin spar cheapo commuters can sometimes out corner things like Hornets (rider skill not withstanding of course), and checking out most sports bikes cept dukes , they seem to have some variation of the twin spar design.

Why does stiffness = better corner attack ability, is it to do with vibration? , being able to stress it more?.

Any suggestions

Thanks.
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craigs23
Mr Muscle



Joined: 08 Jun 2005
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PostPosted: 20:20 - 07 Jul 2006    Post subject: Reply with quote
Since when has a v-twin out accelerated an inline four of equal capacity?

The twin spar design is all about rigity - controlling the stresses exerted through the bikes chassis, how it's acted upon through the susspension/tyres, plus feedback it gives to a rider. Too much rigidity isn't a good thing - even MotoGP bikes has a certain amount of flex to give the riders 'feel'.

It's debateable which setup is best for the road - as the road varies so much in terms of surface, amount of grip available, comfort, etc.
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Jack_Cheese
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Joined: 14 Jul 2005
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PostPosted: 20:53 - 07 Jul 2006    Post subject: Reply with quote
Making it more rigid allows the suspension to do the job it's built for. If the frame starts flapping around, the suspension then shares its job and both the suspension and chassis must be finely tuned, whereas with a stiff chassis only the suspension must be set up properly.

Jack
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V2
Nearly there...



Joined: 06 Feb 2006
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PostPosted: 20:56 - 07 Jul 2006    Post subject: Reply with quote
Surely you want it to 'flap around' a bit for because the suspension cant work correctly when its cranked over?
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Jack_Cheese
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PostPosted: 20:59 - 07 Jul 2006    Post subject: Reply with quote
That's why honda engineered rear-end flex into the rc211v design, to give better feedback to the rider. And due to the fact that the bike is turning whilst cranked over, the suspension still does a decent job, even if it is compressed.

Jack
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V2
Nearly there...



Joined: 06 Feb 2006
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PostPosted: 21:21 - 07 Jul 2006    Post subject: Reply with quote
i thought when the bike hits a bump cranked over, your suspension doesnt work as well as the bump is now trying to push the wheel 'sideways' if you like and therefore the frame has to take a most of the force
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MarJay
But it's British!



Joined: 15 Sep 2003
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PostPosted: 21:23 - 07 Jul 2006    Post subject: Reply with quote
brewer wrote:
i thought when the bike hits a bump cranked over, your suspension doesnt work as well as the bump is now trying to push the wheel 'sideways' if you like and therefore the frame has to take a most of the force


you are correct, but ahead of yourself.

In the old days before stiff frames, bikes did as you suggest. However they flexed in all other planes too. having a really stiff frame is a lesser evil than having one that feels like it is hinged in the middle!! Smile
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V2
Nearly there...



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PostPosted: 21:27 - 07 Jul 2006    Post subject: Reply with quote
Laughing im not rushing to B&Q to get hold of some gate hinges but i would have thought a happy medium would be ideal no?
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MarJay
But it's British!



Joined: 15 Sep 2003
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PostPosted: 21:32 - 07 Jul 2006    Post subject: Reply with quote
brewer wrote:
Laughing im not rushing to B&Q to get hold of some gate hinges but i would have thought a happy medium would be ideal no?


If you can find that happy medium, then work out how to measure it, then reproduce it, then patent it and sell it to Mr Honda for a million quid, cos it aint as easy as that! Smile
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extreme3d
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Joined: 27 Dec 2004
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PostPosted: 21:47 - 07 Jul 2006    Post subject: Reply with quote
craigs23 wrote:
Since when has a v-twin out accelerated an inline four of equal capacity?.


RSV's........ SP2's......... ducati 999........ the list could go on Wink
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NSR Mick
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Joined: 26 Jun 2005
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PostPosted: 21:49 - 07 Jul 2006    Post subject: Reply with quote
extreme3d wrote:
craigs23 wrote:
Since when has a v-twin out accelerated an inline four of equal capacity?.


RSV's........ SP2's......... ducati 999........ the list could go on Wink


Vs a k6 GSXR1000, unlikely. Wink
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quacker_boy
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Joined: 06 Sep 2005
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PostPosted: 21:50 - 07 Jul 2006    Post subject: Reply with quote
craigs23 wrote:
Since when has a v-twin out accelerated an inline four of equal capacity?


I think Itchy meant to say out of corners owing to the greater torque V Twins are renowned for..saying that why isn't your SV 650 along with that list extreme3d? Wink
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NSR Mick
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PostPosted: 21:55 - 07 Jul 2006    Post subject: Reply with quote
Then again my v-twin will beat any 4 cyl of the same capacity. Its a 2 stroke is that cheating?? Wink Laughing
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Jack_Cheese
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PostPosted: 23:55 - 07 Jul 2006    Post subject: Reply with quote
gsxrmick wrote:
Then again my v-twin will beat any 4 cyl of the same capacity. Its a 2 stroke is that cheating?? Wink Laughing


possibly Razz

All this bullshit about torque this, torque that just takes the piss. Out of two engines with the same power output, only one being more torquey and lower revving, and the other less torquey but higher revving, the lower revving unit will generally whoop the higher revving unit due to higher flexibility. However, take a chunk of power of the low revving unit, and it more than loses that advantage. Although they produce more torque, they produce less power, meaning they only beat bikes on which the rider finds it very difficult to stay in the powerband, normally the peakier straight fours.

Jack
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Kickstart
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Joined: 04 Feb 2002
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PostPosted: 00:06 - 08 Jul 2006    Post subject: Reply with quote
brewer wrote:
i thought when the bike hits a bump cranked over, your suspension doesnt work as well as the bump is now trying to push the wheel 'sideways' if you like and therefore the frame has to take a most of the force


The suspension will still work. To be honest I am very dubious about the claims about building in lateral flex to allow the frame to act as suspension. No damping of such flex, and compared to the force to kick the complete bike into the air I would suspect it would need a hell of a lot more force to flex the frame.

Personally I suspect that having a small amount of flex slightly lowers the ultimate limit, but increases feel and makes it far easier to get close to the limit (and makes it break away more progressively.

All the best

Keith
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craigs23
Mr Muscle



Joined: 08 Jun 2005
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PostPosted: 00:24 - 08 Jul 2006    Post subject: Reply with quote
Quote:
All this bullshit about torque this, torque that just takes the piss. Out of two engines with the same power output, only one being more torquey and lower revving, and the other less torquey but higher revving, the lower revving unit will generally whoop the higher revving unit due to higher flexibility. However, take a chunk of power of the low revving unit, and it more than loses that advantage. Although they produce more torque, they produce less power, meaning they only beat bikes on which the rider finds it very difficult to stay in the powerband, normally the peakier straight fours.


Torque is quite important; power is how reguarly an engine can produce its torque (i.e. the torque is the size of a punch, the power is how often it can be dealt).

An inline four makes more power and torque than a v-twin - the way that each bike makes its power is the difference. The low down 'grunt' of a v-twin is attributed to its "power pulses" (the way the engine fires) allowing a rider to lay down more traction when accelerating out of corners.

In a straight line, the fours will generally accelerate quicker, as once they're up to engine speed, they're putting down all their available power (of course, both run the risk of spinning the rear or raising the front) which is more than a 2 cylinder bike can produce. Hence a GSXR-1000 (157bhp, 79lb-ft) will cover a quarter mile time quicker than a Ducati 999 (122bhp, 69lb-ft).

Anyway, back to chassis flex...
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Jack_Cheese
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PostPosted: 00:26 - 08 Jul 2006    Post subject: Reply with quote
Top be honest, i think the flex is engineered in to prevent resonant vibration within the frame, which would add extra vibrations to those picked up from the road, thus lessening the rider's feel. On a totally rigid chassis, the frame would vibrate, and resonate at a cartain frequency. Thinking Humm, now i have a hypothesis! Idea

Jack
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