About Buells...

**coffeejunkie** · 17th May 2009, 10:39

My partner has the 1125R and he loves it, As do i being the pillion, It corners amazingly well around 58 and over the rimmies, We did the 1000K ride on it last year and i found being pillion that it was one of the most comfortable bikes i've been on,

It never fails to start first time and it's just a mean ride,

**jrandom** · 17th May 2009, 10:40

Originally Posted by Big Dave

Why do you think 'Bike' magazine rated the Buell Firebolt 'the best cornering bike ever' in their review Random?

Because:

(a) 'best cornering bike ever' and 'dangerously unstable in turns' are pretty much a matter of how you squint on the day and whether your editor commanded you to write something positive, which leads into

(b) when was the last time you read a negative magazine review?

It's difficult to start pointing out the flaws of the XBwhatever without the whole concept of the bike falling apart, since its design oddities are its flaws, IMHO.

The only other option, I guess, is to laud it as wonderful and come down on the 'best cornering ever' side instead of the 'doesn't turn properly, will probably fall over unless you ride slowly' side, while trying not to mention the fact that the motor runs out of puff at about half the revs it should.

Originally Posted by Big Dave

No shims, no cam chains...

Yes, come to think of it, pushrods were the apex of internal combustion engine design.

**dipshit** · 17th May 2009, 10:44

Originally Posted by Big Dave

It's about keeping the mass lower to the ground and as central as possible.

Which is a stupid idea in itself. Honda were going down that wrong track themselves on their gp bikes in the 90's with trying to lower the placement of mass... putting the fuel tank underneath the engine and such things.

Until they realised a higher centre of gravity actually helps a bike to go around a corner when it is leaned over.

**jrandom** · 17th May 2009, 10:53

Originally Posted by dipshit

... a higher centre of gravity actually helps a bike to go around a corner when it is leaned over.

'Helps'?

How?

A higher center of mass makes the bike tip from side to side more slowly.

Of course, once it's done tipping, it'd turn faster at the same lean angle with more mass further away from the center.

So, yeah. Righto then.

**James Deuce** · 17th May 2009, 10:56

Originally Posted by dipshit

Which is a stupid idea in itself. Honda were going down that wrong track themselves on their gp bikes in the 90's with trying to lower the placement of mass... putting the fuel tank underneath the engine and such things.

Until they realised a higher centre of gravity actually helps a bike to go around a corner when it is leaned over.

The current mass centralisation concept (pioneered by Buell) is to get the centre of mass on the axis of rotation and near the front of the bike.

You guys need to stop confusing centre of mass and centre of gravity.

http://en.wikipedia.org/wiki/Center_of_mass

**jrandom** · 17th May 2009, 10:58

Originally Posted by James Deuce

You guys need to stop confusing centre of mass and centre of gravity.

What's the difference?

**Sollyboy** · 17th May 2009, 11:02

Originally Posted by jrandom

'Helps'?

How?

A higher center of mass makes the bike tip from side to side more slowly.

Of course, once it's done tipping, it'd turn faster at the same lean angle with more mass further away from the center.

So, yeah. Righto then.

The correct C of G and weight bias is varied and changing from manufacturer to manufacturer other factors is braking as well as when its to low the front wheel wont load up properly and tends to push under brakes or sledge during corners, and yes harder to turn side to side when to high, but increasing the weight of a swing arm with oil is retarded if it needs an oil tank then build it from aliminium or just partition the frame .
as for someone say usd fork too, well that was to increase the stiffy of the fork which is an acceptable compromise as it is a performance gain but oil in the swing arn adds no strength or stiffen just adds weight

**jrandom** · 17th May 2009, 11:06

Originally Posted by James Deuce

You guys need to stop confusing centre of mass and centre of gravity.

http://en.wikipedia.org/wiki/Center_of_mass

Tut, Jim.

To repeat the Feynman quote in that article:

"In case the object is so large that the nonparallelism of the gravitational forces is significant, then the center where one must apply the balancing force is not simple to describe, and it departs slightly from the center of mass."

A motorcycle is not so large that when it rests on the surface of the earth, gravitational forces differ meaningfully across it.

From the same article:

"Even when considering tidal forces on planets, it is sufficient to use centers of mass to find the overall motion."

So, yes, for this topic, COM and COG are equivalent.

**Big Dave** · 17th May 2009, 11:07

H'yar I founded that article.

http://www.kiwibiker.co.nz/forums/sh...7&postcount=76

**dipshit** · 17th May 2009, 11:07

Originally Posted by jrandom

'Helps'?

How?

A bike will rotate around its centre of mass when leaning left/right. The higher this point is - the more it will influence a bike to track around a corner.

A low centre of mass will feel more stable when changing direction. Many people do not like the feeling of a top-heavy bike.

**jrandom** · 17th May 2009, 11:10

Originally Posted by dipshit

A bike will rotate around its centre of mass when leaning left/right.

Huh? You might want to re-visualise that.

It will rotate around the tyres' contact point with the road. The center of mass determines the speed of rotation around that axis given any particular tipping force.

Look up 'moment of inertia'.

Originally Posted by dipshit

A low centre of mass will feel more stable when changing direction.

Huh?

A low center of mass means that less force is required to tip the machine from side to side.

**Big Dave** · 17th May 2009, 11:14

Triumph TT600.
There isn't much to criticise any more. Yours regarding stability issues I disagree with.

Pinnacle - maybe. Apparently they can have a different combustion chamber shape due to the actuators. I certainly love them come paying for a service time.

**James Deuce** · 17th May 2009, 11:16

Originally Posted by jrandom

What's the difference?

See above. A bike's centre of gravity is completely different depending on angle of lean or negative or positive acceleration, and is one of the reasons why you can balance a bike while leaned over. The centre of mass doesn't change.

The orginal low centre of mass concept placed the fuel low, which of course sloshed about changing fore and aft weight distribution as well as adding a random moments of rotation once you'd chosen your angle of lean. It also pushed the front under braking, as the the mass being slowed didn't act at the front wheel spindle because the suspension didn't get loaded

A top heavy bike accelerates toward the inside of a corner. Bottom heavy, or top heavy designs make it difficult for the rider to consistently and precisely dial up an angle of lean for a given corner speed.