| Author | Message | ||
     Xl_cheese |
I'm questioning this theory. Seems like in order to fit the exhaust down there you have to make the motor a lil higher off the ground? I'd be willing to bet that you'd have a lower COG if the motor was lowered and the exhaust moved out to a typical spot... | ||
| Darthane |
...what're ya gonna do with the extra 4" above the motor, then?  | ||
| Cityxslicker |
or leave the motor where it is at and get an extra 4 inches of ground clearance. | ||
| M1combat |
And why do you think that a lower CG is by default "better"? | ||
| Not_purple_s2 |
The underslung exhaust is about mass centralization not just low cg. | ||
| Gentleman_jon |
Many people assume that lowering the center of gravity must be best for motorcycle handling. As Not Purple suggests, mass centralization is the idea behind the under engine muffler location. Actually the higher the center of gravity, the less a motorcycle needs to lean whilst making a corner. That does not mean that the cg should be too high either, as a cg that is too high makes transitions more difficult, and slower. Erik Buell has said that there is an optimum location for the center of gravity, and it appears that he and his team may have found it. | ||
| Not_purple_s2 |
It also makes a good excuse for a beer belly. | ||
| Randomchaos |
Mass Centralization is what its called, not lower CG. Mass centralization is taking that optimal CG location, and putting as much mass of the motorcycle at the spot. | ||
| Spatten1 |
I'm questioning this theory. Seems like in order to fit the exhaust down there you have to make the motor a lil higher off the ground? I'd be willing to bet that you'd have a lower COG if the motor was lowered and the exhaust moved out to a typical spot... Agreed. I'd say engine and oil weigh more than a hollow muffler. I believe the Japanese underslung exhaust is mostly positioned behind and below the engine. The bike is built around the sporster derived engine, not the other way around. you can only do so much with the engine position. | ||
| Hexangler |
Would the center of gravity of a motorcycle be best along the line between the two axles when the suspension is halfway compressed? Ask Erik next time you talk to him. | ||
| Sloppy |
A lower center of gravity allows for higher cornering velocities. Mass centralization lowers polar moment of inertia (resistance to turn about an axis). Note that OHC engines likley have higher COG and less mass centralization than do CIB engines. The two combined allow for a bike that travels faster in a corner and quicker to flick into (or out of) a corner. And exhaust systems are not all that light... and you can see that many different mfgs now use an underslung muffler. (Message edited by sloppy on July 22, 2008) | ||
| Not_purple_s2 |
Of course there were compromises with the HD based motor. An obvious one is the oil tank location. In the tubers it was positioned up high. But with the XB's it go lower and closer to center. With the 1125R they were able to design the oil tank into the engine. They even mentioned that this was done for mass centralization in the 1125R literature. With all the preaching of the mass centralization in the "Trilogy of Tech" I doubt that compromised any more than they had to when it came to component placement. | ||
| Iamarchangel |
It also makes a good excuse for a beer belly. Well, no, your centre of gravity is about two inches below your belly button. Big belly is too high and forward creating counterbalance stress on back and knees. Would the center of gravity of a motorcycle be best along the line between the two axles when the suspension is halfway compressed? Yes, sort of, that's why you preload the suspension. Engineering will design the COG with the suspension average in mind, you'll fine tune it to perfect it. You can't have an ideal straight line (axle-COG-axle) since you will never ride on an ideal flat plane.} | ||
| Spatten1 |
Of course there were compromises with the HD based motor. An obvious one is the oil tank location. In the tubers it was positioned up high. But with the XB's it go lower and closer to center. Yeah, talk about reducing unsprung weight, then add oil and lines to the weight of the swingarm.....I know it's not a lot, but ironic nonetheless. | ||
| Bombardier |
True. However the weight has been added close to the fulcrum (swingarm pivot point) and so the real effect is not as much as if it was added at the end of the swingarm. I would have thought that the closer you can get the weight motorcycle to the ground ( the point at which the tyres touch the ground is where the effort to turn the bike from one side to the other is the least) the better the motorcycle would handle. (Message edited by Bombardier on July 22, 2008) | ||
| M1combat |
Just stop it... This thread so far is mostly full of bad info. Y'all need to get a book by Vitor Cossalter and just stop thinking. Most of you aren't doing a real good job of it. | ||
| Court |
WOW Start by comparing the TOTAL MASS of a Buell exhaust system with something like a GSXR (take your choice). You will be amazed and pleasantly surprised. | ||
| Hexangler |
It's Vittore Cossalter. http://www.dinamoto.it/DINAMOTO/books/book%20www%20english_2_version.htm  | ||
| Anonymous |
Simple. Height of motor dictated by lean angle and hard contact points on clutch. Room left underneath filled by muffler. Lowering engine=reduced lean angle=not sportbike. | ||
| Gentleman_jon |
Another excellent book on motorcycle design is : Motorcycle Design and Technology Handbook, by Gaetano Coco, former technical director of Aprilia.  It has a very good discussion of the effect of the height of the center of gravity on cornering, among other things. | ||
| Punkid8888 |
+1 Anonymous I remember seeing this topic here or reading about it in an article. Eric said that the width of the engine was the limiting factor for how low the engine could go. And once that was set you might as well stuff some heavy stuff under it too fill the void. | ||
| Rocksham |
Put the exhaust on top and the rider on the bottom  You wouldn't have far to fall  | ||
| Not_purple_s2 |
Rider under the bike... Is this close enough?  And the exhaust is still under the bike | ||
| Bombardier |
Kiss your nuts goodbye when you rear end that soccer mom in her suv! | ||
| Treadmarks |
I would like to see an undertail exhaust for the extra ground clearance.   This is the pic that made me buy a buell.... | ||
| Darthane |
There's that gatling-gun exhaust from Homecoming, MAN that thing was bitchin'! | ||
| Ducbsa |
FWIW -- Freddie Spencer's GP bike one season in the early '80's (I think) had the gas tank under the motor and the pipes over the top of the motor. Besides being really hot for the rider, apparently it didn't handle very well. | ||
| Djkaplan |
If you ever wonder why Buells carry their XL engines so high, ride a Sportster in the twisties sometime. | ||
| Packrat |
Who gives a ratz-azz...just ride the thing and enjoy it, knowing you own one of the best handling motorcycles made today!!! | ||
| Not_purple_s2 |
Hey, how exactly is a rainbow made? How exactly does the sun set? How exactly does the posi-trac rear end on a Plymouth work? ....It just does | ||
| Jdemoxb9r |
+1 not purple the xb's have been the best handling bikes on the market for the last 6 years...im sure you gotta put a lotta stuff in the right place to get that honor...and keep it | ||
| Djkaplan |
I'm no chassis dynamics pundit, but I heard it proposed that the XB chassis was developed to carry a higher cg with a weight distribution further forward than one would think necessary (or prudent). This was supposed to help the steep 21 rake maintain stability with the short wheelbase.... something like that. | ||
| M1combat |
That makes sense... Higher CG would allow the rake/trail combo to more quickly correct instability. WRT the weight being forward... It allows you to get better drive out of the turns without the front getting too light. I don't know whether there's "too much of a good thing" here or not. |
