Topic: Center of Gravity effects on handling question

[Kong]

With so many Cafe style bikes using specially built, modified, or adopted tanks and owner-made seats and tail sections, not to mention rear sets, a rider's driving position in a finished bike is generally a good bit different than it was when stock.  Considering that the driver generally accounts for upwards of a third of the gross vehicle weight moving the driver around has to have an effect on handling.  Mix this in with long shocks in the front and possibly lowered tubes in the front and one would expect some mighty quick (and twitchy) steering.  But what I'm wondering is how much of this changed (I refuse to say improved) handling is attributable to changes in steering geometry and how much to changes of center of gravity?  In short, what's it do when you move a third of the bike's weight back six or seven inches?

[Big Bob]

I've raced two different CB750s.

1970, stock front end with Progressive springs, stock pegs, clubman bars, 1 inch over staight body Hagon shocks.

1976, stock front end with straight rate OEM springs, Tarozzi rearsets, superbike bars, steering damper, 1 inch over Ohlins shocks with piggyback reservoirs.

Both bikes have 18 inch OEM rear wheels.  Both bikes have 18 inch OEM rear rims laced into a front hub, the '76 has dual disks.

The '70 is now a street bike with superbike bars.

Raising the rear end 1 inch put more weight on the front tire and brought the rake in a little.  The front end on either bike feels much more planted and steady.  I would not call the steering on either bike "twitchy" although the steering damper on the '76 does make that bike noticably more stable.  The clubman bars on the '70 and the rearsets on the '76 puts more of my weight on the front tire which makes it stick even better.  The rearsets also give me a little more leg room and better cornering clearance.

When riding aggressively you should be moving all over the place as the situation dictates, constantly shifting your weight around.  Push on the inside peg in a turn and the bike turns in quicker.  Push on the outside peg to keep the back tire planted.  Hang off to the inside so the bike can turn with less lean, move back on top to keep it planted over that gravel you didn't see.  Move up on the tank to keep the front tire planted, scoot back and lay down to get a good tuck going for speed down a straight.

The most important thing when considering making changes to your bikes setup is to only change one thing at a time.  Change one thing and then ride the bike around for a while to see how you feel about it.  You don't want to get yourself into a "Rubic's Cube" kind of situation where you don't know which change is making you unhappy with how the bike feels.

And not every change is going to make every rider happy, or make the bike fit all kinds of sitatuion.  My '70 is completely miserable in stop and go traffic but really sings out on a twisty country road.  I don't understand why anyone would put clubman bars on a bike that they're going to ride around in a city, they suck in tight situations.  But then anyone who tries to ride one of my bikes thinks the front end is way too stiff.  But it's not set up for them.  It's set up for me.

[nickandkristy02]

^^^^ what he said

[Kong]

Well, that's good and I'm glad we've got someone with expertise to ask. However the question refers to the static state.  The beginning point if you want to think of it that way; how is the bike was before the rider compensates by his own placement.  Does the advantage generally go to the bike with the farther forward center of gravity?  How about its height relative to the road or the axles?  I would think these things have a measurable and measured effect on handling.

As to forcing more weight to the front and 'planting' the front wheel, is that accompanied by an equal uprooting of the rear wheel?  One presumes there is a trade off.  Do you find this to be the case and if so is it bothersome?

[mlinder]

I think that's his point, Kong. The motorcycle is anything but a static tool.

And, while moving your ass back 6 inches would put more weight on the back, moving the handlebars forward 4 inches and down 4 inches will actually put the bulk of the weight, your torso, head and arms, further forward. That is, in a static position, which doesn't happen that often.
The raising of the rear also puts more weight forward, and higher. But, again, it depends on your posture at any given moment, which changes. Frequently.

[Big Bob]

Well, that's good and I'm glad we've got someone with expertise to ask. However the question refers to the static state.  The beginning point if you want to think of it that way; how is the bike was before the rider compensates by his own placement.  Does the advantage generally go to the bike with the farther forward center of gravity?

Assuming we're talking about two bikes of the same model, and we're talking about how well the bike is going to corner?  Most likely.  In the words of Randy Illg of FrameCrafters: "These old bikes were designed to do everything.  Carry the mail, ride to work, two up touring, bring along the dog.  If you want them to handle you need to make some changes."

How about its height relative to the road or the axles?  I would think these things have a measurable and measured effect on handling.

You would think correctly.  It is entirely possible to make a bike handle much, much worse by making bad choices.  It makes me cringe every time I see someone put a 16 inch rear and a 19 inch front on thier "cafe racer."  By lowering the rear you are increasing the rake on the front.  Making your bike corner like a bus.

As to forcing more weight to the front and 'planting' the front wheel, is that accompanied by an equal uprooting of the rear wheel?  One presumes there is a trade off.  Do you find this to be the case and if so is it bothersome?

Keep in mind that these bikes we're talking about tended to be pretty rear end heavy to begin with.  By moving some weight forward you aren't hurting anything.  And you can keep the rear end under contol by applying weight to the footpegs.

If I may be permitted a brief divergence?  Consider dirt bikes.  Very tall bikes, right?  But their riders go around corners just fine.  While standing up!  That shouldn't be possible!  By standing up they've raised the center of mass of the bike/rider unit very high, maybe brought it up a foot or two.  Why don't they go screaming off into the haybales to die in a flaming wreck?

Because center of mass doesn't matter as much as where that mass is applied.  A seated dirt biker may have a lower center of mass but he’s applying that mass in the wrong place, high up on the bike.  The standing rider may have a higher overall center of mass but he’s applying his mass down low on the pegs where it will do him the most good.

Back to your last two questions:  No, I have not found that the bike feels “uprooted” in the rear end with the changes I’ve made.  You could make the argument that by raising the rear to quicken steering you are making the bike less stable in high speed, straight line situations.  I personally have not found that to be the case at, say, Road America or Daytona.  If I did feel that to be an issue I might lower the rear using the adjustable ride height feature of the Ohlins shocks or I’d put on a longer chain and run the adjusters all the way back to increase the wheelbase.

Keep in mind that I don’t have any formal education in this stuff.  I’m just a guy that like to ride motorcycles.  And who currently has a lot of free time without access to his shop.

[paulages]

I think that's his point, Kong. The motorcycle is anything but a static tool.

And, while moving your ass back 6 inches would put more weight on the back, moving the handlebars forward 4 inches and down 4 inches will actually put the bulk of the weight, your torso, head and arms, further forward. That is, in a static position, which doesn't happen that often.
The raising of the rear also puts more weight forward, and higher. But, again, it depends on your posture at any given moment, which changes. Frequently.

...and it matters completely where your weight is settled. On the pegs? That's where your body's COG is gonna be. Against the tank? It's gonna be up front. On the seat? It's gonna be a little higher.

Kong, Big Bob gave a great answer to your question, in my opinion. If you want a simpler answer, decide what position you consider to be "static," figure out where your weight is resting, and there's your answer. I'd say a stock bike puts the rider's COG on the seat and pretty close to the rear wheel. My bike with a long tank and clip-ons, puts it considerably farther forward when I'm not intentionally shifting it around for better control, because my weight is leaned over the tank.

EDIT: BB just elaborated my point with the dirt bike reference.

[turboguzzi]

if the rider is 30% of the total weight, then a 6" shift in rider CG will result in only 2" total bike + rider CG shift.

The subject goes beyond just good handling, there are weight transfer issues, CG height , polar moment of inertia changes, and more.

Not a bad idea to get a thorough education on the subject by going through Cameron's "Sportbike performance handbook" and Robinson's "motorcycle tuning - chassis"

[octagon]

BB, what do you think of lacing a cb550 rear rim on a cb750 front hub? it's an 18 inch rim but narrower than the 750 rear rim. if that would work, what tires front and rear would you use for touring?  

[medic09]

BTW, the recent MCN issue had a pretty thorough article on how to exactly figure the COG/COM for a motorbike, and the effects to anticipate.  Worth the read.