Bike Geometry

[red]

This is a CAD drawing of my Honda CB900F. I plan to monoshock it, and add some modern running gear, etc. This model has 17" wheels, and about 11 degrees of droop in the swingarm (normally about 2 or 3 degs). Increasing swingarm angle does two things to the bike:

**Makes it harder for the engine to compress the rear end under load, which can run you wide out of turns.

**Brings the steering axis closer to the CofG, making it turn quicker.

You can see I have tried to estimate the CofG for the bike, and decided on the location marked in purple. For reference I marked a green line from the rear axle through the CofG.

I read an article recently saying that the steering axis of a bike is (the axis about which it leans over) is about in the middle of the back tyre, goes thru the swingarm pivot, and is somewhere about the top of the front tyre.

For comparison I have included a chopper and a race bike similarly marked.

http://i296.photobucket.com/albums/mm199/moto-journo/CB900.jpg

Because the chopper has it’s CofG so far below its steering axis, it is relatively stable. The trade off being slow handling.

http://i296.photobucket.com/albums/mm199/moto-journo/Chopper.jpg

The race bike is almost inline. It will turn relatively quickly, at the cost of stability – where’s my steering damper?

http://i296.photobucket.com/albums/mm199/moto-journo/Race.jpg

I don’t know what happens when the CofG is above the steering axis, but I’m sure it’s not good…

There's a bunch of guys here who have done this kind of modification, what have you experienced?

[chopiesel]

doesnt matter what you do its gona be a rough ride with those flat spotted tyres.

[WATEVR]

sounds very good but i cant find the race bike example there is a honda there instead.......


cant help with your query just dropped in to be a smart arse!!!

[red]

quote:cant help with your query just dropped in to be a smart arse!!!


You're not alone [:p] :D

[Fatgit]

changing Centre of Gravity won't make your bike turn faster, just less/more effort to turn the bike. The 17" front wheel is going to most significantly effect your cornering ability, smaller front wheel = faster cornering. The length of the swing-arm will also have more effect on your cornering, longer swing-arem = slower steering.

TBH unless you're going to replace the entire frame, I don't think you're swing-arm angle is going to make that much difference. The frame will probably be squirming so much under hard cornering that such fine tuning just won't matter.

[pappas]

Did you draw that yourself Red?

[fimpBIKES]

by increasing the swingarm angle are you pretty much just jacking up the rear of the bike, therefore steepening up your head angle and sharpening up the steering?

great drawing btw! how have you estimated your CoG, would make sure that you have it pretty close before getting too in depth

[Gix11]

Hey Red, what did you base the centre of Gravity line on (the green one). I know you said thre red one went from rear axle to top of front wheel but the green one goes from centre of rear axle through estimated C of G? The end of the green line is irellevant to any point on the bike and just a reference against the red yea? If so, what exactly are you basing potion of C of G on? ......I'm a bit slow today so it will probably be something obvious that I have overlooked.

[fimpBIKES]

it can be measured, you do something along the lines of this


http://www.msgroup.org/forums/MTT/topic.asp?TOPIC_ID=137


i'd do it with you on the bike in a "racey" position, be more realistic that way ;)

[chopaweeza]

Well the chopper one is wrong for starters. Wrong for several reasons. First being it has air ride suspension , the air bag type that has an on board compressor and in the picture it is deflated lowering the bike as a result. Second is you are assuming the line of chain travel is through the top arm of the swingarm. The swingarm pivot can clearly be seen and it doesn't come close to being touched by the line. Third , you are assuming the center of gravity in the wrong place as well . You are generally on the right track but assumption

The frames I build handle well. They handle well because I understand things like Rake , Trail , Ride Height , the effect of the engine I'm using on the chassis and Rear Wheel movement. All these things combine to determine handling characteristics along with a dozen other things(wheel sizes being one). There is no magic formula to make the ultimate handling bike , it's a trade off and it's a matter of getting the best compromise for your needs.

[red]

A few interesting points there. Lets see, yes I did draw the cad file. Sorry I forgot to regenerate the image before copying it - hence the irregular curves.

Estimating the centre of gravity is difficult. I know it can be measured, but I took estimated mass and lengths into mind, and basically stuck a pin in the drawing. I could be way off...

Yeah I know I kinda fucked up the line on the chopper. But I only noticed after posting it. And I don't actually have many choppa pictures to draw lines on (I took the first one I came across in profile). I guessed CofG above the gearbox in the approx location of the vertical seat post. Do you agree?

The green line is a strike between where I reckon the CofG is and the fixed rear axis. Yes, it is a reference for me to understand how different distances between the CofG and the "steering axis" relate. Remember everything about this is an estimation. A bike designer would piss himself laughing at the errors. But for me, this is what I understand.

I believe the bike rotates around an axis as you lean it over (red line). Which is probably the radius of the rear tyre at the back, and possibly the radius of the front tyre at the front. But I am led to understand the front axial point is close to the lower steering yoke. But I don't understand why?

I probably should have said "affects steering effort" in relation to steering the bike, rather than quickens the steering. As in the further away the CofG is from the steering axis, the more effort is required (ie leverage on the bars) to turn the bike. And vice versa.

But I didn't, cause effort takes time. You're not going to rip into a corner on a choppa and drop it on its side, or are you ? [:0] You're gonna enter the corner planning to round that bend smoothly? And I bet you generally get a better ride from a choppa too.

I don't believe sharp steering angles are effective on the road. I have a bike with 30 deg geometry, and for comparison, whatever my Blade has, 26 deg or something. What I believe makes the Blade "quicker" to turn is the proximity of the CofG to the steering axis.

It's certainly way more fucked on the bumps... The old bike needs more pull on the bars, more time to set up, which makes it feel slower. But I just recently rode a bike modded quite like my CAD drawing and it was VERY light steering. Which got me thinking, and this is what I thought. [xx(]

Oh, and it's probably real easy to draw a frame on AutoCAD, but it's a real muck-around to measure a frame and then draw it accurately.:(

[fimpBIKES]

i thought the drawing was pretty good

coming from a mountainbike background i thought that the steering "quickness" was more closely related to the head angle
chopper is raked out so turn-in is slow...

where are you getting your info? i'd love to learn more about this sort of thing!

[red]

Thanks Fimp. I'm just picking up bits and pieces of info. Bits from magazines and bits from the net. Trying to add it all up, and make sense of it. I thought I had enough of a theory to discuss on the board, considering this is a common mod amongst the streetfighter crowd.

I'll get into CAD later on and do measurements of rake and trail on that model.

[Fatgit]

Go read 'Tuning For Speed' by Phil Irving (you have heard of him haven't you? or am I getting real fuggen old?).

Unless you weigh 25 K's wringing wet you're making too much of an issue of CoG. Especially on a roll-a-door. The engine is so wide that you will have no choice but to have a high CoG. Wheelbase, wheel size and steering geometry have the greatest effect. Also what are you doing with the frame? The standard item is not up to the job of keeping your you-beaut front and rear ends in line, which pretty much defeats the purpose.

[fimpBIKES]

thanks for the book title chalky

good point about the engine width, thats why v-twins ROOOL [:P]

[fimpBIKES]

this one looks handy too

http://www.webbikeworld.com/books/motorcycle-dynamics/

Phil Irvings
http://www.webbikeworld.com/books/tuning-for-speed.htm

[red]

quote:Unless you weigh 25 K's wringing wet you're making too much of an issue of CoG.

I don't think so. Ever since the 80's manufacturers have been "mass centralising" and experimenting with CofG. All the modern sports bikes have moved to under engine exhaust systems now, further centralising mass. Engine layouts have changed, gearboxes (as one example) don't so much lay flat behind the motor, but are stacked now.

Hey, why do most racers turn in when the forks are compressed? Lets face it, a bike turns on the radii of it's tyres when leaned over. The further over you lean it, the smaller the radius right? It uses some steering to get the bike leaning (and perhaps this is only where steering geometry influences turning), but the bars are generally neutral while cornering. But how slow do you have to go to actually be turning the bars into the corner to be turning the bike? Under 50km/hr right?

If there is something in Irving's book, I'd be glad to hear it.

[fimpBIKES]

i agree red, i've spent a fuckload of effort turning my TL into less of a pig than the standard one
(shorter forks, lighter wheels and rotors, tucked in exhaust)

but i have sacrificed stability, the wide bars help but a steering damper is happening now too



when i did superbike school they told us about how the bike slows down as you lean
due to the decreasing radius of the tyres
once you are conciously aware of it happening its pretty cool, i spent the whole next session on the track giggling (but then again, im not quite "right")

something i had never really noticed before, but a explains why novice riders brake too much and the bikes stop mid-corner

[Fatgit]

quote:Originally posted by red


quote:Unless you weigh 25 K's wringing wet you're making too much of an issue of CoG.

I don't think so. Ever since the 80's manufacturers have been "mass centralising" and experimenting with CofG. All the modern sports bikes have moved to under engine exhaust systems now, further centralising mass. Engine layouts have changed, gearboxes (as one example) don't so much lay flat behind the motor, but are stacked now.

Hey, why do most racers turn in when the forks are compressed? Lets face it, a bike turns on the radii of it's tyres when leaned over. The further over you lean it, the smaller the radius right? It uses some steering to get the bike leaning (and perhaps this is only where steering geometry influences turning), but the bars are generally neutral while cornering. But how slow do you have to go to actually be turning the bars into the corner to be turning the bike? Under 50km/hr right?

If there is something in Irving's book, I'd be glad to hear it.



Mate if you can 'mass centralise' that engine and frame, then good on you, but I 'll lay bottom dollar that you're gonna find it near impossible to affect much.

And if your serious, go read Irving's book yourself, you ain't gonna get much from one or two selective quotes.

[red]

Fatgit, I may get that book one day. You seem to be the only one with it now. But if you're not inclined to make an argument for your case (or Irving's), your opinion is invalid.

Fimp, slows down in the corners... really? I thought the wheels would speed up as they rolled onto a narrower radius. Can you explain what you mean a bit more?

[Fatgit]

quote:Originally posted by red

Fatgit, I may get that book one day. You seem to be the only one with it now. But if you're not inclined to make an argument for your case (or Irving's), your opinion is invalid.




Aaah no, makes it unproven, not invalid. And as I said, IF you're truly serious then you will need to read the whole thing, not just some selective comments.

And I hardly think that I'm the only one with it, considering that it is one of the most 'significant' books ever published on motorcycle design and theory, and has been in publication since before this old fart was born (Plus the last copy I had fell to bits out in the shed). (FYI Phil Irving was chief engineer at Vincent, responsible for development of the Vincent 1000cc V-Twins, amongst many other things.

What you are trying is commendable, but if you want to replicate what the Japanese engineers have been doing then you're going to need more effort than "somebody please tell me the answer'.
And, of course, no bloody roll-a-door Hondas :D

[Lucas]

quote:Originally posted by red



Fimp, slows down in the corners... really? I thought the wheels would speed up as they rolled onto a narrower radius. Can you explain what you mean a bit more?



I'm not really up on physics but I imagine it would be the same as holding a spinning pushbike wheel by the axle. When it is verticle it spins quite freely and without any real resistance but when you start to tilt it toward the horizontal plane it gets all fucked up and outta shape. As I said, I'm not a phycics buff and I don't know the correst jargon or reasons......

[fimpBIKES]

i think thats more of a gyroscopic effect, i think its a seperate thing happening
they explained it as something to do with rolling diameters

they also asked if anyone in the room was an engineer (i shut up and layed low)
apparently its one of those things you should just go out and experience rather than trying to prove it for hours

basically if you hold a constant throttle, find a nice smooth corner and tip into it you will slow down

[Fatgit]

Smaller diameter = slower speed. ie the wheel travels less distance per rotation. So when the bike leans, the diameter gets smaller. Probably would result in a slowing of the vehicle. BUT what effect modern low-profile wiiide rear wheels have on this I couldn't really say, theoretically they would result in a speeding up of the motorcycle as the effective diameter would grow as the bike leant over.

Plus when travelling in a straight line 100% (for argument's sake) of the power is being utilised to maintain speed, when cornering a certain amount of the engines power is being used to counter momentive force, and probably some to counter gyroscopic forces, ergo less power is being used to maintain speed.

[fimpBIKES]

yes, its GEARING!!!!

duh, you lean and the rear wheel radius gets smaller
so for the same amount of throttle (and therefore engine revolutions)
you have LESS distance travelled by the rear wheel (smaller radius = smaller circumference)

i agree mr fatgit, especially when a wide rear isnt matched with a wide front (you find the rear runs out of rubber FAR faster than the front cos it is a flatter profile and can lean less as a result)

[Fatgit]

Ala Stu on the AFR run a couple of years ago :D

(Gotta admit I nearly took out the same bridge as well, just don't tell him;))

Is this for a Uni course or are you fair dinkum.
Most of the GP race chassis during the 80's were more knowledge of what works more than theory on paper.
And we're talking about a Rolla Door, no matter what you do to it you'll still have to manhandle it into, around and out of any corner, but there lies the fun.of and come to think of it in a straight line.
Do ya mods and you maybe surprised that what doesn't work on paper is A Good Thing and what does work on paper is A Bad Thing. You'll learn a damn sight more from doing than sitting.

And yes get your hands on Mr Irving's books.

Another thing is you'll have to do some serious bracing because modern rubber will tie the chassis into all sorts of knots.
If you want some pointers try seek a look at the CB WG is punting around in Forgotten Era, the rumor has it that it is a trick bit of kit.

[evad]

I think you should go and talk to an Engineer, lol
:D:D:D:D:D:D

[red]

Oops, sorry to bother you guys. Go back to flattening beer cans on your foreheads. ;)

[Lucas]

quote:Originally posted by red

Oops, sorry to bother you guys. Go back to flattening beer cans on your foreheads. ;)




:Dhahahahahahahahahahahahaha

Isn't that what foreheads were invented for ?

[Fatgit]

Cans??????
Cans??????

Fuck fuck fuck,

dat's whoi OI bun gittin' all dese sore heeds, local only sells stubbies [B)]

[pappas]

The centre of gravity should be a singular point on the bike rather than a line through the bike. The closer the CoG to the front of the bike the more stable the bike, the closer the CoG is to the back of the bike the less stable the bike will be. In the vertical plane the higher the CoG the slower you will turn into a corner, the lower CoG the faster you will turn in.

Your trail makes a difference to handling as well....
http://www.streetfighters.com.au/forum/upload/35057963818328.jpg