This is good stuff... bookmarking this now!

Take a look at the illustration below. Been wondering about this how angle of rake can change characteristics of bike. I understand that more angle(or is that less angle. Am no geometry expert. Anyway the idea is when rake is angled further away from perpendicular 90 degrees) will increase wheelbase. Closer to 90 degs. will decrease wheelbase and hence alter characteristics. This I know and is NOT my question.

My question is, since different angle in rake will make the front tyre lean at an angle against the road when handle is turned. Meaning, as angle moves further away from 90 degs., the front tyre will lean more and more and vice versa. In what manner does this affect the characteristic of the bike?

See picture below. It will illustrate my question.

I am no expert but the first two things that came in my mind were the first is the RTR, and the second is the avenger...
Flickability vs Cruising..

Is that right?

Good question..would love to answer this one.
first..lets consider the first picture...when you lean the bike left or right...it directly affects the tyre leaning ( here the "centre of rotation", would be the point of contact between the tyre and the road) as it is more closer to perpendicular. In rotational motion physics...the force acted upon a rotating body(the tyre in this case) is total force acted upon the perpendicular from the centre of rotation and not only the magnitude of the force .
In the picture given below...if rake angle(A) is more closer to the perpendicular..thus small force is required to rotate the wheel by angle B.


Similarly in the second case (larger rake angle)
since the rake angle is more and inturn distance from the normal is more, the force required to rotate the wheel by angle B is more.

This is why shorter rake angles respond more quickly (as less movement is required to produce a large effect) than compared to larger rake angle motorcycles(where more movement is required to produce the same effect).

You can say that. Take the case of the second example (cruiser). If you are taking a turn and lean the bike, the front tyres will be leaning much more than the first example. Not that you would do some knee scraping stuff on a cruiser. Just wondering on the technical aspects and curious as to how much importance is given to it while designing a bike in relation to its handling characteristics. Would a couple of millimeters in a racing sports bike make a difference?

Another interesting thing is how the rear suspension also alters the wheelbase when the suspension goes up and down and can affect handling. Heard about manufacturers design some bike to make wheelbase remain constant during suspension play.

its not only the banking of the motorcycle that decides the turning speed.
30 degree banking on a shorter wheelbase bike covers a larger angle than a 30 degree banking larger wheelbase bike. both travelling at the same speed and in the same interval of time.

Consider the pitcure below. a shorter wheelbase bike following AB path at say 50 kmph at 30 degree banking and a larger wheelbase bike following path CD at same speed(50kmph) and same 30 degree of banking.


since both are at same speed but the distance AB is less than CD...thus the shorter wheelbase bike will cover the same angle in lesser time( though travelling at the same speed).
What im emphasising on is that its only only the banking of a few degrees that is making the difference here..but its actually the wheel base.
So a few degree banking gives a faster turning in shorter wheelbase bikes than it does in larger wheelbase bikes.

Agreed, however as I illustrate im my diagram. A steep angled rake will make the front wheel lean more than a perpendicular one. So lets say, bike is leaning at 45 degs this means rear wheel is leaning at 45 degrees, but front wheels will be leaning more for example 35 degs when turning. Where as if the rake is perpendicular, the leaning of front tyres may more or less be close to 45 degs in the same situation. In effect, the steep rake will have a greater chance of sliding its front wheels. Is my reasoning sound so far ?

Agreed , no doubts.
But what I caught in one of the post was "does a few degrees of banking make much difference"
compared to the handling part...there is more than just the banking angle...which is why a few degrees on sports bike is quite alot compared to that on a cruiser.
But are you trying to just say that steeper(larger) rake angle is more prone to sliding than the shorter one..?

R Series - better denotion for that would be Turning Radius.

- Street bikes have smaller turing radius. Which makes them flickable in tight spots. The sitting stance is more upright which gives you the confidence to easily take tight turns at slow speeds.

- Sports bikes & cruisers have large turning radius..

- However in case of sports bikes the handle movement is restricted.. which causes the large turning radius. Another reason for keeping a restricted Handle movement is.. the rear set footpegs.. which causes more sportier stance which restricts the body movement under tight turns on slow speeds.
Ok.. look at Yamaha R1 pic... imagine yourself sitting on yamaha R1.. presuming the handle movement is not restricted (more handle movement).. and you are sitting in a sporty stance. With both legs up on footpegs.. Can you take a tight U turn without you putting your feet on ground? It would be tough doing that. But on street bike its easy..as Riding stance is more upright.

On a sports bike.. you are laying on the bike. On street bike you are sitting on the bike.

- Where as Cruisers despite of having large handle movement.. have large turning radius due to the longer wheelbase.

Nitro correct me.. if i am wrong anywhere.

I guess I am a trifle late with my inputs about motorcycle geometry but then seeing the kind of confusion prevailing on this thread, I believe this post is justified. Lets take a look at these elements in a way that is understandable to anyone keen enough to learn. I'll use technical terms only where there are no substitute words available.

Motorcycle steering geometry has four principle aspects that have maximum influence over the bike's handling.

They are: refer to the figure below..




Rake: The angle that the steering axis or say of the forks makes with the vertical. The 'less' the rake the steeper or more upright are the forks. The 'more' the rake angle the front wheel seems 'kicked' out.

Trail: This is the distance between a line extended downwards from the steering axis (say the bolt on your triple clamp...the one visible near the ignition key slot) and the contact patch of the tyre with the road.

Offset: This is the distance between the fork axis and the steering axis. (i.e. visually it is the distance between a line connecting the two bolts that hold the forks and the steering head bolt.)

The wheel diameter: something that is self-evident.

The best example of 'trail' working in the real world is watching the small wheels of a shopping trolley 'caster' as you roll it. They always seem to get 'in-line' automatically with the direction of travel of the trolley. Notice that their axis (its vertical) lies ahead of the contact patch.

The same 'castering' effect is responsible for the self-straightening of your bike's steering while in motion. An explanation of why this happens would be a trifle involved and so am omitting it for now. To cut a long story short, large trail has a greater self-righting effect, meaning the bike tends to be more stable in the straight line - which also implies its more resistant to turns - i.e. sluggish steering. Short trail has the opposite effect. So its pretty obvious why sportsbikes carry shorter trail than tourers and cruisers. For race-track use, one needs a bike thats more willing towards quick direction changes. Vice-versa with cruisers where straight line stability is in sync with a relaxed riding style that cruisers are all about. And the 'low slung' design of cruisers limits its ground clearence, thus limiting the 'spiritedness' of the bike through turns anyway. So the trade-off of stability vs quick steering doesn't hurt here. The usual trail figures for sportsbikes are anywhere between 3.5 - 4 inches while cruisers can go to 5 inches and even beyond.

Rake and trail move in proportion as long as the steering offset remains the same. So a larger trail requires a large rake - i.e. a more kicked out front wheel. But there are limits to how large the rake can be i.e. how far out the front wheel can be. Too much rake has a strong and strange disadvantage. On turning the handlebars, the steering head tends to drop with the turning of the handle. This can make the front wheel 'flop' over on its side rather suddenly and so the bike falls. This steering head drop is probably what @kyanmantis was asking about in his earlier posts. Large rake makes the front wheel lean further on cornering than a more upright wheel. This adversely affects a bike's cornering ability by robbing the front off its traction quicker than lesser rake. To limit customisers from making outlandish designs that can be dangerous to use on road, some countries have laws restricting rake and trail values within certain limits.

Over the years motorcycle designers and tuners realized that trail could easily be altered simply by 'lowering' the front ride height. This could be done by sliding the fork tubes further into the triple clamps. And if this didn't give them a bike that steered quickly enough, they could further reduce the trail by raising the rear as that steepened the rake which shortened the trail. This 'dropping the front for track bikes' became incorporated into bike design and allied with low C of G, smaller diameter wheels (for lower gyroscopic forces) and ergonomics like rear-set pegs that made it easy for the rider to tuck in to improve aerodynamics, the 'low' look became a standard design element of sports-bike design.

OF

can someone put up a link to Tony Foale's book: Motorcycle handling and chassis design.

i somewhat disagree with aargee.
you dont need to sit up straight to be comfy. in my observation, sitting up straight sends the shocks of bumpy roads straight up the spine. that imo is more uncomfortable than your upper body weight being distributed through your hands and back.

another point about comfort is that supersports are really designed to be ridden aggressively on the track or somewhere like that. the apperantly awkward riding position starts making perfect sence once you take the bike into its native territory.

and i suppose thats the case with all bikes; you need to set an optimum situation and try to get as close as possible to that optimum. any other approach will land you a compromise.


my opinion on motor cycle classifications is that it is very relative. by all international standards, almost all the bikes sold in india are commuters. maybe the r15 is borderline. the ninja 250 is regarded as a learner bike around the world. my point is that no matter what you are clasifiing, by nature, a classification will only define an entity within a certain framework. and that framework may be subjective. best not to take classification too seriously.

Give a try on this - sit straight in a chair (without backrest) for an hour & try sitting in a chair for more than 30 minutes leaning, you'll understand what I'm talking about. Anyway you don't have to agree with me 'cause its my personal experience; ride a 350 & R15, you'll understand what I mean.

Again, its very true that when you take the bike to its native territory the riding postion makes very sense & that's the reason I was saying that sports bike are not that comfortable as commuter bikes - this is exactly what I was trying to say.

OldFox - Excellent piece of information;appreciate to have put that info in a lame man's language. Thanks.

the experiment is valid. but if you rest your elbows on your knees, the situation changes completely.

disagreeing is the whole point isn't it?
otherwise we ould have only one type of bike.
no that would have been sad. lol!

@Old Fox: Thanks for putting things in such simple manner. And yes you nailed what I was trying to convey. Need to work on my communication skill .

Most people talk about pulsars having heavy front end and karizma as being light in the front. Especially this is quiet evident when one tries to steer.

My small experiment has shown something interesting. I use ZMA-R, I fit in after market clip-on handle bars and was immediately surprised to find the front heavy and difficult to steer (in comparison to stock handles). Yes, its much lower, also the offset of the handle bar have changed due to design of clip-on. Its more forward, meaning further away from rider. Yes, one has to lean more now and may conclude that the heavyness is due to additional weight of the rider against the handle. But interestingly, I also found that it is still harder to steer even without putting on weight. Just sit upright and steer with fingers. Was wondering whether this is due to offset as depicted in 'oldfox' diagram.

BTW I have reverted back to stock handle as the brake lines and other such tubes are touching the fairing when turn to max. So its stock until I can resolve the issue. So currently I have adjusted my stock handle to perfectly align with the rake(shocks) and steering is light as can be.