R-series

The reason is not to give back pain but to give a more aerodynamic posture for riders to go faster.
honestly speaking..i've had the R15 for about 19 months now..but never had issues of back pain. Simple reason is that I keep the spine straight...although its leaned forward.
And more over the leaning position is more of an occurence in sports bikes...not all of them.

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aargee

Oh!!!, let me explain though...
The idea of my street bike is that you can ride it all through the city, do a little to medium touring & ofcourse not compromising on the speed. Like Bonneville or XJR1300 etc.

With R15 the problem is exactly as what you've specified, lean on the bike for aerodynamics which you can as well do with any street bike. My personal experience with my own Rx100 is, when I sit straight enough, I cannot get past the maximum limit & when I lean, yes I get that extra 5-10 Kmph!!!

So I see a clear advantage of a street bike having a good sitting posture, you lean & get that extra speed; while with the sports bike you saddle sore & a free back pain too. While its possible to sit straight in sports bike (as you sit in R15), I'm sure you'll agree with me that it doesn't offer the same comfort as you get in a real good street bike.

Not trying to defend the street bikes, but still wondering there must be some other reason.

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R-series

I completely agree, but this choice would change from person to person.
I mean right now if anyone asks me which is the most all round suitable bike..? my answer would be anything apart from the R15.
but again...the category which the R15 belongs to is something specific. I don't think we should go that way.
Are you saying that apart from sports bike....the street bikes also have a leaned down sitting position...?
(Cos for sports bikes I think you should understand that people like it that way)

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Old Fox

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

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jd666

^^ great info there OF!!

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My offerings to the gods of speed -

- KTM Duke 200
- Yamaha RXZ 5 speed

braindead

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.

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Abhishek

aargee

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.

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Patience pays...The Vulture's virtue is patience and the bird always get it's food - Wise man saying

This is not a bakery. I don't sugar coat anything. If you ask for my opinion...then that's what you'll get. Don't be mad when it's not what you want to hear

braindead

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!

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Abhishek

kaynmantis

@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.

Old Fox

The 'sitting' posture on a motorcycle is a compromise between either loading up the back or loading up the wrists. And this 'loading' is defined more by the relative position of the foot-pegs than the distance between the handlebar and the seat. The one other major contributing element is the front-end ride height.

The higher the speed at which a certain bike is expected to travel through most of its running life, the more is the torso rotated forwards and feet moved backwards. This is in no way connected to the power output of the engine or the ultimate performance capability of the motorcycle. Cruisers have powerful engines but the target cruising speed is low enough for the full-drag spread-eagle position to be viable. Ground clearence issues apart, try riding a cruiser through the twisties at a 150 kph and you'll understand it all. Sportsbikes are not just about fast acceleration, hard braking, quick direction changes and high speeds. Its all this with CONTROL. And so its about a very dynamic and aggressive riding posture with the rider ready for any body movement or control input done reflexively.


Most sportsbikes have riding postures that are like the rider is caught mid-way in a push-up. The forward leaning and tucked in position is not just for aerodynamic purposes as is usually thought. The purpose is to spread the rider's body-weight as much along the bike as possible and also keep him low so as to keep the resultant rider/bike C of G also low. A high C of G poses control problems both during hard acceleration/deceleration and quick direction changes, all the more exacerbated at high speeds that these bikes are meant to travel at. These bikes are NOT comfortable to ride. The ergonomics are not human friendly and yet they remain so for the benefits of handling and control they provide. Also because the rider in not expected to remain sitting in one position for long periods on such bikes. These bikes are meant for taking turns fast and when a rider does that, he hardly sits in one position for long. Weight shifts, foot-peg weighing and moving forwards/backwards on the seat for better control. All this movement doesn't highlight the discomfort of the posture. Use a sportsbike for touring and the 'painful' ergonomics come up front. As during touring, you sit in one position for long periods and any unusual muscle/ligament stresses get amplified.

The 'tourers' have the most rider friendly ergonomics. They follow the middle path between the sports-bikes and the cruisers. The rider is expected to cover large distances with little 'posture alteration' along the way. The 'direction change needs' are for a good response at reasonable speeds without compromising unduly on straight-line stability. The rider sits such that he doesn't need the support of his arms to stay sitting. A small test of 'long-time-on-saddle' friendly ergonomics is (with the bike on the centre-stand) to be able to stand on the foot-pegs and sit back on the seat without needing your arms for support. Try doing that with the rear-sets of a sprots-bike or the forward mounted pegs of the cruiser.

As for the proper (comfortable and control-friendly) arm position on a bike, it is with the rider's elbows level or a little below the handlebar level in a way that gets his forearms parallel with the ground. This is the unweighted position for arms i.e. the arms are there just for the task of steering and controlling the bike and not taking the load of the upper body. Loading with torso weight only happens under braking. And unloaded arms are a pre-requisite for precise steering inputs and feedback sensing. The cruisers do unload the arms but also place the feet so far forward that they no longer can play any role in taking any load off the back.The rider is like a sail catching all the wind at full stretch and this pushes his torso backwards at speed, making him hang on to the handlebars. The reason for large rake and trail with the accompanying long wheelbase is to give sufficient inherent directional stability to preclude a very active role from the rider. They are the 'easy' riders after all....aren't they.

There's an interesting bit of research in this 'research paper'. It is a downloadable pdf document. Just check out the body-position interrelation between various ergonomic/design configurations.

And here's the link to Tony Foale's works.

OF

Kyan: We posted almost simultaneously. The clip-on feeling heavier would primarily be due to the reduced 'leverage' that you are getting from it. I'll site a different example to put this 'leverage' into perspective. The Suzuki Intruder handle is so wide that if used as a lever with a suitably located fulcrum, one can lift more than 2 tonnes with it!.

Reducing handlebar width even by half an inch will appreciably cut down its mechanical advantage as a lever. That and it being positioned lower while being angled 'inwards' increases the 'force' requirement for steering.

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