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Big Bike vs Small Bikes - Cornering dynamics

Discussion in 'General Motorcycling Discussion' started by evader, Oct 21, 2009.

  1. Hi,

    Something that's been on my mind is the choice of going 1000cc or 600cc-800cc.

    My question though is not about power for once or the transition from learner legal to a quick bike.

    I've often read or heard things about 1000cc (or bigger sport bikes) not cornering or flicking side to side as well. The weight difference between new 600's and 1000's isn't that huge these days and really I think they weight of the rider would influence that just as much, so does it really just come down to physics instead of dry weight?

    What I mean to ask is are the larger spinning parts (flywheel) in a 1000cc+ engine are actually responsible for this propensity to go forward instead of flick side to side on tracks or winding roads?

    How much does this gyroscopic effect actually have? Do modern 1000cc+ bike designers try and reduce this effect?

    I really don't know much about this subject obviously, so don't be too harsh if I'm way off.

    Is there an ideal engine size that perfectly partners straight line speed with cornering ability?

    I do remember talking to Johnny Oh once and he mentioned that this is part of the reason MotoGP bikes got quicker when they went to 800cc. Apologies if I've misquoted JO, it's only a vague recollection.

  2. Re: Bike Bike vs Small Bikes - Cornering dynamics

    What's the difference between a Bike Bike and a Small Bike? ;)
  3. Re: Bike Bike vs Small Bikes - Cornering dynamics

    Did you read the post? :)
  4. Re: Bike Bike vs Small Bikes - Cornering dynamics

    did you read the heading (y)
  5. Re: Bike Bike vs Small Bikes - Cornering dynamics

    Yes the gyroscopic effect is large. A perfect example would be too look to supermoto where often the bikes have identical frames, identical engine cases, identical everything except a longer stroke and a larger piston (open class vs 450 class). The weight difference is less than whether the rider had a full bladder or not, but the flickability is markedly different, proven by seat of the pants and lap times. Then of course there's ridability (more power is too much sometimes).

    The gap between 1000's and 600's is drawing much closer than it used to be. But will something like an RS250 still feel like a ballerina in comparison, hell yeah.

    On top of all this again is geometry, steering head angle, offset, trail, wheel base.

    Everyone has a different happy medium. I've heard the 848 called the perfect mix. Same with the 675. But they'll both feel like hulks to the kid that grew up on dirt bikes.
  6. Re: Bike Bike vs Small Bikes - Cornering dynamics

    Devotard, wouldn't the GSXR750 be lumped in the happy medium pile with the 848 and 675???
  7. Re: Bike Bike vs Small Bikes - Cornering dynamics

    I would think so, im testing 600's vs 1000's at the moment thinking about next bike. and when i got on the 750 i was hooked.. more power and flickable.
  8. You can learn to flip a bigger bike very quickly on it's ear - the difference isn't as big as you might imagine once you get the hang of it. The smaller capacity bikes will still feel lighter and more nimble plus they're less intimidating to ride - you don't scare yourself as much, and so your average speed might easily be higher. Either way you choose, you can go the other way later, and probably will want to.
  9. IMRO*, I reckon a lot of it comes down to weight distribution. Larger capacity engines by their nature are taller and wider by a few cms over their smaller kin, and why this doesn't seem like a big deal, it means that the larger capacity engine has to be carried a little higher to provide the same engine casing ground clearance, and the engine being taller as well means that the overall center of gravity may be ~3cm higher in a liter bike over a 600cc bike. 3cm may not sound like a bike deal, but angular momentum is a function of the square of the radius, and when talking about the scales of distances on bikes, that can amount to a 10% greater effort required to get the bike to switch from side to side.

    The Daytona 675 is quoted above. I own two. As an example of the effect of weight distribution, the stock 675's exhaust cannister weighs close to 6kg and is placed under the pillion seat. Replace that with a 2kg after-market carbon fiber item and the bike instantly feels more flickable. Just a mere 4kg difference, and the difference is instantly noticeable in corner transitions. What's the weight difference between liter-bikes and 600's?

    Some of the issue also comes down to the gyroscopic momentum of the wheels. Higher capacity bikes capable of higher speeds need bigger wheels and/or brakes to deal with the added loads/speeds. Front wheel sizes on the 600s vs liter-bikes are the same, but the liter-bike brakes are typically of a bigger diameter and heavier. The rear wheels are also wider and heavier. The rotating mass of the wheels creates a gyroscopic effect that resists the bike wanting to flip from side to side. The heavier the wheels (as on liter bikes), the greater the resistance. It's not a huge difference, but it's there.

    However, and IMRO this is of HUGE importance, steering leverage can mitigate all of these issues almost completely. The typical sports-bike clip-ons are put in a position that is a trade-off for steering leverage and top-speed aero-dynamic efficiency. By top-speed, we're talking about ~5kph differences at >260kph speeds. In a race, that can be important. On the public road, or even at a track-day, it pretty much means jack, and a lot of steering leverage is traded off with the bar position used on most sports-bikes.

    IMRO, the single best (and probably cheapest) thing you can do to make any sportsbike steer better for public road, or even track-day use, is to install a set of adjustable clip-ons, that allows you to raise the grips by around 2cm, and set them wider and flatter by 2-3cm or so. You'll lose maybe ~5kph in top-speed, but you'll have much greater control of the bike at typical public road cornering speeds, and the steering leverage can enhanced by up to 20% or so, which more than offsets any added effort in pushing a larger capacity bike around.

    As to why flatter/wider/higher clip-ons is not THE first modification that any sports-bike rider makes for public road use, I'll never quite understand...

    * - The R stands for Retarded
  10. That's exactly what I was after, thanks [FLUX].
  11. I had a pair of such bars put on my GSXR750 as part of pre-delivery. Mainly for comfort, but the added leverage they offer is a great bonus!
  12. Good read that was Flux, cheers!
  13. Probably because Helibar risers are around $500, and many people with sports bikes don't ride them for long periods of time. Furthermore, raised clipons may not clear the fairing and many require extended clutch & brake lines. They may also interfere with fork attached steering dampers (requiring special brackets). This all can add up to a fair bit and doesn't provide any performance improvement (some riders are obsessed).

    Having said that, I'm currently trying to install a non-specific set onto my bike.
  14. $500?!!! Holy crap! Someone's getting ripped off blind.

    I've bought and use these, which are US$250, or about AU$300 to your door at current exchange rates.

    Given their range of adjustment, I've found that they can be set up to clear most fairings, while still giving a decent raise and width increase for leverage without requiring the need to extend lines.
  15. just google clip-ons. i never thought of it.

    ill admit the road use of my zx6r is not the most comfortable, but im comfortable that the white coats at kwaka have got it right, i would feel uncomfortable changin the dynamics.
  16. Yeah bike specific risers (e.g. Harris) are very expensive, and even more so if you buy them from a reseller who's imported them to Australia.

    I grabbed a set of these which cost me about $130 AUD shipped from some bloke in the US (didn't fit his bike). Haven't put them on yet tho.