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Question about centrifugal forces and scooter wheels.

Last post 03-31-2013 07:03 PM by vortexau. 16 replies.
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  • 03-29-2013 01:45 PM

    Question about centrifugal forces and scooter wheels.

     I've noticed in my reading up on various scooters that there is a limit to the speed you can go on certain size wheels. I owned a very old honda lead about five years ago. It's been my only scoot. I noticed that it got twitchy at 80kph. I've also noticed that of the lighter to. Mid capacity scoots, they all seem to get twitchy at around 90kph. Also the burgman 400 with it's 13in wheels is well known for being non compliant for leaning into turns at speed. From what I've read, most contribute this to it having a low centre of gravity. I believe that to be incorrect but suspect that the real cause is the centrifugal forces that are acting on it's small wheels.(Athough I would guess that the mass of the burgman would have some cancelling effect on those forces. I'm betting that one mass acting on the developed by the spinning forces of a wheel is to some extent is reducing it's effect.)

     One other part of he equation, is the back wheel. This is often made smaller than the front wheel because one, it's not used to change direction persay and increases the storage under the seat. While it's true that it may not steer the bike as such but in fact, every time the bike is tilted, there is inertia that is occurring here that is quite twitchy when it is forced to change direction. (I hope the way I'm wording this is making sense.) 

      To explain how these forces on our wheels are playing out, think of your wheel as a spinning disc. Think of it like the rotors on a helicopter. A helicopter is  a very agile (unstable) aircraft because of the spinning nature of it's rotors. To understand it's effects, think of holding a push bike wheel by it's axle. When the wheel is not turning, it is very easy to tilt (turn) it from side to side. Now spin that same wheel up to a fairly high speed and then tilt it. You will immediately notice 2 things. The first is that it will want to remain in the same plane that it is already spinning in. The second is, that once ou have applied enough force for it to change direction, just how twitchy it is. The first part explains why a burgman doesn't want to lean into a corner and the second part explains why a lighter scooter with 14in wheels gets twitchy at speeds over 90 kph ( which seems to be their threshold of weight of the scooter acting as a dampening effect on the spinning forces that are being applied to the wheels.)

    I'm sorry about waffling on a bit, but just needed to put a frame up for this question. Are large scooters like the Majesty effected in their handling by becoming twitchy at speeds over 90kph? I imagine that the Majesty with at least it's front wheel being 14in combined with it's mass would go Obote this speed, much how much before it or other maxi scooters with I ill Ives wheels, before hy become unstable?

     Thanks.

    Mick Evans.

    P.S. Aploogies if it was. Bit long in he asking.

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  • 03-29-2013 04:32 PM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    I'm going to sidestep the matter of centrifugal force since there is no such thing as centrifugal force :)

    However, two guiding principles in design of performance bikes are mass centralisation and low centre of gravity. Taken to their logical conclusion  they result in something like Gurney Alligator, a design that might not be a huge sales success, but by all accounts despite its oddball appearance does steer very well...

     

    I believe that twitchiness of smaller scooters you experience has a lot to do not just with their wheel size, but also the fact that being built to  design criteria other than performance, they are a long way indeed from the ideals of mass centralisation or keeping their centres of gravity low.

    Conversely, I don't think larger scooters which *are* designed for open roads should have much problem in this department at anything near legal speeds. Certainly my Downtown has no such issues, neither did T-Max or the Integra I tested. Unfortunately I've never actually ridden a Majesty.

  • 03-29-2013 04:46 PM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    Love the artwork Ferrix.

     In regards to centrifugal force; my Funk & Wagnalls dictionary  describes it as:Physics  The inertial reaction by which a body tends to move away from the centre of rotation. End quote. Not trying to be stuck up about it or anything, just clarifying.

     Cheers.

     Mick Evans.

  • 03-29-2013 04:57 PM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    Just an opinion for which I have no evidence, but I think the weight distribution has a lot to do with it. I believe a 50-50 spread of weight between front and rear wheels would be much more stable. Doesn't a scooter get more twitchy when there is heavy stuff in the topbox?

    Mike 

  • 03-29-2013 05:01 PM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    Mick E.:

    Not trying to be stuck up about it or anything, just clarifying.

    No worries, it's me who was stuck up about it, jokingly! It would take a real physics nerd to give a toss about the difference between centrifugal and centripetal forces in practice (which is also kind of the point of the comic)... and I'm certainly no physics nerd - I just like comics.

  • 03-29-2013 09:32 PM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    If I may interject my 10c worth, I ride a 250cc sports scooter. it has an alloy box chassis straight off a 125 gp bike.

    At plus 100 ks in unstable air from other vehicles it can become very unstable. The one thing mine and most if not all other scooters dont have that bigger bikes have is dual triple clamps.

    Derbi GP1 250i a properly fun scooter. Now totally renewed, rebuilt, run in, run Hard and now sold.
    Its name...The Flamenco Dancer by SOLD

    Now on a brilliant Yellow Suzuki SV650 NUDIST crumbs it don't half shift!

    Oh

    And remember boys and girls,

    Stock is BEST!
  • 03-29-2013 11:46 PM In reply to

    • TRA
    • Top 100 Contributor
    • Joined on 05-09-2010
    • Posts 302

    Re: Question about centrifugal forces and scooter wheels.

    I would like to disagree with ferrix, centrifugal force is indeed a force as far as mechanics are concerned :P

    However I agree with the rest of his statement!

    A good bike has an aligned frame, good suspension, balanced wheels etc. Dont expect much from a bike built to a budget. 

    The biggest problem I found with most scooters I have ridden, is the small wheel sizes dont deal with bumps very well, and the lack of suspension travel also does not help. The last scooter I bought was purchased mainly because of the quality of the suspension. Good suspension will suck up the crap the smaller wheels deliver.

     

    One thing I have discovered over the last few years, is that stock suspension on most scooters and bikes is rubbish. Even if its decent, it needs to be set up for the rider. How many scooter riders measure suspension sag and travel? How many get there scooter setup for their weight and luggage combo? HAve you ever adjusted damping and rebound?

     

     

  • 03-30-2013 09:43 AM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    TRA:
    I would like to disagree with ferrix, centrifugal force is indeed a force as far as mechanics are concerned :P

    For those interested in an in depth explanation of the centrifugal vs centripetal debate:)

  • 03-30-2013 10:05 AM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    You got me curious there Ferrix, so I looked it up from my same source. It seems that they are actually opposite forces. Centripetal force: Physics A force attracting a body toward a centre around which it revolves. End quote. There you go, I just learned something new but suspect that you already new that and was having a bit more fun. It always good to learn.

     I didn't know that scooter frames were built so badly. I assumed that they were always built as a properly allined unit. If that is true then it's very sloppy on the part of the manufacturer.

     Also, it is an interesting point about the weight distribution over the wheels. I experienced how twitchy a ute can get when you fully laden it. I removed a large tree from our back yard a few months ago and when i took it to the tip, at 90 ks, it got twitchy. I wonder how much the small wheels are affected by my first question that I opened up this thread on?

    Mick Evans.

  • 03-30-2013 10:53 AM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    Mick E.:
    I didn't know that scooter frames were built so badly.

    Some cheaper ones perhaps are, but even the good ones are more prone to twisting than motorcycle frames - this is the inevitable result of the design that includes a gaping hole in the middle of the frame (i'm referring to the foot well). I also agree with the earlier point about importance of good suspension but if the wheels get out of alignment with each other, the best suspension can only compensate for so much...

    Also, it is an interesting point about the weight distribution over the wheels. I experienced how twitchy a ute can get when you fully ladenit.

    Indeed. And scooter are notorious for having very uneven weight distribution - another factor. Having the entire weight of the engine hanging on the swingarm doesn't help the matters either!

    None of the above is meant as criticism of scootes, btw - I said earlier they are designed to criteria other than performance and I meant exactly that. Scooter frame is the way it is because convenience of step through was considered more important than torsional rigidity and for a small, low power machine it's a fair call. But it is also no accident that larger scooters inevitably abandon both flat floor and swingarm location of the engine.

    ....

    (PS. The article I linked to previously is a little out of date in one respect: towards the end it mentions Higgs boson being a hypothetical particle and as you've probably heard physicists are now pretty sure they've confirmed its existence. But the rest of the article remains valid as far as I know.)

     

  • 03-30-2013 11:31 AM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    Tyre Pressure !!!

    My Daelim S1 I had should of used a tyre pressure of 29psi, front and rear.  I used to have to have it at 33psi, to get rid of the twitchiness.

     

    Same applied to my Xciting. Front tyre used to do this strange wobble, until I increased the pressure a little.  If I match the *firmness* between the front and back, I can avoid the see-saw action that you can get if the firmness isn't matched.

    You might find that the air pressure you have if fine for getting around, but when you want to start doing fast bends and low leans, then you must have the tyre pressure spot on so you don't get the tyre bunching up from under inflation.  Too much and you won't have enough friction, and may bounce off the road.

     

    Also there is the warping of tyres at speed.  This is why Maxi scoot should have Radial Tyres, so to reduce the warping at speed.

     

     

    Something to think about anyway.

    Kymco Xciting 500Ri ( About to sell )
  • 03-30-2013 12:19 PM In reply to

    • awa355
    • Top 150 Contributor
    • Joined on 01-20-2013
    • waikato
    • Posts 194

    Re: Question about centrifugal forces and scooter wheels.

    I would've thought that going up 4lb in your Daelim, (13% increase) would have increased the twitchiness due to the tyre walls having less flexibility and more rebounding off the road surface. A bit like comparing bouncing a under inflated basketball on a hard surface to bouncing the same ball thats over inflated. 

    I tried running far lower tyre pressures in the beewee which worked for INMA but found for me that it felt I was riding on half flat tyres. The ride was softer, but the steering was slower and more vague. My scooter is still on the original Ching shen tyres which are of a quite hard rubber. 

    As ferrix says, most scoots are built to a design ( his word), criteria. I would say built to a budget criteria. I would think that if the right aftermarket suspension components were avalible then the cornering of scooters using 10" wheels could be improved no end. If long term ownership is in mind it might then be worth investigating. For me, I will have the beewee for about 12 months then move to a 150-200 so I will just live with the handling as it is. 

    Has anyone tried different fork oil weights, springs, better rear shock? 

    I believe every road engineer should have to ride a scooter with 10" over any road they have built before okaying the job. 

    Awa and Buggsy
  • 03-30-2013 06:26 PM In reply to

    Re: Question about centrifugal forces and scooter wheels.

    awa355:

    My scooter is still on the original Ching shen tyres which are of a quite hard rubber.

    There's your problem.  If you get some good tyres, you'll be able to notice the difference.  The Daelim came with "Swallow" tyres and they were absolutely POC.  (It's all in the name Wink )

    33psi on the Daelim was pretty good.  At 29 or 29.5, you get the rolling effect, where the tyre bunches up.  After the stock tyres I had Pirelli GTS 23 & 26 from memory, then SAVA MC16's.  The Sava's were very good. http://scootercommunity.com.au/forums/p/15495/113098.aspx

     

    Kymco Xciting 500Ri ( About to sell )
  • 03-30-2013 07:48 PM In reply to

    • INMA
    • Top 50 Contributor
    • Joined on 03-19-2009
    • Posts 818

    Re: Question about centrifugal forces and scooter wheels.

    Scooters seem to reach their limits faster than motorbikes because they have larger unsprung mass in the engine plus they are not really designed for high speed cornering.

    Motorcycles and scooters experience centrifical force when turning rolling around a corner.  The reason motorbike lean into corners is so the force created sideways as you turn in the bend balances the gravitational force that pulls you to the ground.

    The faster a motorcycle or scooter goes around a bend, the more the bike needs to lean to balance the forces.

    When pushing my scooters hard, I often bottom out the rear suspension on corners due to the increased load of the gravitational force plus the centrifical force.  The faster the speed or tighter the corner the mnore load on the suspensions.

    As for scooter's little wheels spinning faster and the centrifical force on the wheel having some effect on handling, the potential heat buildup due to the small mass of the tyres is a more significant aspect than the forces in the wheel.

    At higher speeds the inertia of the wheels and engine of a motorbike or scooter increases the forces needed to steer the bike or scooter.  My motorbike got very heavy when trying to counter steer at over 130km/hr.  Its easy to see how people over cook corners with higher performance bikes, the gyroscopic forces of the front wheel and an engine at 10,000 RPM make a medium sized bike feel like a Mack truck at higher speeds on corners.

    I suspect gyroscopic forces have some effect on bigger scooters like the Burgmen but most people won't punt a scooter like that around bendy roads at higher speeds to find out.

    The GP racers have my full respect when I watch them toss the bikes between corners at massive speeds.  The forces they are controlling are massive and they make it look easy.

    2001 Yamaha Zuma, DR Evo 68 cylinder, Leo Vince TT derestricted, Delorto 17.5 (#98 jet), standard aircleaner, RMS fan, Race CDI, Standard oiler and premix @ 75:1 Shell SX2, Doppler SR3 variator, RMS clutch, RMS R rear shock absorber and Pirelli SL26 tyres.
  • 03-31-2013 10:51 AM In reply to

    • Jobbie
    • Top 150 Contributor
    • Joined on 04-05-2010
    • Craigmore, SA
    • Posts 238

    Re: Question about centrifugal forces and scooter wheels.

    Regardless of whether it's a big or small scooter, the physics doesn't change because it's the front wheel, not the back one, that does all the steering, and most of the steering is dependant on the angle of the front forks. *gasp* Gyroscopic precession (or torque induced precession)- if you tilt a rotating wheel by the axle, the wheel will want to tilt on a axis 90 degrees from the direction you want it to turn. So you need to anticipate the law of percession and use it to your advantage.....This is why to turn left on a scooter, you need to slightly steer right. Force applied: Now it comes down to the angle of the front forks. The more angled, the more the displacement of the front wheel's axle for a given force applied. Bear in mind, of course, that the angle of most scooters front forks are within a few degrees of each other. The diameter of the wheel and tyre, and the width of the wheel and tyre, all play a part. Add to that, the mass of the rear swingarm and motor, the speed, wind, road angle, rain, gravity...In this case....it IS rocket science.

    Look up "gyroscopic precession explained" on youTube. Those guys can explain it better than I can.

    Aprilia Sportcity 125 2007.
    Aprilia Scarabeo 250 2010.
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