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Mick Potter

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PostSubject: Mass Centralisation.   Sun Jan 20, 2013 12:10 pm

Mass Centralisation, an addendum to Frames and Handling.

Mass Centralisation has been a Buzz phrase used by many manufacturers (first used by Honda) over the last few years. I expect many of you will have heard the term but I wonder how many of us understand what it means and how it affects the handling of something as small as a Bantam.

The following scribbling is my understanding of the effects and as always I invite discussion.

Firstly I will mention what Mass Centralisation is not. It has nothing to do with moving the centre of gravity (but attempts at mass centralisation can inadvertently move the c of g to where you donít want it). It also has nothing to do with weight reduction (but weight reduction can both increase and decrease mass centralisation).

As a mind experiment please use your imagination for a couple of minutes and I will describe your bike in the simplest of physical terms. Consider your bike to be a rod that you are holding in your hand at its centre (the rod has no mass). The rod is 2M long and the bike weighs 50KG (a close approximation of a Bantams length and weight). Firstly place 25KG at each end of the rod and swing the rod 30deg from the centre line left and right. Secondly place all 50KG at the centre of the rod and repeat the swinging motion. From your own experience you already know which is easier to swing about. But we donít need our own experience we can use Newtonís Laws of motion. Every action has an equal and opposite reaction. Also, A body either moving or stationary wishes to stay in that state (I leave it to you physicists and engineers to correct my phrasing). The upshot is a bike that is more mass centralised is easier to change direction on quickly. In the past many manufacturers have placed the silencers high up under the passenger seat as first used by Ducati on their 916. A very stylish design and pleasing on the eye but a silencer is a very heavy component to put so far back and high up. All the latest new bikes have their exhaust systems tucked under the engine. Also over the last 15 years or so the fuel tank and air box on sports bikes have changed places. What looks like the fuel tank on most sports bikes is really the air box and the fuel tank is under the rider. These are just two examples of how the manufacturers are improving their bikes.

Once again the same mind experiment but this time thinking about the affects the suspension movement has on the bike. The same rod with the same 25KG at each end. For the sake of simplicity consider the suspension working vertically at each end. As each end of the bike encounters a deviation it transmits the majority of its forces to that end of the bike. The result is a rocking motion rather than a smooth transition. If the mass is moved towards the centre of the bike the suspension at each end of the bike reacts against a greater proportion of the whole mass thus producing a smooth ride.

If any of you have had the chance to ride the best bikes of 20 years ago and the bikes now like I have been lucky enough to you will already know the difference. 20 years ago a bike needed to be manhandled and forced to do your will. Nowadays you only have to think about where you want to be and your there. Some of the improved ride experience comes from better suspension but a lot of it comes from better mass centralisation. If your Bantam is still using steel wheel rims and you change them for ally rims, not only have you reduced the bikes weight but you have also increased its mass centralisation because you have removed weight from the extremities of the bike. Look at every part of your bike. Is that part there because it canít be anywhere else? Any component that can be moved toward the centre of the bike will improve its mass centralisation. Parts like the Battery, water pump, ignition coil, ignition CDI, petrol tank (block off half/move it back),ect. Itís your bike have a good look at it. As Trevor has said in posts on other subjects, 0.5% hear 0.3% there all ads up to a winning formula.

Turning this post on its head. You may have noticed that the vast majority of road bikes these days have bar end weights. The perceived wisdom of the masses is that they are there to damp out vibration and so stop your fingers going numb. This is true but is only a happy side affect of their main purpose. Going back to Newtonís laws they provide inertia which prevent the bike going into a weave or tank slapper. They are in effect star trekís fabled inertial dampers. If your bike has a steering damper try bar end weights instead. If your bike still weaves then you have an underlying problem that you are masking not curing.

I look forward to your comments

Mick.
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dansofield550

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PostSubject: Re: Mass Centralisation.   Sun Jan 20, 2013 8:43 pm

When I first started my build it was very centralised by being in a big box! Sadly I screwed it together ! Have you (mick) any ideas already?
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Trevor Amos



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PostSubject: Re: Mass Centralisation.   Mon Jan 21, 2013 6:21 am

Great article Mick , informative and authoritative , lots to think about .

Spare a thought for us oldies way back in the 60s having to cope with steel wheels , tanks , fork stantions and legs , cast iron brakes and so on , but as we were all controlled by a tighter formula than today , it was no big deal . Making more power always seemed to be more of a priority than any other consideration !

Incidentally , how much lighter is an alloy rimmed wheel than the same with steel , our bike still has steel rims but Mark seems not to mind ?

Regards , Trevor




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mjpowell

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PostSubject: Re: Mass Centralisation.   Wed Jan 23, 2013 6:26 pm

Excellent Mick thanks for that, that should get anyone thinking whose about to make a new frame set up. There is so many thinks to consider.

Cheers Mike
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john bass

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PostSubject: Unfortunate!   Wed Jan 23, 2013 11:10 pm

Great article Mick!

Give this a bashing:-

Unfortunately the front forks with its wheel cannot be centralised somewhere in the nearabouts of the riderīs knees -- where it would obviously be better controlled. The centre of gravity of the front-wheel-and-forks is somewhere near to the top of the wheel forward of the forks which means, at high speed, it can have a pendulum effect about the steering head. Sometimes with nasty consequences.

We had a steering damper on the ABS 250 but on braking it still speed-wobbled which was more frightening for the spectators than the rider. There must be some way of calculating what the damping rate of a steering damper should be? -- I never had time to even think about it in those Good Old Days.

Cheers!

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ted

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PostSubject: Mass Centralisation.   Thu Jan 24, 2013 8:15 pm

JB In all your years you must have seen bikes thet have had the front wheel and forks moved to the centre of the bike. I like Trevor's question as I have trouble getting light weight alloy rims in 17". Your comment "Mark dose not seen to mind" only points out that some riders have the abillity to ride whater they are on. Mark is not your average rider!!
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Ned

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PostSubject: Re: Mass Centralisation.   Thu Jan 24, 2013 10:39 pm

ted wrote:
"Mark dose not seen to mind" only points out that some riders have the abillity to ride whater they are on. Mark is not your average rider!!

Partially true Ted. A good rider like Mark will get the best out of a machine but he could have gone even quicker with a better set up. Wasn't it Trevor that said something like "every .3% helps" Don't forget alloy rims also reduce live and unsprung weight.

Mick I really enjoy reading your articles, very informative and interesting. It's very sporting of you to share your knowledge with your opponents.
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john bass

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PostSubject: Like it, Ted!   Fri Jan 25, 2013 7:19 am

I like it Ted!

Got me thinking about rear-wheel steering. Really would prevent front wheel speed wobble...

Sorry -- being silly again.

I suspect success comes down to how good a rider can diagnose whatīs wrong with suspension relative handling. I read an article by the late Barry Sheene only a short time ago where he said the Susuki -- he was racing at that time -- was far too frisky to be competitive (I forget which circuit it was) and he had 9llbs of weight fixed to the steering head which meant he could be up among the first three ...
Apparently the team went to a circuit -- directly afterwards -- for practice and two other test riders rode it before any changes had been made. Both said it handled so much better and asked what had happened since they last riden the `beastī.

That sounds awfully simplistic and `Suck it & Seeī... yet I guess that is how we were -- a lot of the time -- in those good old days but I think Barry was talking about the 80s era at that time.

Who was it who said, "You steer by the front wheel into a corner and by the rear wheel driving out." Might it have been Les Graham?? .
Cheers!
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Mick Potter

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PostSubject: Re: Mass Centralisation.   Fri Jan 25, 2013 11:32 am

Hi All.

The development of bikes over the years has been slow but incremental. As each year has gone by the next model has only been slightly better than the last one. As with engine development it takes a large improvement before the rider notices the difference (even the best riders donít notice an increase in engine power until it is at least 6%).The rider may not notice the improvement while riding but it will show up as better lap times.

As a road bike is used by its owner it slowly deteriorates and only when it has a service does the owner notice a sudden improvement as it has been returned to as close to new condition as is possible. The same principle applies to handling; it is only when you ride an old bike back to back with a new bike that the improvements become blindingly obvious. All the bikes that I raced in the past handled just fine at the time but with 100% hindsight I can see their limitations.

It is very easy to accept what you have and expect no better because you are happy with what you have. Many people donít change because they are afraid that it will make the handling worse. Donít be afraid of making the handling worse because you will have learned something about your bike. Going in the other direction may or may not improve things but if you donít try you wonít find out how good your bike can be. Some riders always want to change the bike to improve its handling. Others will ride the bike as it is and getting the best out of it as it is. Some riders can ride a bad handling bike and some cant. But everyone will be faster on a bike that handles and steers well.

Mick.
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Trevor Amos



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PostSubject: Re: Mass Centralisation.   Sat Jan 26, 2013 5:32 am

I think it may be true to say , that among recent Bantam race winners , Mark`s is the only one retaining steel rims , so it must also be true to suggest that alloy rims are not a pre requisite for success , who ever lost a race and could put their defeat down to steel rims or not ?
Weight ( sprung and unsprung ) , inertia , gyroscopic , and other fanciful forces all combine to determine how a bike may handle and perform , get these wrong and you will have a good representation of a defective supermarket trolley !
What is beyond doubt is that lighter wheels will allow for greater acceleration from corners and up the straight , but do i spend £500 to get 1/10th sec a lap improvement or get another bhp or so from the engine , for free ? Donations gratefully accepted , then maybe i could get both ! ?

Trevor
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Mick Potter

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PostSubject: Re: Mass Centralisation.   Sat Jan 26, 2013 6:05 am

Hi trevor.

I have in the past been down to my local breakers and rumaged around in his stock. Some bikes have ally rims as standard fitment, old MZ's and most trail bikes come to mind. Its posable to find a sutable rim to mate to your own hub. Iam not a wheel builder by any means but with a wheel spindle and a dial gauge it ia posable after a couple of houre's work to end up with an ally rimed wheel for the price of a second hand wheel.

Good hunting at your local breakers.

Mick.
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mjpowell

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PostSubject: Re: Mass Centralisation.   Sat Jan 26, 2013 7:52 am

Mick once upon a time -about 5 years ago Pete Tibbitts ran a 17" radial tyre on the front of his bantam and a 18" crossply on the back. If a bike came for a MOT like that would you pass it? Whats acceptable with wheels and tyres ? Also Scutty has a bike with a WM2 front and a WM1 rear? and whats best practice ? Cheers Mike

Also Mark had an un-flanged ally rim on the rear of the bantam at Cadwell in June.
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johnSbantam

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PostSubject: Re: Mass Centralisation.   Sat Jan 26, 2013 5:31 pm

I reckon another JB had the right idea ?

http://www.youtube.com/watch?list=UUB_cdRhIDhlavY2I5URSC7g&feature=player_embedded&v=4MbvMqNAFVY
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johnSbantam

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PostSubject: Re: Mass Centralisation.   Sat Jan 26, 2013 5:38 pm



The MCN featured Britten is #6, have been lucky enough to sit on it

I would like to say I own one, well a limited edition model !

http://illiweb.com/fa/pbucket.gif
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PostSubject: Re: Mass Centralisation.   Sat Jan 26, 2013 5:39 pm

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dansofield550

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PostSubject: Re: Mass Centralisation.   Sun Jan 27, 2013 8:40 pm

I guess you water cooled guys could use a small rad flat either side of the engine a bit like a vtr with air dragged out and nice uninteruped air flow over the engine too, still got no ideas for 175's
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johnSbantam

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PostSubject: Re: Mass Centralisation.   Sun Jan 27, 2013 9:55 pm

For the water cooled bikes, how about putting the radiator behind the engine or under the seat ?
On a 175, follow my example and use heavy D14 flywheels and lean your beer belly on the steel tank, a big mass centralised !
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dansofield550

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PostSubject: Re: Mass Centralisation.   Sun Jan 27, 2013 10:33 pm

Biscuit barrel tuning is the easy bit!
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Mick Potter

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PostSubject: Re: Mass Centralisation.   Mon Jan 28, 2013 6:48 am

Hi mike,

A very good question about radial & cross ply tyres on bikes at an MOT. I have never been presented with a bike for MOT with anything other than matched tyres. If I was presented with a bike in that condition I will check the manual but I seem to remember that what fails an MOT is not what you might think. If my memory serves me correctly the best tyre has to be on the back (Wonít be in work until the middle of this week and will post a definitive answer when I have checked the exact wording). As to wheel diameters there is nothing in the test it is down to the manufacture. A lot of bikes have mismatched wheel diameters. Trail bikes tend to have larger diameter fronts to backs where is not been unknown for sports bikes to have a smaller front wheel diameter to rear.

The same applies to wheel rim widths as far as the MOT is concerned. BUT I have never come across any bike from any manufacture that has a larger front wheel rim width than the rear. It is quite common to have wheel rims of identical widths on small capacity bikes but on all larger capacity bikes and small sports bikes the rear wheel is always wider than the front.

Firstly consider what happens to a tyre on different rim sizes. The tyre manufacture deigns there tyre to fit a specific rim size. Some manufactures will give a preferred rim size and an acceptable rim size. On the preferred rim size the tyre will have the profile (at the recommended tyre pressure) that was designed by the manufacture. If a tyre is put on a rim that is wider than preferred the tread is spread sideways. The effect of this is to increase the contact patch of the tyre at about 75% lean angle but reduce the contact patch at 100% lean angle because the contact patch has reached the limit of the tread paten( this is why moto GP bikes have gone to 16.5Ē rims). If the same tyre is put on a rim that is narrower the tread is pulled in tighter than designed. The effect of this is to increase the contact patch when upright and reduce it everywhere else.

Secondly consider what happens to a tyre while lent over. When you initiate the turn you have to turn in the opposite direction (counter steering). Once you have settled into the tern then the bars are turned slightly in the direction that you are going. Because of the rake and trail of you steering the contact patch of the front wheel is moved to the inside of your turn. The tyre manufactures have their matched tyre sets and wheel sizes to work in harmony. Because of the dynamics of front wheels/rear wheels if the front and rear wheels are identical the front will tend to push/drift wide in corners.

If you are correct about Ianís wheel sizes then I think changes are needed.

Mick.
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Trevor Amos



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PostSubject: Re: Mass Centralisation.   Mon Jan 28, 2013 10:57 am

Hello Mick ,
With all this talk of rim sizes WM1,WM2 and so on , could you provide a chart or list of the available race tyres and the designation of rims , that in your
opinion , are suitable for racing Bantam use , please ? I`m sure i am not alone in being totally confused and , will provide much needed guidance to prospective new builders , and if possible , reliable companies to contact to get wheel building done at an affordable price .
This is just the sort of information that ought to be available on the Bantam site as a reference source , and would be most usefull to the surge of newcomers that the current promotion initiative , hopefully , will bring ! It will also keep us oldies ," in the loop " ?

Regards Trevor
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mjpowell

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PostSubject: Re: Mass Centralisation.   Tue Jan 29, 2013 8:35 am

Wheel Rims etc is even more confusing the more you look into it there are WM MT and MT-H2 apparently..
It may be easier to talk rim width between beads (where the tyre butts up too)...

Common sizes are
1.4" (WM0)
1.6" (WM1)
1.85" (WM2)
2.15" (WM3)
2.5"
2.75"
3.00"
3.50"
4.25"

Dunlop KR825 18" recomendation 1.85 - 2.15"

Avon AM26 18" recomendation 2.15" alterative rim 1.85" and 2.5"

Peter Tibbitts - some questions you once ran a 17" Dunlop KR364 what rim size ? Why did you stop using it?

and you ran AM26 tyres one season then replaced the front tyre with a KR825 why?

Regards Mike

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Mick Potter

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PostSubject: Re: Mass Centralisation.   Thu Jan 31, 2013 7:46 am

Hi Mike, Trevor & All.

First, Mike I will finish the answer on Pete Tibbitts tyre choice. Yes it would fail an MOT. Exact wording from the MOT inspection manual is, a bias belted tyre fitted to the front with a cross ply tyre fitted to the rear wheel & a radial tyre fitted to the front and a cross ply or bias belted tyre fitted to the rear wheel are reasons for rejection. From a personal point of view, I would rather have the beast tyre on the front and think the guy who wrote the mot Manuel didnít ride bikes.
You have already answered part of Trevorís question to me but raised another on types of wheel rim. To clarify your (and I expect others) confusion. WM rims have a flat bead seat and are used for tubed type tyres. MT (sometimes designated as TL) has a 5/deg taper on the bead seat to inshore an air tight seal for tubeless tyres. MT was the first type of tubeless rim, later versions improved sealing by having a bead on the inside of the taper and these types have the secondary designation. Other rim types are used but it is extremely unlikely any one will try and use them. There are spoked wheel rims available to use with tubeless tyres and even though the performance of a tubeless tyre is better when it is fitted without a tube the cost of these rims is very high. Obviously it is best to use a tyre on its correct rim type but the change on tyre profile when used on the wrong rim type is far far less than fitting a tyre to the wrong rim size. There is no good reason why you canít use a tubed/tubeless tyre on a WM/MT rim. The difference is tiny. It goes without saying (I hope) that using a tubeless tyre on a tubed rim will mean that a tube will be needed. The difference between the weight of a tubed tyre & a tubeless tyre is negligible. The only main difference between a tubed & tubeless tyre is the weight of the tube. Without the weight of the tube, unsprung weight is reduced/suspension movement is improved, rotational mass is reduced /gyroscopic progression is reduced and mass centralisation is improved. . Tubeless tyres used on MT type rims will always be the best for, grip, feel, road holding, rider confidence, ECT. But considering the level we are racing at even I find it difficult to justify the cost.

Secondly, Trevor I hope I have already given some useful information. On wheel building I have always done my own even though I have to spend a lot of time on it. But I do get a certain satisfaction when I finally end up with usable wheel. At work we have until recently used a local wheel builder who was good and reasonably priced, unfortunately he has decided to retire. So now we recommend Antig Engineering who are in Gloucester and they build speedway /Grass track bikes. There prices are cheaper than some well known wheel builders. I am shore that most of us will have people it there local aria who deal in moto cross/ speedway ECT who can rebuild road wheels cheaper than most (motocross bikes smash up wheels on a regular basis).
As to race tyres I will do some more research so that I can come up with as close to a definitive list as possible not just my own personal experience.

Watch this space.

Mick.
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Trevor Amos



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PostSubject: Re: Mass Centralisation.   Thu Jan 31, 2013 9:41 am

Thankyou for that reply Mick , a typically thorough and erudite post , we are indeed fortunate to have your knowledge and experience so readily available !
Inquisitive visitors to the site will certainly be aware now , that those Bantam guys know a thing or two , and have a level of sophistication that may come as a surprise
and be spurred on to have a go !
I`m eagerly looking forward to the next " Episode " !

Regards , Trevor



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