Attn Mick Potter. Frames and handling..

PS -- what I meant was that I have yet to see an article that shows & explains more than considering the combined CofG (combined centre of gravity of bike plus rider) alone when describing the effects of centrifugal force on handling.
`Forces´ is what I mean.

Maybe I am wrong and there are published articles somehwere.
If anybody knows of such I´d much appreciate more info on this subject.-

It is that all the articles I´ve seen on this, where the combined CofG is not in line with the centre-line of the machine, the combined CofG location is used in explanation and calculation. That´s OK but I think this offset of rider´s CofG -- to bike´s CofG -- is of more importance in respect of handling and comfortable control than the writer´s say in any of the articles I have read.
...

The bike´s CofG is at a constant location although weight is transferred from rear wheel to front wheel on braking and when accelerating vice-versa -- the weight transfer goes to the rear tyre....
These positive and negative accelerations produce two seperate forces i.e. the bike´s inertia force and the rider´s inertia force -- when braking and accelerating.
When the rider´s CofG is not on the bike´s CofG-centre line there is a leverage of this inertia-force times the offset of the CofGs which I consider should be considered as an upsetting-to-steering source in consideration of handling at speed ....

Or maybe it is so small as to be considered neglible -- but I look quickly at the body weight of Colin and myself when racing our Bantams and although we both dieted like crazy for sure when fully race shod we were both around 13stone -- 182lbs.

I´ve seen Colin accelerating from the apex of a bend with his posterior well off the saddle and perhaps his CofG was around 6" of offset.
This would mean if he was accelerating at around 7ft/sec² -- 0.2g -- then his offset leverage would be (from the formula (f) force = ma which is W/g xa) -- f = 182/g x o.2g -- which is 182lbs X0.2
= 36.4 lbs. This, then multiplied by 1/2 foot gives a leverage 18.2ftlbs acting in yaw fashion on the frame
and about the rear trye contact patch to have some effect on steering...

... or am I hopelessly wrong?

Mick! -- help!!

Obviously a heavier rider will produce a greater force and torque.

We all know that the racing motorcycle is never very long at constant speed: it is accelerating or braking most of the time -- and most of the time -- WHEN THE RIDER IS NOT KEEPING HIS OWN CofG ON THE BIKE`S CENTRE LINE... -- this offset levearge (of rider´s CofG) is having some effect on the bike´s behaviour.
The heavier the rider the greater these effects will be.

I just wonder if anyone has ever done any calculations on this aspect -- and as said above -- has there been a publication on the subject??

Comments welcome.

I have already apologised to the Late Colin Aldridge -- he died of cancer in 2002 -- and I did contribute to the Wobblyeman Trophy -- and select it so I feel sure forgiveness is at hand....

Cheers!

Hi, John.

Re your last post, what you say makes perfect sence. But like you I have never seen any papers or articals on the subject. So anyone out there in Bantam land who can enliten us.

Mick.

Hi Mick -- thanks!
I´ve done some simple trig calcs to look at how that comment of yours stacks up about the suspension not only having to be adequate in static (fully-shod racer on board) mode but also having to cope with the load increase (at the tyre contact patch) from centrifugal force when cornering and I surprised myself with the amount MORE it is when considering an 11stone (fully-shod racer on board) and that which developes from having the extra 2stone -- 13stone racer in all his racing gear -- at the mild leaning angle of 30° . It is 13.6%.

So it looks is if it is important -- when selecting suspension-units -- to seriously consider the rider weight that´s added to the poor little Bantam*** already whittled to the bone with weight saving...!

Probably saying WHAT EVERYBODY KNOWS ALREADY and will suirely get another reprimand for being so b... boring...!!

***Addendum! *** When preparing Icarus-1, we were looking for a `Jockey´ and not having a volunteer I decided to take the Bantam to Brands to check out the resonant exhaust and things...
and my mentor & engine-man said that I might just as well have left all the bits on that I´d hacked off because of my (very) personal body-flab-weight. In 7 weeks I got down to 12stone3lbs but was so weak I had to have someone start Icarus-1 for me....

Ow! yaaaawwnnn!
Cheers!

I am surprised?
There have been no reactions to the CAT questions?

Just ignored ... I imagine -- poor old demented JayBee speaking of thrown-up-in-the-air-cats on a motorcycle handling subject!!

Fact is the upside-down cat, as it is travelling upwards keeps the front portion of its body still -- whilst it twists the rear part ABOUT its own CofG. Then it keeps its back part still whilst it moves the front bit ...

...all in the interests of science, of course (it was filmed several times by Professor-Doctors who should have been doing something more useful for mankind -- but it has come in useful for Bantam Racers,*** at last! -- I hope?).

... the cat kept doing this quick short moves of front & back until it then had its feet pointing towards the ground!

What´s that to do with Bantam or any kind of MC racing? you may ask -- well it is just that! the rider´s CofG is constantly moving -- maybe for some `orthodox´ straight men it is keeping the rider CofG on a line, up and down the centre line of the bike whether braking, cornering or accelerating but for many it is a switching from the centre line to Inside-the-centreline and for others -- who lean-up instead of lean-in or hang-in it is the opposite ...

...ah! I think another diagram is necessary because I am describing this action with my hands
and working the keyboard, at the same time, has beome extremely difficult.

Just this last thought before the diagram: During practice -- and possibly during racing -- have you
noticed which side of the front wheel -- or over its top -- are you seeing as you view the road whilst entering, driving thro´and exitting a corner???

I only noticed this because my mentor & engine man during testing at Brands (1968...!!??) one time asked why I "Leant-up" instead of keeping my body in line with the bike on entering Paddock Bend (the old bend before they made it EASY!)...

I then observed -- and this is fixed in my mind -- that on Druids (right handed hairpin) I was looking down the inside (right side) of the front wheel, on Southbank, again the inside, the left side and in Clearways across the top of the front wheel.
... So, I conclude -- after all these decades -- that on Clearways I was in full streamlined crouch and when self-preservation at Paddock Bend took over it had me wanting to push the bike down at excessive angle to look over the top -- where the road dropped away and could not been seen -- well, that was my excuse at the time.

I think it will be necessary for another diagram to explain what I meant about this "Lean-up" style giving confidence -- even though the wheel angle is more acute ... and how much difference there is in forces at the road-contact patch.


UNLESS THERE IS OBJECTION TO MORE `BULLSHINE´ --of course!

Doing some calcs relative this subject and realise I have been way off with a racing Bantam´s weight.+

Good job I didn´t present it on here with the flywheels exercise because you´d all be rolling on the floor laughing -- Peter has the flywheel treatise and has said nothing -- obviously doesn´t want to embarrass the poor old chap -- me -- by pointing out the mistake(s)!

I looked back here and saw that in Mike Powell´s message he says front and rear weights were 37 and 39kg respectively. That is only 76kg -- 167.2 lbs!

Now that I have the opportunity of avoiding Newtons & Metric units I am using lbs and feet -- and even stones! -- and Holy-Crud!! did I make a mistake with the weight of a Bantam !! I used 256lbs and thought that I was being light? Same in the flywheel exercise that is obvious garbage since it refers to acceleration of the bike & rider which is probably half of what is should be...

Onthis `Handling´exercise I also used 11stone for rider fully-race clad and compared those calcs with a rider (fully race-clad) of 13stone. Seems the body weight was also too heavy??

So I am down in the dumps and will have to scrap all the calcs I did on the `Handling´ subject becaues I had a total weight of 410lbs [bike at 256lbs and rider at 154lbs(11stone)] in one exercise and 438lbs using the same bike with a13 stone rider making a total weight of 438lbs.

Time to pack it in -- I guess!

Cheers!

PS -- Just for the heck of it: WHAT IS THE WEIGHT OF A BANTAM RACER? and its rider?

Good evening John,
I thought it was obvious, as light as possible !
all the best Nick

Hi all. More ranting, this time on rear suspension.

I will use the usual twin shock arrangement that most Bantams use for my example (monoshock suspension is fundamentally identical but depending on the type may require slightly different maths). In order that the example is easily understood I will exaggerate the modifications made.
Unless you have spent some money, or are lucky enough to have purchased a bike with them your rear suspension units will have no damping adjustment. For the sake of simplicity I will assume that the suspension units are fitted directly above the rear wheel spindle. The damping will be what the manufacturer has deemed appropriate. The amount of suspension movement will depend on the length of the units themselves (usually a little less than a half of the total length). If the suspension mountings are moved to half way between the rear wheel spindle and the swing arm pivot some obvious and not so obvious changes happen. Firstly the amount of rear suspension movement is doubled (Just because the movement is available doesn’t mean you will use all of it). But the damping is reduced by a half because the speed of movement is halved.
The effect on the spring rate is different. It must be understood that a spring is a spring and nothing you can do to it will change how it performs. When you increase the preload on a rear suspension unit, all you are changing is the ride height. Unfortunately a heavy rider may need to increase the ride height to stop the unit from bottoming out but end up with a unit that has no static sag.
Years ago spring rate was measured in lbs/ft; nowadays it’s KG/cm. If a spring is rated as a 5KG spring it will compress by 1cm with a 5KG load from a free standing state. With 10KG it will compress 2cm. With 15KG it will compress 3cm ECT. If you attempt to measure the rate of your own springs great care is needed as when compressing springs they have a tendency to have a mind of their own, trying to fly off in any direction. Now consider the rear shock in its new position. The leverage on the shock is doubled, but also the length of movement of the shock is now half the movement of the rear wheel spindle. Therefore the spring rate is a quarter that it was in its original position.
Therefore as rear shocks are moved forward on a bike the spring rate will reduce by twice the damping.
Rear suspension acts differently to the front (assuming we are all still using telescopic forks) because the rear wheel spindle and therefore the lower rear shock mount move in an ark. One end of the shock moving in an ark means it is impossible to have a totally linier rear wheel movement. Ideally we need to aim for as close to linier movement as possible but that means initially a reducing rate followed by a rising rate movement. Reducing rate as the description implies means the spring effect reduces with movement, this is to be avoided at all costs. So the best compromise is a slight rising rate. To change the spring rates the angle or the shock is changed. There is no magic angle for the shocks because of the number of variables (length of shocks, length of swing arm, height of swing arm pivot, length of shock movement, position of shock mount on swing arm, ECT). The only way to determine how your rear suspension moves is to accurately measure your bike and do the maths. If the rear shocks are lent over far enough then a reducing rate will always happen (I’ve only heard about it on motocross bikes).

More to come.

Mick.

Yes Nick as light as possible -- and I am keeping to Imperial units of lbs-weight and lbs-force because I am now free to do so...

-- rider and bike -- AS LIGHT AS POSSIBLE!!

-- but is there a racing Bantam weighing-in at 150lbs with its rider -- fully race shod -- at 140lbs??
I ask that because I NOW think that must be a minimum total weight of say: 290 lbs total.

Well I shall work to that...

And as I think of Mick´s treatise on suspension I think of a possible relationship of rider´s weight to bike weight where the rider´s slightly heavier weight might have an enormous influence on the way the lighter machine behaves under control whilst travelling thro´ a bend: the weight is being transferred back-to-front whilst braking and front-to-back whilst accelerating whilst the rider shifts his weight from off the centre-line of the bike -- pushes down, or pulls up on the clips-on and has differing pressure from his feet on the footrests. Maybe I am wrong but I imagine that when the rider´s CofG is offset from the bike´s centre-line CofG there is a moment (leverage) of force against the frame´s rigidity to
distrurb stability considerably...

I come back to the cat analogy where the animal keeps one bit still whilst heaving the other part a bit about its own CofG and then likening that to the total Bike/Rider as an entity, such that whilst the bike´s Cof G is in one plane the rider shifts his CofG into another plane and then moves the bike´s CofG to where he feels it should be according to how the radius of curvature changes ...

Much like the counter-steering, it is done automatically -- by instinct -- without conscious thought. That juggling is happening whilst the radius of curvature is probably changing which is why practice is needed to find the fastest way thro´a curve. Bends are not always perfect curves and the one that starts off wide at entry and tightens up on exit can be disaster and the ones that are tight first and go wide on exit are wonderful to accelerate out of...
In the early days I thought that braking and acceleration should not occur in the bend but having seen Peter Williams overtaking in a bend with the front wheel 3inches off the tarmac, whilst still at a considerable angle of lean, had me think otherwise.

So, back to being as light-as-possible it is the centrigugal force -- `CF´ -- the rider is having to contend with by the angle of lean necessary for equilibrium.

Just to show the difference of CF, centrifugal force, caused by difference of total weight, `Wt´ for one example of a momentary radius of curvature of 650ft and a speed of 72mph:-

From having a Wt of 290lbs (bike 150 and rider 140lbs(10 stone) -- CF is 154.5lbs.
to a disgusting Wt of 380lbs(Bike 180 and rider 200lbs(14.3stone)-- CF is 202.5lbs.

-- which I hope proves the point of "light as possible" -- the heavier bike & rider has 48lbs MORE of force to lean against and either increases the bike´s angle of lean or hangs his own weight -- low down -- inwards towards towards the centre of the curve....

-- but that has not yet addressed the resultant load on the tyre patch when the rider´s CofG is offset from the bike´s CofG (which is on the bike centreline, of course).

I`ll try and send Eddie the diagram (who will show it on here) I did for where the resultant force from offset CofGs for two different angles of lean counteracting the CF for A LARGER MACHINE THAN A BANTAM was used in error yet make the point -- I think!!?.

Cheers!

Changed To Colour

Thanks Eddie!
Sorry! went wrong on me again -- should be in colour, makes it easier to understand particularly when I want to put new numbers -- like Bantam weighs to it.
I´ve tried to send it again -- in colour Eddie,
Cheers1

Edited and changed to colour.


Eddie

Geewhizz!
Looks very lifelike in colour.

The centrigugal force for the Wt -- total at 438lbs-- was 233lbs -- compared with the lightest case I made earlier FOR A BANTAM of 154.5lbs.

So have a bit of sympathy for heavyweight bodies like that of my old buddy, The Wobblyman R.I.P, and I who had to struggle to get under 13 stone....!!

Thank you Eddie!
Sun is shining -- mandate is to go for a walk -- so talk about this Bullshine (really is ey, Nick!!) when I get back.
Cheers!

Re the coloured picture Eddie kindly edited and showed:-

The riders´ Centre of Gravity is shown as a white blob, which shows the Offset (I spoke of earlier) from the Bikes´ CofG -- so which is the more dangerous of the two styles shown -- and WHY?

Point about that resultant force as shown on the back wheel of red and blue machine: 535lbs red and 506lbs force blue respectively, is that the force is shared by both wheels -- the amount being according to whether accelerating or slowing down -- in the old days would be as static measurement because braking & accelerating whilst in the corner at angle-of lean was a NO-NO!

Great posts again people !

Mick Where does optimum wheelbase fit into frame calcs? If making a bantam chassis
today would you go for same spec's as an Aprilia/Honda road racer?

John Does cornering technicque matter that much? as long as you go round the corner as
fast as the fastest rider. And if you can't your technique is wrong??

No, I suppose you are right Mike -- style doesn´t matter as long as you get round the corner as fast as is possible it is to do it -- under all conditions given, of course...

Just that on my second Bantam, Icarus-2 with a very responsive steering set-up I could get round Brands faster than on Icarus-I. Which led me into digression which fitted in with Mick´s responses.

Since Mick was on about suspensions and frame rigidity I thought I´d bring in a question ( for myself as much as anyone on here) about how much body manipulation counts in handling and frame flexing with pressure on feet and clip-ons -- I seem to remember you breaking off a clip-on...
but that was just faulty brazing I suppose?

And when it come to the Resultant wheel load during cornering you´ll see from my drawing that the resultant wheel loading for the two angles of lean have red rider with 22%more loading than the total weight and blue has 17.8%. If at constant speed (no braking nor acceleration) and static weight distributiion is 50/50 then those Resultant loads will be shared equally back & front...

Which means that a rider who sits in the centre-line of the bike probably has the optimum-safest resultant loading for angles of lean ....

What I wanted to get to was something I know very little about and that is selection of suitable suspension units front & rear for racing Bantams relative to rider-weight/bike-weight including inertia effects of the different riding styles and had hoped for some response in that direction.

The little bit of knowledge I remember from many yonks ago is that the acceleration force due to corning at an angle of 45° -- with rider keeping his CofG on the centre-line of the bike -- was always 41.4% MORE -- in the resultant (the force in the centreline of the wheels) than the total weight

--and I used this criteria to select the rear units of both my Bantams -- since both were never fast enough to have more than 45° of lean...!??.

From what I have read ... I think, sometimes the selection might be more exoctic (and costly) than necessary for the job in hand...


Cheers!

Final tie-up (by me) on this subject and in answer to Nick´s comment on "...as light as possible..." and my bit about selection of my suspension units early on using the criteria of 45° lean angle *** ...

The centrifugal-force equals `g´-force at 45° angle of lean.

The LIGHTEST `Wt´, Total Weight of rider plus Bantam, I think, could be 290lbs.

When the total weight (of bike + rider) equals 290lbs (bike 150, + rider140lbs) the centrifugal force is 290lbs AT 45° angle of lean.
The resultant tyre contact force (at 45° lean) on the road is 410lbs. (1.41 X static weight).


The most heavy Wt, bike and Bantam is possibly 200bike + 200lbs rider????

When Wt = 400lbs & the Fcf 400lbs -- the resultant tyre contact force (at 45° lean) is 567lbs.

***It was a criteria given me by a mechanic at Gander & Gray, Stratford who sold Royal Enfields.

The same mechanic also sold me a pair of Girling rear units for my grass tracker ...

9 years later I used the same reasoning for my first Bantam -- and added a few pounds to the calcs for luck -- in thinking the angle of lean might be a bit more than 45 degrees!!

...so maybe that was a ´con´ but I have since done calcs to prove the theory that he held of it being satisfactory for a starter....

















[i]

Mick after re-reading your posts:-
I see you say do not go for multi-rate springs on the front or rear!

If building a frame now what rake/castor and offset would you go for?
and wheelbase come to that! I realise you would be going for 17" wheels
and rubber!

If I was to steppen my rake/castor angle say 27 to 25 and shorten fork offset say
70 to 50mm would it make much difference to front/rear axle weights?

Waiting for your next installment.....

Mike

Hi All, further ratings.

Once again bear with me, I don’t wish to teach Grandmothers to suck eggs. But not everyone has expertise in every aria and I hope to share my knowledge with those of you may not have considered such things.

This time on the length of the swing arm, the position of the swing arm pivot & the effect that that has on anti squat.

When power is applied to the rear wheel forward motion is imparted to the whole bike at its centre (the rear wheel spindle) and the power applied is parallel to the ground. If the rear wheel spindle & swing arm spindle are horizontal to each other then all the power imparts a forward motion, WRONG. The forward motion power is transmitted through the drive chain in a direct line from the top of the drive sprocket to the top of the rear wheel sprocket; at 90deg to each sprocket (so every time you change you’re gearing you change the equation (Robbie?) slightly). Now because the force of the rear wheel drive and the force applied to the drive chain are equal the resultant force will not be in this example what is desired because as power is transited to the rear wheel the rear spindle will be pulled up causing the rear end to squat. The most undesirable effect of this can be felt when trying to accelerate after passing the apex of a corner. As power is applied the rear wheel rises resulting in less weight on the rear tyre and the tyre starts to lose traction. The upshot of this is that the rider has to wait until they are upright before applying any meaningful power.

If the rear wheel spindle is lower than the swing arm pivot when power is applied the rear wheel is pushed down onto the track. This downward movement is obviously opposed by the upward force of the drive chain. The rear suspension of any bike is obviously a dynamic environment and it is possible to move from a position of anti squat to squat just by going over a bump. The relationship between where the gearbox sprocket to swing arm pivot and rear wheel spindle is as infinite as any triangle can be. All modern bikes are designed to have the gearbox sprocket as close to the swing arm pivot as possible and a swing arm as long as possible to prevent the possibility of moving from a state of anti squat to squat. The Bantam was not designed with any outer thought than it was a simple workhorse. Consequently we have a bike that isn’t well suited to racing as the gearbox sprocket is too far away from the swing arm pivot. It’s important to have dimensions that give enough anti squat to push the rear tyre into the track when accelerating out of a corner (in order to improve traction and therefore speed at the start of the strait) without causing too much jack up that may cause the suspension to top out and /or make the bike unstable (both unlikely due to the limited power that a Bantam produces, but must be taken into consideration).
To calculate where the forces of anti squat / squat act on your bike it’s necessary to draw your own bikes dimensions. Draw a line through the rear wheel spindle and swing arm spindle. Then draw a line along the drive chain from the top of the rear wheel sprocket to the top of the drive sprocket and continue it until it bisects the line from the swing arm. Where the lines intersect will be in front of the drive sprocket and this is where the forces of anti squat / squat act on the bike. If comparisons are made between the extremes of suspension movement you will be able to calculate where your bike changes from anti squat to squat, or maybe all anti squat / squat.
Whatever your calculations for your set up try to ensure that at the exit of a corner, however far you expect your rear suspension to be compressed that when you apply power anti squat is in effect


Everything I have posted on this and previous posts on handling are covered in by far greater detail in published papers and articles.

If anyone has any questions on anything I have posted on the handling subject I will do my best to point you in the right direction.

Mick.

Mike, in reply to your last post.
Linier rate springs give greater control of suspension movement when undulations are minor, as in race tracks. On the road the majority of the time the surface is as smooth as a race track, but occasionally a large pot hole needs to be traversed. This is when a duel or multy rate spring works.
In dealing with the pot hole the duel rate spring has compromised the linier rate spring when the road is relatively smooth.

On your second question , to many variables, and it totaly depends on what the rider needs.

Third point, yes front end weight will improve but not enough information. But its only a matter of maths. Firstly you need to know your wheel base and also how much each end weighs with the rider . From this information its posable to work out your c,of,g front to rear. Asuming only the front wheel spindle moves backwards then the percentage change can be calculated, but the percentage depends on how far front to back the c,of,g was originaly.

Mick.

Hi Mick on my bikes the swinging arm is angled down about 20*(guess) from
swinging arm pivot to rear spindle but the g/box sprocket is a long way
infront of the s/a pivot. The top chain run is parrallel to the floor (on
Cadwell gearing anyway). The chain run line and swinging arm line cross
before the g/box (between s/a pivot - g/box sprockrt).
The only downside I see is the extreems of chain tension!

How much more information is to come Mick??

Hi Mike.

I have obviously looked closely at your bikes in various race paddocks (as well as every other Bantam). Without taking measurements & doing the maths I will have trust to memory. Your bikes have swing arm angles that I think are close to if not correct (is that design or coincidence). On a gut feeling if any change is needed, maybe a little less angle to avoid jacking up the back end. Obviously a Bantam wasn’t designed to go racing so all we can do is the best compromise possible. Many motocross bikes have extremes of chain tension & one way they get around the problem is to have a fixed jockey roller directly under the swing arm pivot, this prevents the chain becoming to slack without sapping power as a sprung loaded one does.

I have now finished my planed ranting on handling & await comments, questions plus other people’s ideas on the subject.

Mick.

Last one on this subject --

Counter steering:-

On big heavy bikes I think it happens -- on Bantams I am not so sure because there´s unequal pressure by both feet, on the footrests, and by both hands on the clip-ons which is effected by the rider´s weight (rider as much as, or more than, the bike´s weight...!!) which creates lean... The rake-angle then causes the front wheel to turn into the bend although the amount of turn is small, relative the amount of lean. It always seemed to be so -- to me, when racing Bantams("riding" rather -- both my Bantams were either slow or unreliable!!).

With grass track & speedway (training) I definitely pushed hard on those wide bars and put a lot pushing weight onto the footrest opposite the foot that was stuck out ...!!

Mike Powell showed a pic of his right clip-on after it snapped off at Mallory. Was that fracture attributable to his controlling the road bump oscillations -- or was it a heaving on the clip-ons?

-- or probably, poor brazing??

I hope to hear a comment on the above?

I promise -- last from me on this subject:

-- I think that much body manipulation is responsible for the attitude (lean & steering) of the bike as there comes from counter.steering. In other words, à la the cat being thrown into the air upside-down; the cat rights itself by twisting one part relative the other part until its four feet are pointing downwards. This is much the same as the Bantam rider where the bike-&-rider become a single entity. Often with the rider weighing as much as -- or often more than -- the bike, forces the machine into attitudes it would not go by just counter-steering.

Colin you will find information/pics in the previous pages .. Mike