Attn Mick Potter. Frames and handling..

This is a post in which I'd like to as Mick Potter about Frames. Perhaps we'll all learn something.

I've run basically the same Mick Scutt designed frame(001/003) for 26 years with few changes. When Mick Potter raced 004 it was converted to 17" wheels/tyres and slab fork yokes - bringing the front wheel further back(getting rider weight further forward?) and I think shorter rear shocks. Now knowing Mick P all this was done for a reason. Mick can you give us your thoughts on ideal head angle/ wheelbse and the like and why?

Cheers Mike

Excuse me butting in:

There´s an ancient book, published by Osprey ISBN 0-85045-560-X
in 1984, called, "Motorcycle Chassis Design: theory & practice" by Tony Foale & Vic Willoughby which contains a lot about handling but was out too late to be of any use in my experimental efforts with Icarus-2, because I left England in 1974.

Mike! perhaps Mr Potter has missed your message -- there´s a lot of traffic on here lately -- and if one comes on and only looks at the latest Postings then the ons that have gone earlier are probbaly missed.
Send him a personal e-Mail ...
Cheers!
JayBee.

Hi John

I agree with you, "having worked with walter Griffiths and David Spate at Welworthy, AEPP/ GOETZE, I am aware of the aspect your talking about censored in a piston. I dont understand most of it, so sadly cannot contribute a meaningful responce, although I would love to be able to.

Anyway keep them comming.
regards

Derek Betts

Hi Mike, and all those who want to hear my meandering theries on frame geometary.
I will endevor to answer you but with a wife & 2 kids hogging the computer all the time I cant log on every day.
Every one tries to make thier bike as fast as possable and we may be reaching the limmits with the fastest bikes in the club.
The possability of improving performance by improving handeling and more importantly grip in the corners to improve lap times seems to me to have been neglected by most but not all riders.
We are probely all aware that we can out corner every outher class in the B,H,R,C but improvments can still be made.

For those of you who dont know me i think I need to explain that Ihave been a mechanic in the motorcycle trade since 1987 and have ridden more bikes than the average racer, plus I understand the evolution of bikes from then to now.

It is VERY,VERY important to understand that how you like a bike to feel & handle is not the same as everybody, that is why at the higher levals of racing there are so many adjustments avalable to the riders. Do not just ride the bike to get the best out of it as it is now but think about what is wrong with the way it feels / handels now. Verry rairly will you be totaly satisfied.

Ted & I decided to use 17 inch tyres a few years ago after I reseched thier dimensions. The diameter of the 17 tyre is only 3mm smaller than it's 18 inch equivelent but because of its profile and the smaller rim size it has a much larger contact footprint when on the edge of the tyre. Also I belive ( and I have no evidence for the folowing statment ) that NO develipment is going into 18 inch tyre carceses whearas 17inch carcases are currant and still being deeloped. Dunlop or any other tyre manufactuer will use the same compound of rubber for all thier club race tyres. Therfore the 18 inch tyres will improve as compounds improve but 17 inch tyres will improve as both carcases and compounds improve. Personaly I find that the 17 tyres gives me more feel in the corners and when they start to slide they are more controlable. Everybody who has ridden one of Ted's bikes with the 17 inch tyres has been impresed with the feel and performance of the tyres.

The forks that most of us are using are almost as basic as they get. They can be improved but only up to a point. Back in the 1980's many top teams of the time tried anty,dive forks because they din't understand why they dived so much under brakeing. Back then The fork spring was as long as it could be, this produced a spring that was as consistnt as it coud be. Now forks have very short springs with spacers to make up the length, the efect of this is that as the forks compreses spring rate increases much faster,stopping the forks diving so far and also increasing the weight onto front end, so improving front end grip.

Most bantams have a rake that by road bike standerds is quite normal. A rake of 26/25 deg seames to be quite average. It is more important to get the trail right. Most Bantams that are racing at the moment (by my observations , not by mesurment) have too much rake and compensate with to much fork ofset to get the corect (or near) trail. I think 22/23 deg is optimum this will produce a fork yolks that have hardly any ofset to the stearing stem. If you dout me go and look at any moden sports bike. If you wish to look throgh the internet at the last honda rs 125 you will find a stearing head angle of 21 deg.

Time for me to give this a rest for a while. next fork oil grades, air gaps, swing arm pivot points, swing arm/wheal base lengths, c, o,g, ect.

Excellent Mick can't wait for the next installment - but give it a week to let all that
info sink in!

My head angle is 27 and weight of my bike is bias to the rear so quite 'old hat' but
of course I am use to it? How did you cope with 004 as it had a 27 head angle and
shorter rear shocks? (est 28 head angle).

The only bantam with 17" wheels i've raced was Teds a/c 125 great until I suffered
my only ever highside! (cause footrest grounding- suspension unloading)

Cheers

Hi Mike

If I remember corectly when you rode Ted's 125 we were trying out Pirrely's. Ifound they gripped reasonably well but let go without worning. We changed to Dunlops the year after, 10 percent more grip, 80 percent beter feedback.

Really interesting and a great post !

Thank you Mick

Mick S

Wobblyman (Colin Aldridge) -- when he was working in the Midlands -- called me to join him for a lecture by Ken Sprayson on frame building and setting up the bike for personal, optimum handling.
I got hooked on -- if I remember aright -- "Centre point steering" -- with precession being the initiator of lean when entering a bend. One needs a diagram to explain this phenomena and I cannot do pictures on my paraffin-powered PC so could someone out there show this please --

-- I know James Cook was attracted by this subject when James was looking after this site -- SO James? would you be so kind as to show the line thro´the steering head bisecting the rolling-radius at above, below or spot-on the centre-point to effect the `steering the wrong way -- a bit -- to initiate´ the lean....

As I have mentioned before on here -- during experimentation I got Icarus-2 to Flop into corners and be hard to pull upright, which had Wobbleyman get quite angry at Llandow and accuse me of attempted murder....

Cheers!

John,

I think you are referring to counter steering, a reasonable overview can be found at

https://www.youtube.com/watch?v=C848R9xWrjc

If the 2012 engine and gearbox set up works to design I am hoping to have a reasonably competitive 175. Then the next great challenge will be to sort out the handling, but so far the chassis has never felt under pressure.

I have to confess that despite my best intentions I have never measured the static sag on my race bike or looked at resolving the excessive dive on the front end beyond heavier oil. I do think many aspects of a bantam could be improved for example Rake, trail , wheelbase and swingarm length and pivot point just for starters.

James

Mick ref 004 when you rode it (very well) I assume you didn't change the head angle on the frame, so what was your reasoning
of changing rake/trail with the slab yolks? I realise the change of forks from 70's to 90's - better. Also shorter rear shocks would increase head angle ???

Yours in puzzlement Mike

ps did run it on 18's ever ?

Hi mike & all

This is my second try to reply to your last post i hope this one dos'nt get lost in the ether.

When Ted perchased 004 from Kieth Woods, it was still on 18 inch rimms, riding it I found it was very slow stearing compaired to my expectations, but very stabel in a straight line. There were also some handeling issues & a serious lack of feel / breaking efect compared to the imput to the front brake lever.

During the off season Ted fitted the same forks as he fitted to the bike we made our comeback to Bantam racing (honda nsr125) also fitting 17 inch rimms. The new brake sorted all my breaking problems. The new yolks had less offset than the ones origanaly fitted so to compensate we droped them down the fork legs. This had the efect (And hear I think it appropriet to remind people that everything to do with chasis dimensions is only the best compremise you can come up with) steapening the stearing head angle and lenglhening the trail slighly. not nesiseraly what we were aiming for but the best compremise without changing the stearing head angle. The changes made transformed the bike. it was still slow stearing & stabel (I have always prfered a bike that is quick stearing but that dosn't mean it has to be flighty / unstable) but the handeling was greatly improved. The way a bike feels under you is verry personal but I think it felt better because, No1/ fork tube diameter increase (a bigger tube resists flexs better). No2/ shorter distance between fork slider & bottom yolk (less flexs because of reduced distance between two parts that cant move.) No3/ greater overlap betweenfork slider & fork tube(the larger the overlap, the smaller the chance of manufacturing error). No4/ more modden fork with better damping controll.

I have hade several people ask me about front end patter. There can be several causes of this. No1/ suspensionn bottoming out under breaking. No2/Fork/Frame flexs when on rippled serfaces. No3/ uncontroled dammping. Plus many more, I AM NOT A SUSPENSION EXPERT but I can give you the best of my experiance.
The diferance between over sprung & over damped is extreamly dificult to understand. It is important to remember that a spring is what it is you can't change its spring rate by adding preload to it. to find if you are over sprung first attatch a cable tye to your fork leg. Then go and do the hardest breaking you can do, compare the amount of travel of the cable tye to the total travel avalable to your forks, you need to be looking for between 10/15 mm of reserve travel anything less too soft, anything more too hard.The amount of travel in your forks will need to be between 115mm max to 90mm min. Next you need to find your static sag, do this by bouncing the forkes up and down several times and mesuring the diferance between where they come to a to a rest and totaly toped out, the differance about 15/ 20mm. There will be a missmach. You have 2 springs in your forks, the steel spring and an air spring. Only use a linnier spring for racing not a duel or multy coil spring these are only for road/ ofroad use.As I said erlier you cant change the rate of your spring but that only applies to your steel spring, you can compensate (up to a point with your air spring). When filling your forks with oil pump them several times t expell air & then compleatly comress them & messure the level of the oil without the springs in place (it is VERRY important that both forks have the same air gap). A steel spring is linier, a air spring is explonential therfofe you cant compensate for a week spring at the start of its stoke & vise versa. If your steel spring is to week reduce the air cap , if to strong increase the air gap. Only so much comramise is avalable, if you cant hit a happy medium then you will need to change your steel springs(just like all the top teams). After getting your spring rates correct then it's time to look at dammping rates (dont be afraid of mixing diferant fork oils from the same manufacturer to come up with a custom mix if thats what you feel you need (I do it all the time at work to fullfill the needs/ wants of customers). when you do this you may find you need to reevaluate your spring rates( who said racing at the top level was easy).

hope this answers your questions Mike.

outher rammbellings to follow

Mick

Hi all.

To continue my rammbelings.

Back in the mists of time telescopic forks replaced girder forks (I'm not going into the benifits or drawbacks of of differant types of front suspension. We have to use what we can get hold of & you can make up your own mind about witch is best). Damping was nearly non existant. Then along came damping rods, followed by fulley adjustable fork legs. Early forks used engine oil, even some of the forks withh damper rods. As fork develipment improved specifications for oil changed to 20w oil then 10w. As forks moved into the fulley adjustable forks oil specs changed to 5w, and now unspecified oils that are over £25.00 per lt. How is this infomation helping me I can almost hear you saying to your selvs. The reason that fork oil has become thinner & thiner is because it has taken engenears a long time to to come to the logical conclusion that firstly that oil thins with heat & second oil thins with age degredation. Modden forks use verry thin oil because when you start with a verry thin oil it cant get much much thinner due to heat or age. Our fork oil will not get old but it has to work in the cool of sping/ autum & the heat of summer. Most of us will be using forkes with damper rods, these will be using realitivly thick oil meaning in summer they may be under damped and in the spring/ autum over damped (for those of you who have adjustable damping & you know who you are (Ian) you must make the most use of what you have). It may be posable to modify existing damper rods to give a more consistant damping efect over the whole season. The damper rod has large holes at the lower end this is the compession damping controll. At the top there are usuely 2 smaller holes slightly staggered that are the rebound dammping controll. Its posable to weld up these holes and replace them with smaller holes, thus replicating more modden forkes (the theary sounds a lot easier than the practice because unlike adjustabe forks you cant undo what what you have done untill the next meating /prctice). If you decide to modify your damper reamember that only 0.1mm can make a large differance and only a shock dyno can acuratly mesure the differance.

More rambeling comming soon

Mick

Mick

many thanks for posting all this - really interesting stuff


especially for those like me that know nothing on the innards of modern forks.

How do you rate the nsr forks you use ?


Mick

Hi Mick

The NSR forks are obvisly not the best forks that can be obtaind but of the ones that are readaly avalable & sutable for a Bantam I consider them one of the best choses. They have a tube diameter larger than earlier forkes making them stonger with added advantage of coming with a stonking brake. They obviesly come from a bike that is heavier than a Bantam so we hade to reduce the preeload, we also changed the oil from 10w to 15w. Then we changed the oil level until I was happey with the result.

There are many forks that are sutable but most are to long as road bikes tend to be higher than Bantams. Most road bikes have suspension travel that is longer than we need on the track so it's posable to modify them to increase the overlap of the tubes & therfore reduce thier length.

Hope this information helps.

Mick

I agree with the point that for road racing the suspension travel for road racing
is not that required for normal public roads and particularly far different for Off-Road racing. I seem to remember shortening the standard (uppside-down) Bantam front forks by over an inch and had AMC slim units on the rear. I don´t think Icarus-1 Bantam ever went fast enough to ever feel uncomfortable with its handling... Icarus-2 was quite different but that was attributable to `experimentation´ ...

I lived thro´ the period of going from girder thro´ Teledraulic and unbelieveable rear suspensions and the worst ever piece of crud engineering (in my experience) was the rear suspension of the ´52 RE Bullet Scrambler. Top tube entering bottom tube with a Snubber valve*** and a couple of rubbers seals... Cheap & nasty!!

At my second Scramble both rear units had the top tubes stuck in the bottom tubes and pulled out of their (upper) brazed mountings -- causing the springs to go into terrible distorted shapes and finally loose all semblance of spring thus locking the rear wheel against the mudguard and putting the rider (poor old JayBee) on his head!. In the process of bottoming, the lower tubes (on the outside) collided with their respective mountings to burr the ends and grip the bottom tubes hard enough to tear them out of their brazings...

There is no doubt that on the road these units would be fairly reliable and I tell this because BAD engineering should always be exposed -- RE should never have sold the BULLET AS A SCRAMBLER --- and there was/IS no doubt that AMC made the BIGGEST strides with suspensions at that time! And there is no doubt that Motor Cycle Racing was responsible for the fantastic modern suspenions we have today.


***plate with a small hole to act as damper when oil was forced thro´ the hole!!

john bass wrote:

the worst ever piece of crud engineering (in my experience) was the rear suspension of the ´52 RE Bullet Scrambler. Top tube entering bottom tube with a Snubber valve*** and a couple of rubbers seals... Cheap & nasty!!

I thought you were a man of nostalgia John !
Obviously forgotten the sprung hub Triumphs !

Good Old Days times tho´ weren´t they Ned!

Sprung hub reminded of the Enfield cush drive in the rear hub-- what a disaster!! Three moving vanes pressing against hard rubber segments caused the rubber to swell out against the thin plates & holding bolts which snapped because of the excessive strain. Bill Lomas had some severe criticism of RE´s Chief Engineer with justification....

Yet the Bullet is still being made -- in India, of course -- it is to wonder why the Bantam production was left to die, or that some German hadn´t claimed it back -- it was DKW to start with....

fantastic posts on the fram and forks, This information is so helpful, I would have just picked up a pair from an auto jumble, that looked about the right size,

but blimey now I'm going armed with all this new information, it's so helpfull and much appreciated.

well done "Mick Potter".

best and kind regards. TERRY

Hi Mick
hoping to get a new frame done over the winter going to work my way back from the trial and play with head angle what about wheel base ???

You can calculate the trail fairly easily and try different fork offset distances or head angles in the formula to see what happens

If you use spreadsheets see the image below for a simple example to set it up (distances shown in mm).
This was done in Open Office but should be straight forward to do the same in Excel or any other spreadsheet.

Head angle can be measured either from the vertical or the horizontal. I prefer measuring from the horizontal and thats the way the formula works but you can calculate one from the other by subtracting what you have from 90. It is often quoted from the vertical eg 24 degrees from vertical which is 66 deg from the horizontal.

if you want to use a calculator instead of a spreadsheet then set it to degrees and -

1) multiply the cosine of the head angle by the wheel radius

2) subtract the fork offset distance

3) divide by the sine of the head angle

these 3 steps assume the wheel radius and fork offset are in the same units and gives the trail length in the same units.

Mick

Hi robbie.

My next post will anwser your question butyou will have to wait until after the AGM.

ok mick see you there

Hi all.

An addendum to my last post as some important information was left out. Damping increases with the square of the speed of suspension travel (Not the amount of travel). On the compression stroke some modern road suspension units have low speed & high speed damping settings. Race units now have low, medium &high speed adjustments. If when riding your bike you don’t notice your suspension then it’s not far from what you need. If you can feel your suspension (either to soft or hard) when at the limit of cornering then you will need to make adjustments. Only change by one fork oil grade at a time whatever your decision about to hard or soft, After you have changed the oil you will be tempted to bounce up & down on the new settings. You will feel almost no difference because the speed at which you can move them is only low speed. The only way to compare is to ride the bike fast to compare it over fast bumps. So don’t be tempted to change more than one grade at a time because you can’t tell the difference when bouncing the bike.

Now how the centre of gravity affects the handling of your bike. I will assume that for this example that your suspension has perfect front to back balance.
Firstly the height of the c,of,g If you have been lucky enough to have ridden many of the sports bikes that have been used for world superbikes or world supersport in the last 15 years, you will have noticed the riding position getting steadily higher. The reason for this is because as power levels have increased the need for lager rear tyres in order to transmit that power to the ground has meant larger and larger tyres. When a bike with large tyres is leant over the contact patch with the ground moves from the centre line to the side by half the width of the tyre. The real angle of lean passes from the contact patch through the c,of,g. The perceived angle of lean passes through the centre line of the bike. This means that for any given corner a bike with wide tyres will appear to be leaning over further than a bike with narrow tyres for the same speed even though the real angle of lean will be the same. If the c,of,g is moved up the difference between the real angle and the perceived angle decreases. This is why rider height has increased as the rider is a large proportion of the c,of,g. The rider also has the option of moving their body weight to the side of the bike moving the c,of,g into the corner. If you watch moto GP I’m shore you will have noticed that all riders will be hanging of the side of the bike. Compare then the riders on the 125’s, they hang of less because they have narrower tyres and don’t need to move as much to have the same affect. The problem with having a high c,of,g is that quick changes of direction become a problem as the c,of,g has to move in a larger arc. Therefore a compromise between speed of changing direction and angle of lean in the corners has to be arrived at that you are happy with. If you can master hanging of the side of your bike whilst cornering then you can have the best of both scenarios. If you are like me and can’t get on with hanging of the side of the bike then you will be at a disadvantage, but still probably quicker than any other class the club.

Secondly the position of c,of,g front to back. Consider a bike at the limit in a corner on a neutral throttle. If like most Bantams you are using the same size tyres front & back then the amount of grip needed will be the same. The amount of grip is proportional to the weight on the tyre (assuming both tyres are identical). Therefore both tyres need to have the same weight on them. If there is a rearward weight bias then the front will let go before the rear (I have lost the front end many more times than the rear). One problem with the Bantam is that it is very short in standard form. The result is a quite pronounced rearward weight bias. I can’t think of a single case wear a bike is heavier than the rider (including me at 63 KG). Moving movable parts on the bike towards the front will help the weight balance but I don’t think that it will be enough. Some bikes have had their wheelbases lengthened by stretching the frame, at least one (maybe more) by lengthening the swing arm. Lengthening the swing arm I think is a far better solution for many reasons. The original swing arm is very short. The amount the wheelbase needs to be stretched depends on the person riding the bike. I estimate somewhere between 2 to 4 inch (I assume the rider is as far forward as they can get).To determine the weight at each end it’s necessary to weigh the bike with the rider on the bike in the position they adopt though a corner. As the wheelbase is extended the weight is transferred to the front. If you have a rear tyre that is larger than the front then it will have a larger contact patch consequently more grip so more weight will need to be moved forward.
Please now consider the effects on a bike when it’s not on a neutral throttle when cornering on the limit. If you enter a corner on the power and shut off the weight will be transferred forward. The result is the front end will wash out and you end up with a low side crash. If you enter the corner trailing too much front brake then the rear end will be light. The result is a high side crash. Exiting the corner and opening the throttle depending on the balance of your bike you may lose either end. These effects are explained by pitch and yaw. Pitch and yaw (front & back rock) is affected by c,of,g. If the mass of the bike is close to the c,of,g then pitch & yaw is reduced this is why all the manufacturers are concentrating on mass centralisation.

More machinations to come.

Mick.

Neat-oh Mick!
I often wondered why I often never exitted a corner -- cornerS I should say -- !!

I liked the bit about big tyres/wheels on GP bikes -- had my mind flitting back to the bit about Rossi being upset with the Ducati having too-light flywheels -- or was it too heavy flywheels?? -- No! it was light -- because he gassed it too hard, flipped off Stoner and himself in a bend.

What I am getting at, is confirmation of my point: the wheels are acting as flywheels and their effect -- bigger and heavier-- dwarf that of the engine´s rotating parts effect -- and it was TORQUE in Rossi´s case -- about 45ftlbs of it -- Poor young Hedgehog doesn´t know the difference...!!

Point though Mick is that it takes a fast rider who can FEEL what-is-wrong for the reason to be put right. I remember they used to say of Mike Hailwood that he could take a complete unknown machine out onto the track and within a few laps tell that either the front or rear suspension were wrong....

Cheers!