Frames & Handeling

Hi All Bantam enthusiasts.

An addendum to my earlier post’s on suspension. I have just had this web site recommended to me by Rob my suspension expert (he has recently discovered it himself). In it you will find the workings of front forks described in finer detail than I did and with diagrams that help your understanding.

I bring to your attention the Emulators kits for vintage bikes that they produce. With these fitted it is possible to make our old-fashioned forks operate like modern fully adjustable examples.

The theory is fully explained, so if there is not one suitable for your forks it is possible to make a pair from scratch if you have the machining capability.

The web site is http://racetech.com/ Race Tech parts are available from PDQ motorcycle Developments in the UK.

Keep it rubberside down.

Mick.

Mick search your memory banks... which is the best handling BANTAM you've raced? ? and why?

Regards Mike

Hi Mike.

I read your question yesterday and have been thinking about it.

It’s a difficult question to answer easily because not any one Bantam has done everything better than all the rest. Firstly I think it important to make the distinction between road holding and handling. A bike that has good road holding doesn’t mean that it has good handling and vice versa.

It goes without saying that what is ideal is both.

The Bantam that I am riding this year (Kindly loaned to me by Mike Redhead) has the best road holding of any Bantam that I have ridden so far. At the beginning of this year I spent a couple of months adjusting spring and damping rates at both ends to suit my weight so that it is now so well balanced that I can slide both together. But I find the steering a little slow for my liking.

The best handling Bantam I’ve ridden is Ted’s 175. When we were building it Ted was VEERY sceptical about the dimensions I was asking him to make. The original Bantam was a work horse, not intended for racing. Why do so many maintain the original dimensions. Ted and I raised the steering head by 2 inches but more importantly we raised the swing arm pivot by 4 inches. This steepens the steering head and razes the centre of gravity also improving anti squat. Steepening the steering makes turning into a corner quicker. Raising the centre of gravity makes changing direction easier. Improving anti squat makes corner exit better.

Both these bikes can still be improved, but so can all bikes as our understanding of suspension and handling gets better.

Mick.

Sounds good Mick -- unfortunately 44 years too late for JayBee,...

Lifting the C of G was a no-no in the late 60s but I am sure you are correct when it comes to a Bantam.... My feeling was that I liked to think I was chucking -- not only my own weight about -- but throwing the bike´s weight about as well....

...the latterr might be controversial!!!!??

Cheers!

Hi ,
Shortly after Mark`s return to Bantam racing , and after a Cadwell meeting where he managed to pick up a couple of wins , i asked him how the bike handled ?
He paused for a while , ermed a bit , and replied , " Didn`t think about it , i just went round the corners and if i needed to go faster i just leaned over a bit more,
no problem " !
Subjective , objective , dismissive , i have no idea but he never seemed to have any problems , just did the business !

Trevor

Wünderbar!!

Hi Mick i'm surprised you didn't mention SMS004 in your reply as when you rode it, you
were the most competative you have ever been on a bantam! Not sure if it was 'flawed'
in some respect with the 17" wheels and small off-set yokes? Though i recall you raced it
with 18" wheels and TOM yokes.. Must admit never rode 004 so cannot compare..

Regards Mike

Hi Mike.

004 handled quite well but had slow steering. The forks sometimes suffered from chatter but I have a better understanding now and believe I can cure it. The 17 inch rubber defiantly had more grip than the 18 inch rubber that I have been riding on this year .But I have never raced on a bike that changed from one to the other without making other changes as well, so I can’t comment on the effect on the handling. Ted’s 11bhp bike handled very well, but it never had enough power to tie itself into knots.
004 is the most powerful Bantam that I have ridden and it’s always easy to look good on a fast bike.

Mick

MCM (5 Sept) has a piece on Rossi’s new bike which also has a high centre of gravity.

The last paragraph says ‘A high Cof G therefore means greater agility and faster direction changes, but stability is worse. Being able to change this on a MotoGP bike helps in adapting it to the demands of different circuits.’ Can Mick’s bike be adjusted in any way or is this not allowed in the rules?

Hi Arther175.

No adjustments were built in but nothing in the rules forbides it.

Mick.

I recall a former Bantam club member thinking he was onto a good thing when his Dad bought him Kenny Roberts 250 Yam. He found the way Kenny Roberts had the bike set up to his liking made it un-ridable for him and he could have gone quicker on a standard TZ.
King Kenny had the handling very lively and it dropped into corners.
What suits some wont suit everybody.

Icraus-2 FLOPPED into corners -- Wobblyeman never got used to it, reckoned I was trying to murder him. I purposely had the steering centre-point a 1/2" under the Rolling Radius bi-section -- just to see what happened. As Mick said you steepen the steering head a bit and the front end gets more lively.
Like a lot of theory that seems back to front at first...
... that´s because if you have the steering head vertical (90° to road) and turn the bars the bike won´t flop over. If you take a normal 63° steering head angle and turn the bars the bike falls over --- unless you hold it up....

So, if the steering head is steepened the tendency to fall over should be less BUT that ain´t so and the question must be why does a steeper steering head angle make the front end feel twitchy (can´t think of any other suitable word?) ? and I reckon we are looking at the interaction of the front-wheel-weight and its CofG distance from its pivot -- front wheel torque, if you will? -- with the total weight (rider and bike) in the angle of lean.... Smaller wheel -- less of this twitchy torque***?????????????

OK! so that´s a load of crap but I proved the point to myself and if Icarus-2´s motor had had a bit more reliabilty...???? Funny thing was that although Colin Aldridge hated Icarus-2 he got round Llandow a good second faster a lap before the motor ate up its own piston and he neverr fell off it -- and I did not either.

That might prove some point or other about, 'How to race a motorbike on the road...' .

***pull it apart -- come on? and what about the shifting of the rider´s CofG ...???

I´m off to Blighty on Tuesday 18th Sept -- if Lufthansa allow it (been striking lately)
hope to be at Cadwell on Sunday 3oth...

Cheers!

If head angles can be altered, swing-arm pivot points raised, and frames re-tubed, why then can special frames not be built while keeping the same wheelbase?

Let's face it, crankcases apart, racing Bantam engines are now highly developed and bear little similarity with the original lump, but the frame is frankly agricultural; so why not ditch it? After all, the modern rooster racer is conceptually all about upon re-engineering a product that is now far removed from its humble beginnings and yet we see exotic cycle parts having to bolted onto a bedstead.

Howard Smith

undredseesee wrote:

If head angles can be altered, swing-arm pivot points raised, and frames re-tubed, why then can special frames not be built

RULE - 3. FRAME: Must retain Bantam main loop, but may be modified.

The wording of the rule as intended means you can alter the original tube not replace it.

The wheelbase is something you do want to change, as it will be hard to get a Bantam with road frame geometry to handle well under racing conditions. Unless that is you have a low power motor, like my old 11BHP bike.

Ted. What is the wheelbase of a Bantam - I'm guessing it is around 47/48"? And with reference to the regs is the swinging arm free, since altering the length of that component is the easiest way to adjust the wheelbase?

A short wheel base helps the bike to turn in quicker while a longer one contributes to rear trail and straight line stability. I believe a several riders steepen the head angle to quicken the steering, which will shorten the wheelbase. Am I correct in thinking 50" would be nearer the ideal?

Following on this theme, has anyone any thoughts or measurements on the amount of front end trail that helps a Bantam racer handle well.
I have just measured this on my D14S, zilch ! My D5 framed SSWC 125 with Bultaco TSS front end 22mm, my D7 framed 185 (B44 shallow offset yokes, B40 forks 50mm shorter than standard) 90mm. Guess which one turns in and handles better ?
I remember my old 125 (ex Chas James) when fitted with D14 Bushman forks, had shallower yoke offset, but was 2" too long, had to hang over front axle to get round sweeper at Pukekohe flat out.
Also remember the MP/SMS002 round Cadwell, ? higher centre of gravity, thicker fork yoke offset (Presumably TomMs) was great handling.