A Bantam for BEARS?

Hi John,

There's still a fair bit of clutch slipping going on at Bantam races especially at these 'orrible little 'bus stop' chicanes that seem to have popped up all over the place.

I'm interested in your comments on clutches - are you saying that when slipping the clutch at its optimum position if there were 21.3 ft lbs of torque going into the clutch there would be 10 ft lb transmitted to the gearbox?

Yes, it was the Hoop & Grapes in Farringdon Road - I don't think I ever fell asleep there - it might have been the stimulating conversation or maybe it just wasn't as comfortable as your garage floor

Mick

Thank goodness I got the pub name right, Mick...

When Mary was there after her having been to the ACU HQ it was like a thunderbolt had struck. Why, she demanded, did we just accept the ACU mandates without a murmur of protest about track safety...?? Mrs Styles, after one such visit to ACU-HQ, exploded flaming wrath across the table such that all seated were momentarilly dazzled with shock. Mary told us that the bunch of dozy old dunderheads at ACU HQ had asked, did she know how much straw bales cost? and she had told them, you lot´d put the cost of a few straw bales before the life of a rider...

Mary was good for the club.

Re slippimg clutches, Mick: not quite that wayround -- it has to be an average value of torque over a period of time. Maybe my reasoning is wrong and I´ll gladly submit it to examination. If you look at what happens with a fluid flywheel (on automatic transmissions) the effective torque being transmitted is at a radius arm, from the centre of the fluid cups to the centre of the shaft which is then a mulitiplication of the torque being transmitted by the gear ratio. That is the amount of torque at the gearbox output shaft less THE AMOUNT OF SLIPPAGE in the fluid flywheel.

What I have assumed is that the radius arm from the centre of the Bantam clutch inserts to the shaft centre is doing the same. ie. That dimension you gave me for clutch inserts to shaft centre, relative the gear radius I calculated as 2.13:1 which I am saying is the torque multiplication of the final drive ratio you happen to be in -- which, of course, does multiply the 11ftlbs (or whatever the torque happens to be) by the final gear ratio and then by the 2.13:1 but does not allow for the loss due to slippage which goes out in the form of heat generated... Fully coupled (hmmm??)(ie no clutch slip) the maximun Tractive Effort -- in 1st gear (Wheel´s rolling radius, 11.4" -- circumference 6 ft) ratio at 11.787:1 at 60mph is 167 lbs.force (using Trevor´s previous diagram info...) and at 40mph the TE is just over 92lbs.f ... It all looked nice and clear ABOVE 40mph -- so I asked myself what would the Tractive Effort be at the start and up to 40mph ?? Would riders do what I did so cruelly with Icarus-1? Keep the revs near max and feed in the clutch so that Icarus-1 would be almost competitive off the line.

I have made a couple of diagrams -- based on Trevor´s info -- showing what you mentioned about gear changes and shall have to dream up a diagram to explain the bit about slipping the clutch at the start -- for say 4 seconds -- at the start from standstill -- until fully coupled. Zero to Full Torque in 4 seconds and then average the tractive effort transmitted?

As I visualise it I had (with Icarus-1), maybe, about 8ftlbs of torque at the crankshaft x 2.13:1 clutch slip drive x 11.787 final push ratio at the rear wheel contact patch = 200.85lbs force??? Which when expressed as an AVERAGE force is 100.4lbs.f... and sounds about right when considering the acceleration off the line....

The whole trouble when racing is that whatever happens is felt in the subjective manner -- and sound estimates of speed and acceleration -- particularly acceleration -- cannot be objectively reported. I do know that with Icarus-1 it always felt as if it was accelerating better when I was slipping the clutch, which actually backs up my reasoning in calcs and diagram....

No one has yet given me a definite figure for how long the clutch is slipped during take off at the start?? 4 seconds is my guess with memory clouded a little by the passing decades.

Is there a simulation programme that gives calcs & diagrams for Tractive Effort (and Tractive Resistance) versus vehicle road speed? Would save me showing my diagrams and what I have dreamed up....??

And your speaking of slipping the clutch at chicanes reminds of the Cadwell Club Curcuit (circa `68 -- `74...) where at the hairpin I was changing down & braking whilst slipping the clutch and having the revs up at around 8,000...! Horrific!

I looked at that chicane by there -- last September -- and wondered what ever was in the minds of the organisers of a race track....??


If you read this yesterday you´ll see I have editted and added a bit.
Cheers!

Rodney Dummkopf.

Sorry! I should not have posted anything technical so close to Cadwell ...

Hope the weather was good today -- it was summer here! -- and that you all have a good one tomorrow.

CheerS!

Hi John

thanks for taking the time to explain your thinking on clutches - I need to read it a few more times before I can make a sensible reply!

The reason I was interested is that one of the simulators - Engmod2t - has a page where it calculates the time for a standing start 1/4 mile taking into account the torque curve for your engine as produced by the simulator, the mass of bike and rider, gear ratios, the rpm you slip the clutch at and MOI of flywheels etc.

It looked like a good way to compare engine output curves but a Bantam only does one standing start per race and most acceleration is done from some rpm above clutch slipping speed so I wrote a similar program where you can specify a torque curve ( by selecting an output file from the MOTA simulator) and a starting rpm. I was quite excited when you mentioned transmitting power through a clutch but realistically I doubt if I can incorporate it into the program in a reasonable amount of time.

Mick

Thanks Mick! Not giving up, although I might be derisively laughed at for doing so ... That is, the acceleration of the Bantams I have seen I estimate to be between 5ft/sec² and 12ft/sec². We know that there are two opposing factors against achieving the latter (12ft/sec²) -- a heavy bike-&-rider -- say, above 310lbs -- and a low torque output -- say below 11ftlbs with a final drive ratio of 11.5:1 -- or therabouts...

Theoretically -- that´s ignoring transmission inefficiency -- WITH the ABOVE values we´d have 11 X 11.5 ftlbs of torque at the rear wheel -- plus the total weight of rider plus bike at 310lbs weight. Assuming the Rolling Radius of the rear wheel is one foot then the Tractive Effort will be a 126.5 lbs push at the rear wheel tyre patch. Using the formula for acceleration `a´, where `f´ is the force (at the rear wheel) and `g ´ is the force of gravity, 32.2ft/sec² -- we have: F = ma which is --- 126.5 = 310/32.2 x a ....
Therefore a´ = (126.5 x 32.2)/310 ...
Therefore `a´ = 13.1ft/sec² .

Which is theoretical but when we take into consideration of transmision efficiency being around 90% then 13.1 x 0.9 = 11.8ft/sec² which `nice´figure fits and sort of suggests when one juggles -- like a Rodney Magician -- with numbers -- then Rodney can make anything mean what you like (and what Rodney likes).

That is backed up by the fact that below 6,000 - 7,000rpm the torque is much smaller than its max of 11ftlbs -- and seems never to be measured by Bantamites anyway -- which means to have max torque at the start the rider has to holding the revs at max-torque speed -- which according to most Bantam dyno curves is somewhere around 8,000 -- 10,000 rpm.

So the slightly heavier blokes-&-bikes must have more than 11ftlbs of torque at their crankshafts to achieve the magic 12ft/sec² ... Might it be even 15ftlbs??

But back to the clutch slip idea of it being a torque multiplier. By earlier reasoning I arrived at the ratio, 2.13:1 thro´ a slipping clutch (dimension of friction insert to shaft centre), and then said that because it is slipping it will have to be an average torque transmitted, added to which is the amount of heat generated being lost to transmission...

BUT! there are several clutch plates driving on their respective `both-sides´ -- is it three plates on a Bantam??? -- so we are now into say, 6 surface areas sharing the drive of the crankshaft torque which is where I stopped this daft exercise....

Last word (Really!!). Say the 1st gear ratio is 11.7:1 and the torque at the crankshaft is 11ftlbs at something like 9,000rpm, with the rider slipping the clutch from standstill (with 9,000rpm on his rev-counter) then that Tractive Effort could be more than 126.5lbs during clutch slip -- it could be over 200lbs force -- COULDN`T IT??

I await the brickbats with great enthusiasm.

Cheers!

Rodney D....

Rodney Dümmkopf!

Had a rethink about this palaver I started with Sticky ...

"A Bantam for Bears" was not the right topic to be on for this technical Bullshine (as Nick B would say...) -- it belongs in Trevor´s "Phenomena Plus" which went to a graph Trevor displayed,"Power Through The Gears" where -- I think -- some confusion developed about the gears and the `jump´ between bottom and second. Then there was the interesting point on the graph that at 30mph there´d be about 6 horsepower available ... which I questioned and received no answer that then led me into clutch slipping from the standing start.

So, having sounded Sticky out for a simulation calculation programme for, "Tractive Effort (and Tractive Resistance) VERSUS Road Speed" -- with again no response (except it would take too long...much appreciated, I understand) -- I did a little exercise (for my old Rodney D... brain) involving the transpostion of "Power Thro´The Gears" horsepowers into torque and then into Tractive Effort by using gear ratios that Sticky published earlier, along with 14 x 48 (gearbox to rearwheel sprocket ratio) which shows very clearly that from 40 to 105 mph (clutch fully coupled, of course) the amount of force available -- (tractive effort minus tractive rseistance) for acceleration. This graph shows beautifully that the force available for acceleration diminishes thro´out each gear -- from bottom to top -- and that with a high-revving engine which `revs-on´ it is a mistake to change into a higher gear too soon.

Of course, you all know that by Intuition & FEEL -- and this is a `numbers proof´ only -- and hardly worth the bother -- except that it kept my Rodney brain from falling off its gimbals into the sump....

Eddie will publish it and if so I must apologise to Trevor for manipulating his lovely diagram.

Sorry this is so long!!

Cheers!

Have not forgotten your questions John

am modifying my program to show the results graphically and in your choice of units

Hope to post some graphs but it may take a little while.

Loved the story about Mary Styles!

As you said, she was very good for the Bantam Club

Mick

I think its Mary style's you are talking about,? my Godness -"Man" good for the Bantams" !!!! she was one of the pioneer's.

I have seen "as a boy", grown men tremble when ever you spoke her name in the Paddock,! but she was an absolute darling and as polite as can be, "to me", I was only seventeen at the time, so maybe she has changed, ! my grany use to say, a lepod never changes hear spots, or "he who dares" will never win, especially against a woman, only a fool would stand against such oddS, and Im sure she absolutly does not suffer fools. LOL

regards Derek

At a committe meeting in the H&G Mary said, "You have got to do it JayBee, speak to him about his overdue subs and what he owes for an entry. He´s entered for Snetterton in the Production Class race. Can´t miss him -- he´s a big bloke."

He was lifting his big Proddy bike out of his van with no ramp. I shivered as I said, "Mary Styles would like to speak with you..."

Apparently he paid up on the spot.

GOD days they were -- Good Old Days!

Cheers!