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Terry Andrews
United Kingdom
546 Posts |
 Posted - 27/03/2008 : 18:24:02
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Hi Everyone,
Back in the 1980’s I did a lot of research on P types. At one time I had all the P chassis files at home. Reading the chassis files was VERY interesting. In the Year Book article I mentioned that some cars were fitted with the “Colemore Clutch Stop”. At the time nobody really came up with what I thought was as satisfactory answer. The consensus at the time was a different style clutch stop as fitted to the firewall. If anyone can come up with the DEFINITIVE answer I would be very interested to know!!!!!!
Regards…. Terry
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Bob Stringfield
United Kingdom
854 Posts |
Posted - 28/03/2008 : 09:01:57
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For Colmore Clutch Stop - See the 'In defence of......' post.
Bob. |
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David Allison
United Kingdom
665 Posts |
Posted - 28/03/2008 : 11:02:25
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What is wrong with snatch changes?
I have seen a "Colmore Stop" on a motorbike too but found the gearchange easier without it.
I think the idea is a good one but in reality the reason for needing the fix was that the gear ratios were not really suited to the duty.
On older motor cycles the hand gearchange could be tricky to operate at high speeds and I can see a need for the "Colmore Stop" - but as the bike designs improved the need for a fix was gone.
The device is mentioned in a Morgan sales pamphlet in 1924 as an aid to selecting reverse gear on the "Family Model" three wheelers.
This makes a lot of sense because they had a relatively small amount of space for a gearbox and the speed difference between forward and reverse gears would be huge and cause problems to the chain drive.
Many vintage (pre 1930) sports and saloon cars have truly terrible gearboxes and gearchages - making quiet changes a thing of rarity and in some cases impossibility and I can see that in competition use the device could be usefull.
I know several trials exponents who use a snatch change - this is tricky to master but when perfected - is seemless and non damaging.
Steven Dear was capable of ultra quick seemless shifts in all 5 gears - even selecting reverse while travelling forwards with only a minor snick to be heard.
The secret is not to try too hard and feel the gear engauge - easier said than done it has to be said.
I recently drove a standard PA for the first time in 20 years and it took me quite a while to pluck up courage to try what Steven and my Dad had taught me.
When I went back to snatch changes I actually ground less gears than by conventional double de-clutching - eventually as my confidence grew I even achieved the seemles changes.
The best way to explain a snatch changes is as follows.
First of all get used to the engine revs required for each gear.
Use the clutch to disengauge the gear,lift off the throttle, then feel the lever into position - as it starts to engauge firmly push (or pull) the lever and apply the throttle.
It is vital not to either hurry the gearbox - or be too slow and also to match the engine revs to the road speed for each gear.
The technique is similar to a good double de-clutch change - you just dont use the clutch - simple!
Err No - It aint simple - but great fun when you get it right.
Probably best to stick to double de-clutching thinking about it!
The F type crash box has much larger gears making the snatch change much easier - indeed you often perform better changes without the clutch than with.
C types and J4's also have a similar box (with closer ratios I think?) and a lighter car which makes it even easier.
Unfortunately these gearboxes are like hens teeth and coveted by AC owners, C type and J4 replica builders (amongst others).
It is difficult in these modern times with synchro gearboxes, automatics and paddle shifts to get ones brain in gear when driving with a crash box again.
I am spoiled because the NA has a pre-selector and I guess that is the reason why so many people modify their cars and fit the heavier more complicated gearboxes in place of the crash box.
That said the crash boxes are very rewarding to drive when everything is right and I doubt if a "Colmore Stop" is really suitable for a MMM car.
I think that the engine and gearbox are probably rotating too fast for the device to be of any benefit - although this is a personal view.
Some of the vintage specials have slow rotating engines with heavy flywheels and this might be more suited to the device.
I dont know of any MMM cars fitted with a "Colmore Stop" - perhaps the owner of such a car might like to own up (although I doubt it as they are frowned upon by the VSCC)?
I would also be interested to hear from John Reid and Mike Rushton on the trials front in terms of if they think that "Colmore Stop" is of any benefit?
The A Series engined MG Midgets had no synchro on 1st and reverse gears and most owners would select 2nd before reverse to slow the gearbox down.
Changing into first gear at speed from second was also done using the snatch change technique.
Hope this answers the query.
Regards David |
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Gordon
United Kingdom
692 Posts |
Posted - 28/03/2008 : 11:23:37
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I had assumed that the clutch stop was only there for upward gear changes and a friction device slowed the clutch shaft when the pedal was fully depressed and not as Cymber reported in his post re a RR diesel engine. I understand the method advocated by David and it is very satisfying when you get it right. An excellent reason for driving your MMM every day!
Gordon |
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ags
United Kingdom
275 Posts |
Posted - 28/03/2008 : 14:00:39
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Hi All,
Once again I shall chime in after David with some confirmation and amplification. I personally would never consider using any thing but the Wolseley crash box on a PB. When you consider the weight, size and complication of the alternative there is really no doubt, for me, which is the optimum choice, and even treated as I shall describe the box never at all gave me any mechanical problems, though the magnet in the drain plug collected a fair amount of swarf each season. The gear box was not unduly noisy either, (another advantage of straight SAE 140 oil).
As David says, the crash box in the right hands is capable of extremely fast changes possibly because of the lowish momentum of the small parts within the box. On the road I always used to double declutch, and one of my happiest moments was when approaching a roundabout in a hurry I managed a completely silent down change from third to first across that relatively large gap in ratios. However on the track I used to snatch when going up, particularly from first to second because of the straight gear lever travel. The dog clutches made third to top fast by any technique, but the slight dog leg second to third did need more respect. Equally the bigger 2 - 3 gap on a PA box might take a bit more care.
The technique is all a matter of timing and confidence. The best explanation which I have read was a quote somewhere from Freddy Dixon, who said that he waited until the moment was right and then shouted, "Gerrroop you booger", to himself as he moved the lever. My version of this was to get the revs in first to where I wanted them, generally 5800 - 5900, ease back the throttle and kick at the clutch whilst moving the gear lever absolutely as fast as I could manage back into second. Easing the throttle was nothing like a complete lift, but just to take the driving load off the gears so that they did not stick on disengagement. (As an aside, driving home from Prescott to Bucks in a 1960 Mini with absolutely no operable clutch, taught me the need to get that part right). I have no clear idea as to whether the clutch was engaged or disengaged at the moment when the gears met, as it was only a brief kick, and my gear stick movement speed indicated that my timing phrase should be "Whoopee". This speed was helped by my lightened flywheel, done to the Cream Cracker "soup plate" design down to 11.5 - 12 lbs though the counterbalanced billet crank was heavier than standard. If I hesitated, either disengaging or engaging the gears, I was lost and ended up in graunching confusion.
If you get it right then I doubt if the change is any slower than a preselector. The only advantage of the latter might be that one could keep more power on through the change, though I have to say that I have only driven a preselector once, a Riley Merlin in the Sixties with the owner beside me so that I was not going to be too adventurous.
Just reminiscences from,
Andrew Smith MMM571 |
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David Allison
United Kingdom
665 Posts |
Posted - 28/03/2008 : 15:45:13
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Just as an aside - reading what Andrew very ably added to my earlier notes - NEVER full throttle change with a pre-selector!
The ENV 75 in a blown 4 or 6 cylinder MG is running beyond its maximum capacity and full throttle changes get the internals hot and bothered.
They dont need any excuse to get hot and bothered and a full throttle change gives the box exactly that excuse!
The heat build up in the indirect gears is enormous and I was always told - full throttle change and you pay the bills and as a result I dont full throttle change!
I have seen the internals of an ENV 75 racing box that had spent 1 day at Silverstone with full throttle changes and its insides were blue with heat. Most of the bonding on the gear bands had broken down and there was terrific damage to the planat gears all caused by heat.
On rebuilding the box and returning to throttle off gear changes the box gave many years service without issues.
I know there are those who believe the pre-selector gearbox's main advantage is that you can full throttle change but I was always told never to do it.
I also heard similar advice given to ERA drivers and that box is considerably stronger than the ENV 75.
But Andrew is quite correct there is no pre-selector that can change gear faster than a crash box with a sweet set of ratios.
The main reason for the pre-selector was it's ability to change gear while the driver maintained two hands on the steering wheel. Plus of course the fact it is almost impossible to select a wrong gear - by that I mean the gearbox will only change into the gear selected (driver error is normally the cause of wrong shifts). Also you cant crunch the gears - however infallible they aint!
The NA has a pre-selector because that is what was fitted in 1935 for the Monte Carlo Rally - when Dad bought the car it was a lightweight special with a crash box, he only changed back to the pre-selector because he broke the crash box and pre-selectors were cheap and easy to come by in 1964.
I would never fit a pre-selector to any car from choice - they are great fun but a good crash box is lighter and takes up less room inside the cockpit.
Just thought it worth mentioning the first couple of paras before others think it is OK to full throttle change with pre-selectors.
By all means do so with your own car - but ask the owners permission before you do it in someone elses!
I am sure a lot of owners of cars with pre-selectors will back me up here?
Regards David
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Cymber
United Kingdom
966 Posts |
Posted - 28/03/2008 : 21:05:34
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Sorry about the gaffe folks, it's a rather vague memory from a long time ago. I should have thought about it a bit more before going into print. A simple case of not egaging ones brain before openimg ones mouth.
Maurice Blakey. |
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John Reid
United Kingdom
704 Posts |
Posted - 31/03/2008 : 00:02:31
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David and others,
Gordon has succinctly described the correct setup of the clutch stop such that you can still speed up the the driven member when changing down with a small depresion of the clutch pedal, yet a full dperession will bring the stop into operation. There is a short section on Clutch Stops in Newton and Steeds book "The Motor Vehicle".
As for its benefit in trials, I doubt if it would be of much use, though for a more informed answer we should ask Bill Bennett, Gerald Burridge or Alan Grassam. All I do know is that if you drove a Musketeer special with a blown 1408cc engine you certainly didn't have to worry about changing gear - they had enough torque and flexibility to climb many hills in 2nd off the line!
The reporter of the 1935 Gloucester Trial in the Autocar writes:
"...There was a restart at Guiting Wood on a good surface on only a moderate gradient. Fastest here were those who later proved to be the fastest on every observed section of the trial - namley the MG Magnette Three Musketeers Team, driven by Macdermid, Bastock and Langley... Finally came Nailsworth Ladder, which produced surprises. The Magnettes romped up again, Macdermid making one of the most spectacular climbs that can be remembered, going up the hill in a series of power slides and leaping like several goats on the big bumps. Admitting the fact that his car is specially built for trials, its performance is still astonishing. He climbed every hill in 2nd Gear!"
A bit off topic, but an interesting period observation.
John R |
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