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+--- Thread: understanding 3 speed gearbox conversions (/showthread.php?tid=4537)
RE: understanding 3 speed gearbox conversions - Mike Costigan - 16-06-2020
I'm with you on this one, Bob. My RM with standard 3-speed and 4.9 diff matches a 4-speed RP under most circumstances - better in some places, not so good in others, but it evens out over a run. I certainly wouldn't want a higher bottom gear in Derbyshire, on some of our local runs it can be touch-and-go whether I can make some inclines!
RE: understanding 3 speed gearbox conversions - JonE - 16-06-2020
Damian and Charles' point about the constant mesh impacting both clarifies perfectly. I'm glad this post will exist to help others now.
I can see the pre-planning is fraught. But that on Terry's chart, JUST the second gear amendment keeps the initial range which may address some the concerns re. Yorkshire hill 1st gear use and so on?
RE: understanding 3 speed gearbox conversions - Charles P - 16-06-2020
(16-06-2020, 11:30 AM)Mike Costigan Wrote: I'm with you on this one, Bob. My RM with standard 3-speed and 4.9 diff matches a 4-speed RP under most circumstances - better in some places, not so good in others, but it evens out over a run. I certainly wouldn't want a higher bottom gear in Derbyshire, on some of our local runs it can be touch-and-go whether I can make some inclines!
That's what reverse is for Mike
c
RE: understanding 3 speed gearbox conversions - Mike Costigan - 16-06-2020
I prefer to keep forward momentum; in many cases I have in mind there is no room to turn round and resort to reverse!
RE: understanding 3 speed gearbox conversions - Parazine - 16-06-2020
(16-06-2020, 10:55 AM)Bob Culver Wrote: I am intrigued by the comment that 3 and 4 speed cars do not mix well on runs, because for stock non sports rpm in 3rd of 4 and 2nd of 3 are about the same. The 3 speed rev lower in top but are mostly much lighter. I dunno to what extent the crash boxes hamper change speed but I usually double declutched downward on the 4 speed syn anyway and lost neglegible road speed when pushed. Upward changes are certainly often a handicap on large old cars as the delay can be very long. The local owner of Sports 20 claims Sevens often do better on hills as his car once committed cannot change up until the road distinctly flattens.
The near doubling of rpm 3 to 2 of small 3 speed cars is not uncommon. Ford Y, Ten etc.
Bob et al, the problem with four and three speed cars on a run is this.
My 3 speed car is capable of climbing virtually any gradient but it needs to do so at speed to keep the engine "up on the cam". On a run, with a mix of cars and driving abilities, we usually wait at the bottom of a steep hill to allow the car ahead to clear it, setting off at 1 minute intervals to tackle the incline. However, despite these precautions, on occasions I have stormed up a hill in 1st gear at high revs, rounded a 1 in 3 hairpin and almost run into the back of a Box or Ruby, sedately managing the gradient with his 1st gear but at slow speed.
Once baulked by a four speed like this you have the choice of trying to restart the three speeder on a fierce gradient or reversing down to somewhere where a restart is at least possible without burning the clutch out. Another alternative is using reverse and I can remember cars reversing up Porlock years ago because they couldn't manage it in 1st! However, my neck is too stiff to reverse very far these days and anyway, the engine overheats when you reverse at speed, uphill.....
The "Andes" gear helps because at times, my three speed, locked into second by a lesser gradient would be similarly baulking a four speeder in third who would be able travel faster. The result is that the four speed car is now unable to hold third gear and has to select second resulting in him dropping way back. With the "Andes" ratio, I can make progress in second gear, almost equalling a four speed third and that evens things up nicely.
RE: understanding 3 speed gearbox conversions - Malcolm Parker - 16-06-2020
I ran my RL with an Andrew Bird 'Andes' box for a couple of years with the 4.9 axle, but it struggled on one of the Pennine Runs on some of the very long climbs out of the valleys ( I am thinking of Holmfirth up past Compos Café) when I was forced to use first gear for mile after mile because 2nd was too high. The car was two up with luggage. I have reverted to the standard ratios which on the saloon are better for North Yorkshire. The Ulster also has the Andes box but with more power, less weight and the 5.25 axle will climb normal hills in top and romps up steeper hills in 2nd, even the fearsome Sneck Yate with only me on board.
On the Coast to Coast Run there was a long straight hill up onto the Yorkshire Wolds from a village called Flixton. Being only about five miles from the start in Scarborough there was usually a large bunch of Austin 7's. The road was quiet and wide enough for four Austins abreast. It was a wonderful sight to see four lanes of Austins, all positioned according to their gear ratios, climbing the long hill. The speed of the cars in lane two may only have been 1 mph faster than those in lane one but by the top of the hill most cars had sorted themselves out. Happy days!
RE: understanding 3 speed gearbox conversions - Parazine - 16-06-2020
I have to admit that my Chummy is running a 5.375 (post war Regent) axle with a lightly tuned engine for more power.
My RK is mostly standard, except for the 5.25 axle and 24T2 carb. This combination, with standard gear ratios works well.
RE: understanding 3 speed gearbox conversions - AustinWood - 16-06-2020
I have Andes gears on my RK fabric saloon. It suits The Scottish Borders perfectly as we have long hills but not as steep as the Pennines. I can hold 30-40 mph up these hills were I would be stuck at 20-25mph with standard 2nd gear.
On Malcolm's runs through the Pennines I was frequently reduced to 1st when those around me were OK in 2nd.
The worst combination was a run in the Borders for many makes. We got behinds a 1940s P3 Rover 75 who drove at a steady 30mph up hills and down. Very frustrating as I would have been doing 45mph+ down the hills in order to get up the other side in 3rd.
I call it Conservation of Momentum.
RE: understanding 3 speed gearbox conversions - Bob Culver - 17-06-2020
I was considering stock non sports cars on reasonably normal roads. I have only driven a chummy a short distance but was notably more lively then a stock 1935 Ruby I had at the time.The lwb 13 bhp cars are soon forced on to 2nd when progress is tedious and when the earlier cars should outperform. With the later head and SU etc much improved if the revs can be maintained. The enormous number of modified cars makes it all now confusing. I have observed few Seven runs but our local Jowett club runs used to degenerate/elevate into Bradfords fighting it out at a giddy 55 mph. (Years ago a colleague had a Bradford; compared with my RP it was like a sports car! no cornering roll)
RE: understanding 3 speed gearbox conversions - Mark R - 18-06-2020
Just looking at the Milk delivery car brochure on the online archive and interesting to see that it quotes the ratio as 5.6, is this the large banjo Ulster type axle, that'd probably make it quite drivable.
Regarding gearbox ratios I only rang Andrew Bird the other week and to say the least he was very helpful and informative, after my call I think I'll get him to do the gearbox for me rather than try it myself. Problem is there really doesn't appear to be a one size fits all solution. The engine, weight and terrain planned to be tackled are unlikley to be the same in two cars. I know my old 31 saloon was hard work with the original gearbox and axle combination.