405 G/box 1st gear freewheel???

GlennBurnage · #1 (**permalink**) 21-10-21, 04:51 PM

Can someone please explain exactly what the 405 gearbox first gear freewheel is/meant to do/reason for Bristol doing it etc. I can't seem to do anything in 1st gear that 'freewheels'!!! Can it be disabled? Can it go wrong and then not function?
So many questions.
An opportunity to educate the ignorant.

David C · #2 (**permalink**) 22-10-21, 08:44 PM

Quite nice when crawling in traffic and coasting rather than being slowed down by the engine when the car overruns.

They are fairly fragile units. There used to be a Clumsy Oaf Club in the BOC for those who had over revved in first gear and burst the freewheel.

GlennBurnage · #3 (**permalink**) 23-10-21, 10:16 AM

Many thanks, Chris, for outlining what it does.
I suspected as much and have tried to see if it freewheels in first gear - it doesn't, so it's either Kapput or ..........?
So, can anyone enlighten me as to what happens when the freewheel unit goes wrong? Will I see swarf etc when I drain the oil?
Can they be disabled, or if it is out-of-action, does it mean that its broken?

David C · #4 (**permalink**) 25-10-21, 05:38 PM

I was waiting for someone better qualified to help. I have rebuilt one of these gear boxes but I'm no expert. I would expect that if the freewheel had burst you would have no first gear. It could well be that a non freewheel option was available as a repair. I'm not aware that a 4 synchro box was ever available.

Julian Caples · #5 (**permalink**) 28-10-21, 10:17 PM

The simplest way to see if your freewheel is working is to (carefully) see if the car rolls forward with the engine off when in first gear (brakes off, foot off the clutch pedal)

The freewheel works like the freewheel on most bicycle chain drives, but it doesn't use chains and ratchets. It will allow the prop shaft to run free if it is running faster than the main shaft in the gearbox. This means that you won't get engine braking in first gear and the car will roll with the engine off in first.
It clearly is only on first gear (it is a part of first gear) and uses rollers that run up ramps inside the first gear - there is a picture on p20 of the gearbox section of the 405 workshop manual

it can go wrong - with wear the freewheel can fail to function but that usually means it freewheels all the time and doesn't engage (ie no first) but it plausably could "lock up". the usual failure is "bursting" in which the drive is taken up too violently, the rollers run up the ramps too hard and physically burst the first gear cog. This usually results is loss of first gear altogether or a locked up gearbox (if the first gear pair lock together and prevent the lay shaft from turning) - car should not be driven at all with a damaged freewheel (and not towed)

as to why Bristol did it - who knows - it was used by several manufacturers in various forms (Rover for example) and in this case its really just a replacement for synchro on first - it actually would have been just as easy to put synchro on first when they moved to BW baulk ring synchro - may have been space constraints in the early drum synchro boxes

There were "fixed" first gears in period (mostly in sports gearboxes - eg AC Ace and Aceca) and they have a higher ratio too. There are some modern "four synchro" conversions that replace the freewheel with conventional BW baulk ring synchro

Julian

GlennBurnage · #6 (**permalink**) 29-10-21, 09:26 AM

Many thanks to those who have replied with most helpful advice.
I may not have made it apparent, but this car is a Werner Oswald Frazer Nash Le Mans Replica 'replica' using, almost exclusively Bristol 405 compnents.
Julian, As I suspected, it doesn't roll forward freely in 1st gear. 1st gear works fine and is quite a high ratio, so I would imagine it is standard and the non-syncro is very evident. Any thoughts about the fact that the freewheel doesn't work but the 1st gear does?? From what you mentioned in your reply I might suggest that it may have been 'modified' to over-ride/remove the freewheel?
The high ratio of the 1st gear makes me suspect that it's not an ex-AC box and I can't find anything in the car's history file that mentions anything at all about the gear box, let alone any deviation from standard or any modifications.
You mentioned that there were 'fixed' 1st gears in period - do you know anything about how, without dismantling, one might tell? I could see no alternative 'Fixed 1st' parts in the manuals, or mention of it. I have no info regarding the donor car and not sure, yet, if gearbox/overdrive has any numbering on the cases. Some time after the car was assembled (mid 80's ish) the engine was completely rebuilt, but I feel that nothing was done to the drive-train and there are a few 'whirling' noises, depending on the speed/gear/load etc, which merit investigation. I intend to have the engine/gearbox/overdrive unit out once the first salt is down.
Once again, thanks for taking the time for the detailed info and any pointers would be helpful

Geoff Kingston · #7 (**permalink**) 29-10-21, 03:18 PM

Glenn
As the car was assembled around the early/mid 80's this was a period were a few of the specialists were offering fixed first gear conversions, the late Paul Burd in Bristol being one person I know who did this. I acquired a 403 through him which had this done to it, I remember it was very difficult to engage first gear unless the car was stationary or almost.
My recollection of a burst first gear freewheel is that you lost drive in that gear and had the added bonus of bits of unwanted metal flying around in the gearbox, avoided if it was treated with respect.
Geoff.

GlennBurnage · #8 (**permalink**) 29-10-21, 04:11 PM

Geoff, that's very interesting. I also have to be as close to being stationary as makes no difference to engage 1st gear when the car is moving. As 1st works but the f/wheel doesn't, I can only assume that the f/wheel has been removed.
I've yet to change the gearbox/overdrive oil as I've only received an oil dilivery this week. I look forward to inspecting the oil!!!