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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

405 G/box 1st gear freewheel???

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Old 21-10-21, 05:51 PM
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Default 405 G/box 1st gear freewheel???

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.
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Old 22-10-21, 09:44 PM
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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.
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Old 23-10-21, 11:16 AM
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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?
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Old 25-10-21, 06:38 PM
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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.
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Old 28-10-21, 11:17 PM
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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
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Old 29-10-21, 10:26 AM
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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

Last edited by GlennBurnage; 30-10-21 at 09:06 AM.
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Old 29-10-21, 04:18 PM
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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.
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Old 29-10-21, 05:11 PM
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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!!!
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Old 29-10-21, 10:02 PM
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Someone who knows more than me may confirm but I would expect a fixed first gear to be straight cut and therefore make the same noise as reverse - a fair bit noisier than second, third and fourth.
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Old 31-10-21, 11:16 PM
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Glenn

It does seem likely that you have a first gear from an AC Ace or one of the replacements that was made in the 80s or more recently
These are higher in ratio (2.92 vs 3.61 - lower number=higher ratio) and "fixed" in that they have neither the freewheel nor synchro

The "fixed" bit means that they engage by simple dog teeth with no means to match gear shaft speeds (synchronise). Engaging at low speed is possible without double declutching, or you could double declutch

They are helical gears (not straight cut) - there was (and is) no reason to make them straight cut, and as has been pointed out a straight cut gear will scream.

The Bristol "fixed first" were not modified conventional first gear sets, but properly done first gear and layshaft pairs (you need the matching layshaft) as the ratio was changed (as were sets done in the 80s and more recently). In theory it would be possible to modify a freewheel gear (to fix it), but it would be a bodge, would be hard to get to work properly, and wouldn't change the ratio.

The higher ratio would be perfect for the FN, the lighter car would be woefully undergeared in first, particularly if it still wears the 4.22 ratio diff from the 405
The FN (and all AC Ace/Aceca) had the higher ratio first ex factory

Cheers

Julian
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Old 01-11-21, 07:41 PM
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Julian, interesting. As my car uses, as far as I can tell, only components from a 405 including an overdrive gearbox, I'm assuming it'll have a 4.22:1 diff?
I can use second in almost all 'pulling away' situations, first is only really usefull for getting things rolling on an incline before second is swiftly taken. Although my car is probaly considerably lighter that a 405, as is an Ace, I would have thought that a modified first gear would be closer to second than mine seems to be.
15.24:1 seems like a stump-puller compared with second at 7.71:1!!
I can check the diff ratio without having it to peices, but will have to wait till the drive train is removed and the box opened before I can see what's what.
I think my ACOC forum memebrship is still active and I might put a request for info re modded 1st gears on Ace/Aceca Bristols. (I thought they had Moss boxes??)

On the subject of ratios, I've had conflicting 'education' on how gearing is expressed from highly educated engineering types over the years. It seems to me that refering to a ratio as high or low depends from which angle one is looking. Depending if one is looking at the work going in, or the work coming out, if you get my meaning?

Last edited by GlennBurnage; 02-11-21 at 08:23 AM.
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Old 05-11-21, 09:18 AM
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Hi Glenn

If your car has a normal 405 first gear it will give you about 5.3 mph per 1000 revs (may be some variance due to tyre profile) (this is the 15.24 ratio you refer to).
If it has an ex AC Ace first (or equivalent) it will give you about 6.7 mph per 1000 revs (again possibly some variance due to tyre profile) (a 12.26 ratio)
In a light car this will still seem very low (as you seem to be experiencing) because the diff ratio is low (4.22)
As you say, being a light car first is only marginally useful, and second at 7.7 or so is achieved very quickly.
These are NOT modified gears, they are stock ratios that were available in period and were delivered in the sports gearboxes at the time.
It is straight forward to check what ratios you actually have by driving the car at (say) 2000 rpm and seeing how fast you are going in first gear (see mph per 1000 rpm data above).

AC Aces delivered with Bristol engines all had Bristol gearboxes (usually BW CR 6/9/12) with "fixed" (non synchro, non freewheel) first gear with an internal ratio of 2.9:1 which would give 11.38 with a 3.9 diff (or as possibly in your case 12.26 with a 4.22 diff). This is all well documented.

AC Aces with AC engines did have Moss boxes - that's a completely different kettle of fish (and a completely different set of ratios).

It is conventional to talk about gear ratios as "higher" if they have a smaller number and "lower" if they have a higher number. This is because the gear ratio divides down (reduces) the revolutions per minute as you progress along the drive train. For example the diff in early (pre 405) Bristols is 3.9, this is higher than the diff on the 405 (and 406) which is 4.22.
To illustrate - if the engine was turning at 3000 rpm in top in a Bristol with a 3.9 ratio diff, the half shafts (the drive shafts out of the diff to the wheels enclosed in the rear axle) would be rotating at about 770 rpm and the car would be doing about 62.7 mph. If the Bristol had a rear axle with a 4.22 ratio (405) then the half shafts would be rotating at 714 rpm and the car would be doing 57.9 mph. "higher"= faster for given revs, "lower"= slower for given revs. This terminology convention applies for all gears in the drive train.

PM me if you want more detail

If your gearbox has ratios that are too low to be useful, when you have someone look at it, it may be useful to talk to Ian Nuttall who can do additional (higher) ratios for first gear (I presume you are in the UK, I'm in Australia so unlikely to be much practical use to you even though I can do a number of ratios and synchro for first). Again PM me if you want more details

Cheers

Julian
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