BB Baboonface
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| posted on 19/2/17 at 11:44 AM |
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Dumbest drive train question ever?! Probably definitely...
Hi all, been signed up for a while but generally read about what folks are building more than anything. Some awesome imaginative stuff btw!
Don't have a build as such, just (very) slowly modifying my Civic EV1 with its stock K20A3 engine, hoping to drop in a K24 with a 6 speed at
some point, but as it is comparatively "plug and play" i guess it does not really warrant a project thread.
Do fancy trying a real project at some point, but time, space and money are not plentiful right now.
So cut to the chase, dumb question time! I don't have a great grasp of how stuff goes together yet, still learning many of the basics, so please
be kind! lol!
This concerns using a transverse engine in a longitudinal mounted format, not exactly new ground i know, but looking at various AWD drive trains made
me wonder about something.
It has to be stupid/impossible because no one else seems to have contemplated the idea as far as i can see, but in theory, could one retain the
transverse transmission and have what was the front diff of the transverse gearbox essentially become a centre diff, and replace the former
driveshafts/axles with longitudinal prop shafts to front and rear diffs?
Yes, someone just asked that! Oh dear!
To sort of flesh it out a bit, it seems some existing systems that power the rear wheels (well at least the Mitsi EVO, or Honda RT4WD) use the
transfer case/PTU to over spin the prop shaft compared to the final drive rpm of the transmission, then the rear diff would alter the ratio/rpm again
to match the front end.
So my half baked theory was, if it was necessary to "over spin" the props, then use an appropriate final drive in the transmission/gearbox
to increase shaft speed slightly, then use the (matching) front and rear diff ratios to return the wheel rpms to something more like those used in a
conventional setup.
Really feel stupid posting this, but it is bugging me and asking people with real world knowledge and experience seemed like the best way to put it to
rest.
Guess it would never work or someone would have done it by now.
Thanks for reading, and sorry to ask such a silly noddified question!
James
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Sam_68
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| posted on 19/2/17 at 12:46 PM |
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Your problem will be to find components that offer the necessary gearing.
Obviously, the final drive in your FWD gearbox already steps down the rotational speed to something sensible for the road wheels, so you'd be
looking for front and rear diffs that had a 1:1 ratio, which simply don't exist - the tallest diffs commonly available are around 3.14:1.
Even if you could find, say a 3:1 final drive for the main gearbox and 3:1 difs, that would still be an overall reduction of 3x3 = 9:1, which would be
ridiculously short gearing for a car.
You could potentially use a step-up transfer box to step the propshaft revs up before reducing them again at the diff., but it would be complicated,
heavy, and result in substantial transmission loss.
You might want to Google the Ford RS200 drivetrain, though, to see how they did something similar to what you suggest, albeit with a longitudinal box.
Ford weren't worried about budget, though, and could afford the bespoke transmission components.
[Edited on 19/2/17 by Sam_68]
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BB Baboonface
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| posted on 19/2/17 at 01:47 PM |
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First and foremost, thank you for furnishing the question with a considered response, much appreciated.
Got the pdf owners manual of the RS200 on an old hard drive somewhere, really interesting setup (talking Group B, was always fond of the 6R4 for the
amusingly savage contrast to the Metro in original form)
On topic though, it's obvious that my grasp of the fundamentals is sorely lacking.
Was reading about the EVO system and kind of hoped you could bypass the step up from the transfer case by spinning the shafts a little faster with the
main final drive (the example i was looking at had the shaft spinning at about 6086 rpm when the engine was at 6000, not seemingly a huge increase)
After that step it down with the diffs as with a conventional system, there seem to be a few configuration and ratio choices with the R200 Nissan
units for example.
Definitely missed something big reading the posts about the Mitsi system methinks...
Being a noob to this there must be some glaringly obvious reason why bypassing the transfer simply wont work, only started trying to learn the basics
a couple of days ago and it definitely shows! Perhaps should google how a car works! Haha!
Should perhaps spend time more usefully looking into something feasible, RX8/1UZ/CD009 combo could be pretty funky (apart from the stupid
CAN'Tbus system grr!)
Anyway, thanks again.
James
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mark chandler
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| posted on 19/2/17 at 02:03 PM |
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Yes you can, it has been done before.
If you want to build a lightweight 4x4 vehicle, limiting factor being gearing as you are dropping two sets of ratios.
Malcolm here pioneered a few of these running golf engines and transmissions
http://www.whitbread-offroad.co.uk/news_offers_view.php?id=66
[Edited on 19/2/17 by mark chandler]
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BB Baboonface
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| posted on 19/2/17 at 02:21 PM |
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Ooh! So the plot thickens... 
Thanks for the link, will have a scan at Malcolm's site and see what is what, though the application i was thinking of was more of a road car
kind of thing. Interesting tat the idea has been implemented in some form though.
Really wish i understood enough to converse at the level you guys are at, obviously the site is for doers rather than dreamers and you don't
need to be wasting time explaining basic principles to plonkers like me lol.
Definitely want to build something for sure, it is a royal PITA trying to modify/work on a daily driver when it needs to be operational within a few
hours, no room for errors, and that occasionally makes things a little hair raising.
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mark chandler
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| posted on 19/2/17 at 02:30 PM |
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You will probally need to call him, this was 20 years ago now!
Limit is the top end, good for trialing, not very successful on the road as you end up being geared down to 50mph.
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BB Baboonface
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| posted on 19/2/17 at 02:44 PM |
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50 mph? Ah. lol
Tbh, i probably won't bother the poor fella with flights of fancy, his site says he's taking a back seat from the business after all, so
he'd probably like a bit of peace and quiet!
Have to say you guys have been most kind so far, may rock up with something of interest in the projects section one day, but likely not using the er,
interesting ideas of AWD i had above.
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obfripper
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| posted on 19/2/17 at 04:52 PM |
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You could use vw syncro rear diffs, the ones from a golf rallye and a passat g60 are 1.05:1 ratio, the later 4motion diffs are similar but in the
1.6:1 ratio area
They're not going to be cheap though as they're pretty rare - expect at least £500 each.
You would need to invert one diff to make it drive in the correct direction (which one will depend on the orientation you choose for the engine/box)
and the viscous coupling would need to be removed/locked. Inverting the diff should be simple due to design, and afaik the vc replacement parts are
available from syncro specialists.
The syncro diffs also have diff locks built in, which would make for an off road capable vehicle.
The only disadvantage is the width of the diff unit, which would make the driveshafts quite short.
Another option would be 4motion diffs and a diesel gearbox, the longer diesel gearbox output ratios will offset the shorter diff ratio.
This would prove alot cheaper as a second hand diff can be had for £50, there is a wide choice of diffs from audi vw seat and skoda.
The tricky part may be removing the haldex unit and replacing with a drive flange - i don't think there is one available, and the haldex unit is
torque limited to 2000nm.
Whether that limit would matter in a light car is another thing, and it would be trick to have switchable fwd/rwd/4wd.
Dave
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BB Baboonface
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| posted on 19/2/17 at 06:14 PM |
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Interesting thoughts again, cheers Dave and you other guys.
Bet the VCU's would probably be snapped up by Syncro owners who need replacements, pretty pricey as service parts go i believe.
Looked at all sorts, do think electronic control would be beasty, but something mechanical and dependable would suffice i guess.
Need to get the old noggin round this ratio thing really, might not have had to trouble the good folks of LCB then. That said, it seems to have got a
few cogs whirring (pun intended?!)
Being a fan boi, this all started with looking at AWD Civic builds over in the US, but the whole transverse to longitudinal thing just sprang from
that as a sort of unrelated tangent.
Reckon even if the theory were sound, building and setting up such a system would take some serious skills. Did train as an ornamental Blacksmith and
welder some years back, and tiddled about doing ball joints and coilovers on the current car, but homemade AWD is a couple of steps up from there!
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mark chandler
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| posted on 19/2/17 at 06:15 PM |
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"You could use vw syncro rear diffs, the ones from a golf rallye and a passat g60 are 1.05:1 ratio"
Now those would be perfect, most cars are over geared so a slight drop would not go amiss, one of the advantages using such a narrow gearbox you can
get the engine right back, knock up an interesting clubmans RWD type car by welding the gearbox diff, sitting the engine next to the driver and
running RWD!
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BB Baboonface
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| posted on 19/2/17 at 06:34 PM |
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Haha! That sounds pretty bonkers, in a good way though.
If anything a RWD project seems much more likely, using something with FR layout from factory.
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