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Sept 11, 2018 15:04:09 GMT
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I've had this idea in my head for a while to do a mid-engined AWD conversion on a Peugeot 106 GTi using the regular transverse FWD engine stuck in the middle and flipped around so it's longitudinal. The FWD gearbox stays where it is, and the differential now becomes a centre diff running propshafts fore and aft to regular diffs. So far so gravy. The to-date insurmountable issue has been that because you'd be driving the car through two normal ratio diffs you'd end up with a really high diff ratio and top out at 70ish mph which is less than ideal. There are two ways I've thought of to overcome this. The first is to run monster truck tyres which necessitates a lot of other compromises. The other is to try and find a diff combination that results is an acceptable final drive ratio. Lowest diff ratio I've been able to find has been 2.29 in a Ford 7.5" (I think only live axle, but should be able to make it fit a Sierra diff). That's pretty low, but even combining with the diesel BE gearbox final drive of 3.47 you end up with a final drive ratio of 7.95 which is too high. Short of getting custom 2:1 final drive gears made for a BE (if that's even possible), the only option is to try and find a standalone diff that has less than a 2.29 ratio. So, does anyone know of any super-low ratio diffs? Ideally something with a 1:1 ratio would allow for a wide variety of applications, but anything less than 2:1 would probably work if it exists  |
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Darkspeed
Club Retro Rides Member
Posts: 4,706
Club RR Member Number: 39
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Sept 11, 2018 16:21:16 GMT
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Have a look at the Audi Haldex ratio's - something in the JB Ape thread makes me think they are very low.
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froggy
Posted a lot
Posts: 1,099
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Sept 11, 2018 21:23:10 GMT
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E46 and later diffs go down to 2.3 fd but the front would have to be run upside down pushing on the coast face of the CROWN wheel to work . Seems like a lot of extra weight unless it’s for off road
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Sept 11, 2018 23:28:54 GMT
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Hi, The front diff doesn't have to be upside down, if fact it's better it isn't otherwise it would be competing against the rear one.
Colin
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stealthstylz
Club Retro Rides Member
Posts: 14,841
Club RR Member Number: 174
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Sept 12, 2018 0:15:16 GMT
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VW Haldex rear diffs are 1.8:1 from memory. I still can't see it working at that though, a 106 GTi does say 140mph flat out as standard. With a 1.8:1 diff you're still going to be flat out at 85-90mph.
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Sept 13, 2018 10:42:08 GMT
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Ooh now a Haldex diff I haven't looked at. Is it a standalone thing or part of a gearbox? Either might be workable actually.
I've just googled and apparently Haldex diffs are 1.58:1 which is definitely workable. You're right that on a standard GTi 'box and wheels that's going to be too short. However, with a BE gearbox from a 205 diesel you can get a bolt-on final drive of 3.471 (as opposed to 3.94 for the GTi).
With standard GTi wheels that gets us a top speed of 110mph. Bit short geared still, and a little way away from the 136mph standard gearing limit.
Bump up the wheels to 225/50 16s and we're getting somewhere at 125mph!
I need to work out the old headphysics model for running diffs at the wrong end of cars. I've read they need to be flipped upside-down to get the gear teeth to mesh on the right face and stop them trying to push themselves apart. What I need to work out is what that does for the rotation of them as flipping them around to point the opposite direction already reverses the direction.
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Sept 13, 2018 11:02:58 GMT
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Nope, just worked it out with some scribbles. If you put a RWD diff up front and just flip it around so the pinion flange points towards the centre of the car then that makes sure that your diffs are rotating in the same direction like so. CROWN-wheel winds up on the other side, driveshaft from centre diff stays anti-clockwise. Car moves forwards, but the front diff teeth are pushing on the wrong side.  Flip it upside-down and the front diff now has the pinion on the same side as the rear diff. Given the same rotation of the pinion, it'll want to drive the wheels in opposite directions.  Yet I've definitely heard of people flipping diffs upside-down when converting cars to AWD. Am I missing something? Did they initially think that and then work out it doesn't work? |
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Sept 13, 2018 13:38:00 GMT
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land rover transfer box is 1.15:1 final drive, and 2.3:1 even in low range. i can see your issue as most FWD hatchbacks will be in in the 3's !
re: direction of rotation, theres nothing that says you cant have gearbox infront of the engine (if that gives you favourable direction)
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Sept 13, 2018 14:44:35 GMT
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Is the Landy one as low as 1.15:1? Neat. I've got a Ranger Rover transfer case (1.222:1) that I plan to stick in a B GT with a Jag rear end and Subaru R160 front end to make an AWD Baja B (that's the next plan after the Spitfire). You're right with the issue though. Most transverse engines wind up with 3s for the diff ratio. Even diesel 'boxes are still relatively high compared to what we need. Haldex rear diffs front and rear would probably be workable with bigger tyres. I've done some reading on what people do when converting drivetrains to run the other way around which has been enlightening. Basically, the issue I posted above about the diff rotation is right. If you flip a diff to point the other way around then the direction of rotation is right, but the gears engage on the 'coast' side of the teeth which tries to push the pinion and CROWN wheel away from each other. A fair few manufacturers (like Land Rover for instance) just say 'sod it' and run them like this anyway to simplify manufacture. It works, but it reduces how much torque you can put through the diff. Some cars that use front diffs do have reverse-cut gears so you can run them in the opposite orientation with correct teeth mesh (the R32/33/334 GT-Rs have R160s cast into the sump with reverse-cut teeth), and you can buy reverse-cut gears for a number of diffs that are common in 4x4s (Ford 9", Dana60, whatever Landies use). The other option which I've come across is how to convert a whole WRX drivetrain to rear-engine AWD (a chap in Australia did this with a Manx). What he did was flip the whole shebang around so the engine was hanging out the back, and machine up a reverse-cut gearset for the Subaru transaxle. Because of the design of the Subaru integral front diff you can flip it around inside the casing to reverse the direction of rotation. He then stuck an R160 up front flipped around and upside-down which works in his favour this time. So, options would be: 1. Haldex diff at the front and rear with the clutches locked together, with the front flipped around and see how quickly the front one blows up. 2. Haldex diff at the front and rear with the clutches locked together, with the front flipped around and upside-down, pinion oiling sorted and a custom-machined reverse-cut gearset. |
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