Diff Swap Question

[wiltsathome]

Hi All, has anyone reverted to a Sierra diff from a BMW. I have a 420R that has blown the diff and am considering fitting a Sierra unit as I have a couple of spares albeit without an LSD. 

What parts would I need and does anyone have them? I'm guessing prop or prop flange and some of the drive shaft components??

 

Thanks Tony

[DJ.]

Sorry duplicate post, I swear I only touched the screen once!

[DJ.]

I built one of the last Sierra based kits and was offered an "upgrade" to BMW. If I remember correctly, it was just the diff/carrier, new prop and new drive shafts. So you are correct

Duncan

[ChrisC]

What gearbox do you have, 5speed?   Has there ever been a Caterham prop made for a Mazda 5 speed to Sierra diff?   If not you might need a custom prop.

[wiltsathome]

Mazda 5 speed. Its a 2016 car

[wiltsathome]

Thanks, I'm led to believe that the chassis will take the Sierra diff still, but its the bits that hook it all up that need to be compatible. I'm also told that the BMW casing is machined to fit the Caterham but I have no details so if anyone knows that I would be interested

Cheers. T

[ScottR400D]

Doesn’t the 620S come with 5 speed Mazda? 

[ScottR400D]

BMW casing is heavily machined. It then fits into its carrier which replicates the Ford fitting so they’re a straight swap. 

You need new drive shafts and suitable prop.  

 

[ChrisC]

You are correct 

The BMW casing is machined to fit the Caterham diff cage.   A Caterham back plate is applied to clear the DeDion tube. 

The Caterham diff cage uses the same bolts as the Sierra diff, so fitting the diff isn't the problem.   The drive shafts and prop on the other hand is another question. 

Drive shafts are available from CC, at about £300 per side.  The prop is more difficult, because I am not sure if a Mazda gearbox to Sierra prop was ever created by CC.  Maybe a 620S uses it, but all other cars with a Mazda box (introduced 2013) will have been connected to a BMW diff (introduced in 2010/11 ish).     If its available then it will be in the region of £260(ish).   

Why are you looking to change to the Sierra diff, is it because of the threads about the Titan diff?   

A 420R 2016 car will have originally had a carbon plate Titan diff in a BMW case, which unfortunately seem to be expiring.  

If I had a 2016 420R I would perform a preload check on the LSD to see where I was on the service life.   Others has posted Phils wisdom from RRT, 40ft/lb when new, 20ft/lb rebuild needed and 15ft/lb impending doom.

You then have a choice in cost order

1, Rebuild with Sintered plates (as per CC post 2018),  but this will make a lot more noise than your 2016 carbon plate version.

2, Replace the LSD with other brand, probably where I would go, I will be looking from a LSD with molybdenum plates like the Tracsport.  It will be more expensive than a Titan rebuild.

3, Convert to another solution. 

As a side note, the Sierra diff wasn't without its faults either.   Many converted from Sierra to BMW when it was first release because of the noise they used to generate, so do your research.  You don't want to jump from the frying pan into the fire.  

[ScottR400D]

That’s about the size of it. 

[wiltsathome]

Chris, 

Thanks for your comments, and yes, my carbon plates have broken up after 7k miles so I need to fix it. I have spoken to RRT who were super helpful and are looking at a new diff and rebuilding the Titan with sintered plates which is the most straightforward option but.......

A) I would prefer a lower maintenance and quieter LSD such as ZF or SPC or maybe Tracsport, and would also consider a Torque biasing option such as Quaife or MLR

B) Talking to the BMW motorsport fraternity they absolutely rubbish the durability of the BMW '168' diff as very weak when faced with any sort of power. I recognize the shortcomings of the Sierra units as well but at least i have two proper Ford ones in my garage, and did 35000 miles in my last 7 with a Caterham one, which wasn't quiet but did survive and didn't deteriorate over those miles

So I'm looking at options with durability and maintenance in mind as I do relatively high mileages, 7k to 8k per year. What does amaze me is that Caterham chose such a poor replacement for the Sierra diff.

Clearly its great to get a range of opinions and experiences from you guys to help with the solution, so thanks again for any input

Tony

 

[ChrisC]

Hi Tony 

I would not be so down of the BMW 168, granted its only used for 4 cylinder BMWs, and that must be a dirty word in the BMW motorsport fraternity, but the vehicle weight must have an effect on the ultimate load the CWP components are under. 

I have been a member here since the BMW diff was introduced by CC, and I can't remember an instance where a member reported an open BMW diff failure, or the CWP has caused a failure.    Obviously CWP sets have been damaged when ;-) other components have failed. 

Chris,

  

[wiltsathome]

Thanks for that, I have machine tools so can copy the diff I have unless someone has a drawing, but as I learnt about how poor the life of the BMW diff is from the BMW motorsport guys then I think buying used is an unwise option

[ChrisC]

All this does prove is regular preload checks, oil changes and magnet oil fill plugs are an essential part of Titan / BMW diff ownership. 

[ScottR400D]

Wiltsathome, what exactly is wrong with your diff casing? If it's only the carbon plates that have disintegrated, unless you're  unlucky like I was, a rebuild with new bearings plus either an overhaul of your Titan LSD, a new Titan LSD, or a new SPC LSD would be the options IMO.

The first of those being least expensive, the last the most durable.

[wiltsathome]

Scott, my diff casing is OK but as the CWP is damaged I'm advised it is cheaper to buy the complete assy from Caterham than buy all the parts including bearings etc. Having said that, I haven't got all of the info back as yet. What I haven't been able to track down yet is the supplier of the SPC LSD but have seen positive info on them, any leads please

Cheers. Tony

 

[ScottR400D]

Ah, unfortunately you've had the same issue that I had.

The guy you need is Steve Perks

SP Components Ltd 

Lighthouse Works 
Feckenham Road 
Astwood Bank , Redditch, Worcs B96 6DS 
Phone:  01527 894232
E Mail tracsport@hotmail.co.uk

His mobile is +44 7538 382729 and he’s on WhatsApp. 

I’ve found him really good to deal with, helpful and interested. Since I fitted his diff he’s contacted me twice to see how it’s going. I think he’s very confident in his product. 

[aerobod - near CYYC]

Talking to the BMW motorsport fraternity they absolutely rubbish the durability of the BMW '168' diff as very weak when faced with any sort of power

It is torque that kills the diff. As far as I can tell, the highest torque the 168LW diff with ball bearings has been used by BMW with no issues is the E90 320i with N43 engine and automatic transmission. In this application the maximum torque in first gear is 210x4.171x3.91 (210Nm Max engine torque, 4.171:1 first gear ratio, 3.91:1 final drive ratio) equals 3425Nm.

From a Caterham perspective the 420R / R400 has 203Nm / 150lbft max torque with the plenum, likely 220Nm with the roller barrels, final drive ratio is 3.64:1 and first gear in the 6-speed is 2.69:1 for a total maximum torque of 2154Nm, quite a bit lower than the maximum BMW has specified it for.

 

[wiltsathome]

Many thanks Scott

Tony

[wiltsathome]

James, thanks for that and understand your point. What I have learnt is this diff uses ball bearings that obviously can't be preloaded, instead of the more conventional taper rollers, and from my own experience of CWP life in racing Hewland boxes, once you lose pre-load you are on your way to CWP and / or casing failure, So accepting that this isn't the root cause of my current failure, I'm not a fan of living with flawed principles, but that's my engineering background irritating me I guess

Cheers. Tony

 

[ScottR400D]

#18 Good explanation Aerobod. It does seem that the CWP, bearings etc are OK, just the LSD clutch pack that's overloaded. 

[aerobod - near CYYC]

Wilt,

From an engineering perspective the ball bearing solution in the BMW diffs is fine for the torque that has to be transferred. The crown wheel and pinion mesh is controlled via various thicknesses of circlip behind the driveshaft seals that eliminate any lateral movement of the crown wheel and provides a small preload. I'm sure one reason BMW uses steel cases for their diffs with ball bearings is the elegant circlip solution for setting the mesh is helped by dimensional stability from consistent thermal expansion of the case and the internal components.

Bearing in mind (pun intended) that BMWs on the road for the past 20 years or more have used this design and it is used in current high torque cars such as the 550i with the 215mm diff (not sure if ///M cars use taper roller bearings, but non-///M cars use ball bearings), they have been well proven on tens of millions of cars.