Living with a welded diff on a BMW E39

RIGHT! The time has come to do a write up on what living with a welded Diff is like on an E39.

So last week I took the plunge after literally MONTHS of debating if I should do it or not after hearing so many stories (both good and bad) from people on the internet and wanting to do some drift events next year I took my 530i to be professionally welded by a BMW drift team that was local to me.

First though let’s talk about why I wanted it done, having had a MK1 MX5 with an LSD I sorely missed the way a car would perform when it was able to lock both the rear wheels up, A car with an LSD or welder feels SO much more alive on a twisty road! You can really feel the car move about and squirm when you’re near the limit or simply the fact you can put ALL of your power down when you exit a corner/ leave a junction.

An open diff in my opinion if you like to drive spiritedly is rubbish! Trying to power out of a corner can leave you spinning one wheel and understeering into the nearest static object while making a hell of a racket and causing uneven tire wear and if in the rare case you actually manage to provoke a slide it’s very snatchy and actually quite scary to control!

Of course, DSC on our cars make a VERY good job of keeping us on the straight and narrow and correcting mistakes if you go too fast into a corner or prod the throttle like you’re an ape BUT they mostly kill all the potential fun of what is a remarkable chassis that the E39 has.

How has a welded diff affected my car?

First of all, if you want to run a welded diff on an E39 you NEED to lose your DSC entirely, As you know DSC can never fully be turned off in the E39 and leaving it functioning will result in a car that just won’t move properly and could end up actually damaging your diff with how it brakes individual wheels, This also means as the ABS/DSC is on the same module you will lose ABS too… So basically, in an E39 with a welded diff you are 100% on your own with zero driver aids!

Older cars that only have ASC I believe are a different story as that should just try to kill wheel spin and shouldn’t try to brake individual wheels. I have however been advised you shouldn’t run ABS with a welded diff anyway and when it comes to an MOT you would want your brakes to be road tested rather than on a roller as that apparently can also cause damage to your welded diff.

Let’s talk about the cars road manners and daily driving, You’ve probably heard many horror stories about how cars with welded diffs are noisy, uncomfortable and downright annoying and to that I say ‘’sorta’’, The E39 does a very good job at masking extra road noise created by a welded diff as it’s quite a heavy well built car so compared to say an old Mazda with one it’s simply not going to transfer through as much, I only really notice the wheels making skipping noises when the windows are down or it’s very quiet and even then it’s not enough to irritate me.

The wheel hop however is a bit of a different story! It can make parking on certain surfaces a bit irritating as the back of the car will want to hop about and if you’re at the right angle on dry concrete then it does become a bouncy castle! Taking all this in mind you can still park just fine, You may need to add in a bit more steering lock and add a couple of point turns to your manoeuvres but it’s certainly not a deal breaker.

What’s it like to drive in town/ at lower speeds?

The first thing you’re going to notice is your steering is going to be a tad heavier and you’re going to need to add a bit more lock to make the car turn out of tight side roads and sharper turns, This is due to the fact the rear wheels are now 100% locked all the time and want to push the car forward but as with parking you get used to this very quickly and it stops being a problem, I can comfortably drive my welded car round town all day long as your brain simply adjusts to the fact you need to change your steering input slightly. It does sometimes when it’s wet feel a tad understeery and the back end will hop a bit but add more lock and you’re generally golden.

What’s it like to drive at faster speeds/ on a back road?

Simply put… FANTASTIC! As I stated earlier the car feels much more alive and 100% more predictable, As the diff is always locked you pretty much know that the car is going to want to push and that feels devilishly fun, And when the rear end does push out it’s wonderfully controllable compared to an open diff which snatches and jerks about, This simply slides out, you use throttle and steering to catch it and a big grin washes along your face.

A lot of people say a welded diff will make a car understeer horribly when going fast and to that I say (at least in the case of an E39) How bloody fast are you driving? At spirited but sensible and legal speeds I don’t feel any understeer AT ALL and the car performs fantastic! Maybe if you were dragging it round a track at 100+ MPH you would do but on the road it’s fine!

Why not an LSD? Simply put there’s no easy and affordable way to get an LSD in a non M5 E39, Yes they can be hacked together from the BMW parts bin but it’s a hell of a lot of hassle and expense.

Will a welded diff try to kill me or my car?

Let’s talk about the latter first, A welded diff will put more stress on your driveline! Subframe bushes will eat themselves to pieces, drive shafts and half shafts can snap, chew input shafts, diffs can blow up and strip the teeth, gearboxes can explode and tires will need changing more often! This is something you really need to be aware of and that would mean if your car is subjected to poor maintenance you’re going to have a bad time! So treat your car with respect, get the diff welded and plated by a professional and replace ropey looking parts and you should be golden.

Will a welded diff try to kill you? Well unless it fails from poor welding the answer is simply NO! The horror stories you hear about welded diffs making cars into death machines isn’t down to the diff but down to the driver! If you have never driven a RWD car with the traction off or experienced a LSD on a car or had any experience getting a car sideways and you think that running a welded diff is going to make you a drift god then you are 100% wrong!

On a car with a welded diff you need to be able to exercise restraint, understand throttle control and be able to predict when a car is going to step out, Also you need to be prepared for the car to let go if you’re driving on the edge, You need to take weather and road conditions on board and generally try to use your common sense, If you follow these rules then you won’t have any accidents.

Is a welded diff illegal/ going to get me in trouble with the police?

As far as I am aware there is NO law saying you can’t run a welded diff on the street BUT it is classed as a mod and you should notify your insurance of driveline changes, Doing mods and not declaring them if you get in a crash will cause you a serious headache, Would an insurance company check for a welded diff? Unlikely BUT you never know! If you’re involved in a fatal collision then you bet your bottom dollar your car will be gone over with a fine toothpick from head to toe!

Will it get you arrested? Well that depends! Do you drive about like an absolute idiot? If you do then you probably don’t care about getting in trouble with the police anyway! The bottom line is control yourself and don’t go drifting on the street and making a commotion to bring attention to your car, Not only is street drifting illegal BUT it’s reckless and could cause serious death or injury!

You need to remember there is a time and a place to get your car sliding and in the middle of town at 5PM is not the place and your local industrial estate probably has CCTV too! If you want to go sideways get yourself booked into some drift days or find some private land you can get permission to play on safely and legally.

This concludes my write up on how a BMW E39 performs with a welded diff and has information that could be applied to other cars with welded diffs, I will update you all on how it handles on the track after I get some drift days under my belt.