17. M3 suspension parts & Quaife limited slip differential
If you want your car to drive faster than it can do in stock form, increasing the power output of the engine is only one part of the equation. If driving faster in a straight line on motorways is your only goal, then this might be enough; however, if your objective is also to be faster around corners, bends or even on a racetrack, then an improved suspension and better traction is a no-brainer.
Let me start by saying that the stock suspension of my car (it's a 335i without M sport suspension) is actually quite good, once you ditch the awful runflat tires and upgrade to Michelin Pilot Sport 2 as I've done (see previous posts). But still, it's a suspension the main objective of which is to make the 335i a fast luxury sedan, and not necessarily to be on the sporty side - pursuant to BMW, that's what the M3 is for. For me, the stock suspension of the 335i has a bit too much body roll in corners, it does not react as fast and precise to steering input as an M3 and does not provide a very good feedback from the road to the driver. You can still drive it quite fast, but you have to sort of guess where its limits are or what you have to do in order to go where you want; I wanted to improve this.
Also, once you increase the engine power and, in particular, the torque, you are faced with another problem: traction! I experienced that, after having flashed my car with the Evotech flash, it got increasingly difficult to transfer the power on the road, the tires spun too easily and much of the desired forward momentum went up in black smoke, decreasing the life span of the tires. This was even more true in corners or in the wet - unless the road is dry, there's wheelspin sometimes even in fourth gear, and even when dry, powering out of bends is near impossible in second or third gear.
Why is that, you may say, doesn't the 335i come with a differential? Well, BMW has labeled a function of its DSC "electronic differential", but that is quite a misnomer: All it does is brake the rear tire that has lost traction, thus decreasing the life span of your rear brake pads but not helping with forward momentum. The 335i unfortunately is not equipped with a limited slip differential as all M models are, for marketing reasons in order to differentiate it from the M3. As a consequence, it has an open differential which means that the power of the engine is always directed to the wheel with the least resistance. In corners, that means obviously the inner wheel as the weight of the car is transferred to the outside - and if you apply full throttle now, the inner wheel starts to lose traction and spin, the DSC cutting in to prevent this, and you get sort of a hickup-driving-style. That's not fun!
You can't determine the curve radius with your throttle, you can't drift, and most of all you can't properly apply the power of the mighty N54 to the road. I experienced that on numerous occasions during my trip to the famous Nürburgring in August 2009 and by driving through mountain passes in the Italian Alps, and this led me to the conclusion that a limited slip differential
(LSD) was definitely needed here.
What does it do?
An LSD senses that a wheel loses traction and transfers the torque to the wheel with more traction. In the example above, if you corner with an LSD, it will automatically transfer the engine power to the outer
wheel which is under load and therefore has more traction, enabling the car to apply more power to the road. Such differentials are also called "torsen" (torque sensing) or "automatic torque biasing" (ATB) differentials. They can lock up the normally open differential until 80% (a complete lock would be too dangerous) to enable the torque transfer from one wheel to the other. The effect is typically progressive, making it easy to drive.
A) Limited slip differential:
There are several limited slip differentials available for the 335i - e.g. from Drexler Motorsport
, Quaife Engineering
. Drexler is a "plate type" LSD that uses clutch plates to do the torque vectoring, whereas Quaife relies on gears for their operation. The advantage of the latter technology is that, as there are no clutches that are used, such LSD are almost maintenance free, "fit-and-forget" solutions. For me that was the decisive factor, as I did not really see fundamental advantages of a clutch type LSD (except in motorsport which was irrelevant for me). So then, Quaife or Wavetrac? There's a lot of discussion going on on this forum about the advantages of one over the other. Wavetrac is supposed to work even if one wheel is completely off the ground - but that seemed a rather irrealistic scenario for my driving style, so that argument was of no concern for me. On the other hand, Quaife had a long-standing track record (decades, really) and first rate reputation, in particular for BMWs, and I knew several forum members personally who had it installed in their cars. Furthermore, Wavetrac didn't have an option for a welded differential like mine (I later learned that there may be a workaround, but I wasn't interested in a solution that wasn't perfectly established), so the choice was easy in the end - Quaife.
Now, where to obtain it? If I was living in the US, I would have gone through HP Autowerks
as I had already dealed with Harold in the past and was quite satisfied with his services. Being in Europe, however, I was recommended Birds in the UK
as they are the European distributor of all Quaife differentials and have unmatched experience in this sector. As I wanted to have other modifications done at the same occasion and wanted to benefit from their experience with BMWs, I combined the installation of the differential with a nice trip to London where I hadn't been for a long time - killing two birds with one stone! You can look up their prices here on their website
- due to my welded differential I needed a final drive unit which cost me (installation included) around 1800 EUR. I also thought about obtaining an LSD with a shorter gear ratio which is available as an option (for quicker acceleration), but in the end I thought that it was unnecessary as I'm not into 1/4-mile-racing and wanted to retain a high top speed (unlimited Autobahn!
So, last February I took the ferry over the Channel and drove on to Birds whose shop is located in London, not far from the Heathrow airport. They're really nice guys, and I got shown around their shop where several cars were in surgery, the most impressive being a conversion of a Z8 from left- to right-hand drive (wow!
). I was also shown what the LSD looked like and where under the car it would be installed (as I don't really have on-hand mechanical knowledge). - The installation itself was routine for them, and the new LSD went in without any problem.
The decision on how to upgrade the suspension was on several levels more complex. Springs? Dampers? Sway bars? Coilovers? Which type of each? I had test driven the Bilstein B16 Ride Control coilover
on a similar car to my own last summer in South Africa (thanks to my friend Charles! here's his review
) and experienced it as a passenger on Tony's car (see his review under this link
), and had been very favorably impressed with this coilover. However, it also was rather expensive (2000 EUR without install), and together with the LSD that was slightly out of my budget at that time.
In the meantime, however, I had discovered that it is possible to transfer a certain number of components of the M3 suspension onto our cars, thus integrating parts of the superior handling characteristics of the M3 onto the 335i. In particular, this link about various M3 components
as well as this review
had been quite helpful and instructive in this matter. These parts were somewhat less expensive overall and could also be combined with different springs, dampers or the Bilstein B16 coilover at a later date. Also, none of these upgrades (except the sway bars) exist as aftermarket items, making them even more desirable and indispensable if one is really serious about increasing the handling capacity of the 335i. Another plus is that as they are OEM items, so almost no one will be able to tell that they're not stock - something which is rather important to me as my car needs to go through the TÜV inspection at some point in time.
It was mentioned by those who already had these pieces installed that while mounting the LSD, it makes sense to install other parts for the rear axle at the same time, in order to avoid duplicate work later on. It seemed therefore obvious to me that I should have at least all rear axle items installed (rear subframe bushings, rear sway bar, rear guide rods, rear upper links); but (yeah, the mod bug got to me…) in the end I just thought "why not do all the rest too if the car is on the jack anyway?" and added the front axle items as well (sway bar, tension rod, lower wishbone). However, I left out those that needed different dampers (rear lower camber links), as I wanted to change them at a later stage (see above).
I somewhat hesitated as far as the sway bars were concerned, as Birds recommended the Hartge sway bars instead of the M3 ones, the reason being that the M3 ones still induce some understeer while the Hartge ones are stiffer and provide a tendency for mild oversteer. However, I had driven an M3 and found it very well balanced, and a more or less neutral steering appealed to me as I do not want to pretend to have sufficient driving skill to counter any sudden movements from the rear end. Also, the Hartge sway bars seemed excessively expensive to me (around 730 EUR = almost 1000 USD), and I really had to set a limit somewhere.
I obtained all items except the sway bars from HP Autowerks
, as I could then be sure not to miss any vital part. However, I have in the meantime tried to put together a list of all parts and part numbers that were used, as a means of reference. Here it is, along with some explanations for each part (some borrowed from the HP website) - no guarantee is given, of course, and these are the parts for an E90/E92 (most words in brackets are the German words for each part):
1. Front anti-roll bar / sway bar (Stabilisator vorne)
Diagram see here
31352283515 (Stabilisator vorne)
31352283516 (Gummilager Stabilisator Unterteil, 2x)
31352283517 (Gummilager Stabilisator Oberteil, 2x)
31352283037 (Haltebügel Stabilisator, 2x)
31352283441 (Pendelstütze vorne links)
31352283442 (Pendelstütze vorne rechts)
07119904295 (Bundmutter selbstsichernd, 4x)
33326768884 (Sechskantbundmutter, 4x)
That's what it looks like:
2. Rear anti-roll bar / sway bar (Stabilisator hinten)
As delivered the E9x 3 series has excessive under steer and limited roll control. The M3 rear sway bar increases rear roll stiffness by reducing mass transfer forces in corners. That should give the car crisp, quick turn-in response and reduce understeer, making the car feel more planted. M3 anti bars give the driver the ability to rotate the car on corner entry and steer with the throttle when necessary. It also makes the suspension (front or rear) stiffer, which will reduce the grip.
Diagram see here
33552283655 (Stabilisator hinten)
33552283709 (Gummilager Stabilisator Unterteil)
33552283710 (Gummilager Stabilisator Oberteil)
33552283714 (Haltebügel Stabilisator, 2x)
33556764428 (Pendelstütze, 2x)
07119906077 (Zylinderschraube, 4x)
07119903931 (Sechskantschraube mit Scheibe, 2x)
33326768884 (Sechskantbundmutter, 2x)
That's what it looks like:
3. Tension strut / rod (front) left+right (Zugstrebe Vorderachsträger)
That's what they look like:
4. Lower wishbone / control arms (Querlenker Vorderachsträger)
These add 0.75 degrees of camber, an alignment of the suspension after the install is therefore mandatory. A different xenon light regulation rod is needed (provided in the HP kit).
37142283867 (xenon regulation rod)
That's what they look like:
5. Rear subframe bushings (Gummilager Hinterachsträger)
The soft stock rear subframe bushings are replaced with stiffer, high performance bushings for a more predictable handling and more control.
33312283382 (front, 2x)
33312283383 (rear, 2x)
That's what they look like:
6. Rear guide rods (Führungslenker Hinterachsträger)
Original guide rods were made to deflect under load, a bad thing for good handling and traction. The M3 guide rods are made of all aluminum, a lightweight component thereby reducing wear and tear on other, more critical parts (rear subframe, control arm bushings etc). Each guide rod weighs just over 1.5 lbs making for a total of ~3 lbs for both parts (stock guide rods weigh 2.1 lbs each). Bushing deflection with a rubber material at one end is replaced by a sealed joint for deflection and noise free operation. Bushing deflection is unwanted because it leads to excess suspension movement. This is bad for handling and traction due to constant camber and toe changes. Plus, any power from the engine can take longer to get to the ground because it has to windup the bushing first.
Diagram see here
That's what they look like:
7. Rear upper links / wishbones (Querlenker Hinterachsträger)
Original upper links were made to deflect under load, a bad thing for good handling and traction. The M3 links are made of all aluminum, a lightweight component thereby reducing wear and tear on other, more critical parts (rear subframe, control arm bushings, etc). Each link weighs just over 1.7 lbs making for a total of ~3.4 lbs for both parts (stock guide link weigh 2.5 lbs each or 5 lbs for both). A weight savings of over 1.5 lbs from the rear suspension.
That's what they look like:
Here's also a photo of all parts before the install:
The installation procedure of most items was (pursuant to Birds) very straightforward, in particular the tension rods, control arms, rear guide rods and rear upper links were really easy to do - take out the stock part, put in the M3 part, basically plug&play. The rear subframe bushings are a bit harder to do, apparently some force is needed to squeeze them in. Due to the LSD that was being installed, the exhaust had to be lowered anyway, so that access to the bushings was provided for. An alignment was done afterwards (this is a must
due to the different camber induced by the lower wishbones in the front!), but no complete KDS (there was not sufficient time).
Here are some photos of the installation so you see where at least part of the pieces ended up:
Comparison of stock and M3 subframe bushings:
Unfortunately, it proved somewhat difficult to install the sway bars: Pursuant to Birds, the M3 sway bars did not fit into the OEM endlinks, and the bushings for the sway bars that I provided supposedly did not fit either. For the rear sway bar, Birds therefore fabricated custom bushings by re-machining the OEM bushings; but they gave up on the front sway bar. Now, as several here on this forum have confirmed, everything does
fit, even though it's a tight affair and you may have to apply some force to get everything in place. Fortunately for me, my local shop in Germany where I have had all my other modifications done, Daum Motorsport
, managed to get the front sway bar installed. I must admit that I was somewhat disappointed with Birds over this (also because they charged me extra for the re-machining which I assume would not have been necessary), but am nevertheless glad it worked out in the end.
Now, let's come to the part that certainly is of most interest to you - how does it drive now? I held off with my review for some time as the weather was really bad and I wanted to spend some time on a race track - the famous Nürburgring - with the modifications to evaluate the changes properly. Here are two photos from the two days during the Easter week-end that I spent there:
Even though the weather was not ideal, I was able to get almost 15 laps done (around 300 km), and have also driven an additional 3000km on normal roads. Immediately upon taking delivery of my car from Birds, the change was very noticeable. The car felt much more planted, body roll in corners was substantially decreased, and it was much more responsive to any steering input. Although even in stock form I didn't have any serious complaint with the steering response (in particular compared to some other cars I've driven in the meantime…), the car felt much sharper, more awake when going round corners, and any slight change in radius was immediately transferred to the road. Combined with the active steering I have, it is really much more fun to drive now! Going around the 'Ring, the decrease in body roll was also noticeable, the car also felt much more stable at high speed cornering (there are some bends on the 'Ring where you are faster than 150 km/h). In general, you have the feeling that you are more connected to the car and, through it, to the road than before. There is less of a "filter" that delays your input and the car's feedback. It now feels more like a sportscar than it did before.
B) Limited slip differential:
The LSD was the most cost-intensive modification I had done so far on my car, and I was therefore quite curious to see whether it was worth the expense. First of all, it's completely noiseless, I could never hear any sound whatsoever coming from the differential, under any circumstance and load condition. I attribute this to the excellent install done by Birds.
As to driving - while driving normally, you don't feel anything different. But as soon as you push the throttle harder, there's no blinking christmas tree in the dashboard, no power cut off through the electronics, just acceleration pushing you into the seats. Under dry conditions, with good tires and off the race track, it's much more difficult than before to get the tires to spin - in a straight line, that's basically only possible in first gear or in second but at very high revs. The biggest difference can be felt in corners: Whereas before traction control cut power and the car hobbled around, now it just zooms along and you can feel how the torque is transferred to the outer wheel. It practically grips the road and pulls the car along, and long bends or motorway ramps are much, much more fun now. That's as it should have been as a factory car, in my opinion. I also believe it's safer to drive, as the situations where suddenly and unpredictably the DSC cuts your power and you're left with a car that doesn't accelerate as it should will be much less frequent.
Of course, the LSD can't defeat physics - when it's wet and the road slippery, wheelspin can still be induced easily, but that's to be expected with a 400+hp rear wheel drive car. However, wheelspin and traction control intrusions happen to a considerably lesser extent. I was able to experience that on the Nürburgring as it was mostly wet there last time - without an LSD, it would have been close to undriveable, with it I could still go quite fast around the corners, as long as I avoided second gear and full throttle in third.
• Problems / disadvantages?
For the LSD, the price tag is probably the first deterrent for most people, at least if they lease their car. If it's your own car, however, it's definitely worth it and I would not consider the price performance ratio to be bad. For installation I would recommend a shop with experience, and that can sometimes be slightly difficult to find; in Germany, for instance, Evotech distributes and also installs Quaife differentials.
As far as the M3 suspension components are concerned, ride comfort will be slightly (and I mean slightly) decreased. This is due to less cushioning in the front, so that you'll get more feedback from the road through the steering wheel; also, the stiffer rear subframe bushings (and upper links + wishbones) lead to a firmer rear suspension, road imperfections will be felt a bit more than before. It's not much, though, comparable to switching from 18 inch tires to 19 inch tires. - Lastly, I also noticed that (probably a consequence of the stiffer sway bars and the LSD) if the rear end slides out, it does so less gradually than before and you have to react quickly, even with DSC turned on - but that usually only happens if you drive like you should only drive on the track, and then you're supposed to know what you're doing.
As a summary, I can say I'm very, very pleased with these modifications and they have transformed the car in a lot of ways. They contribute to my driving pleasure each time I drive a bit faster on curvy roads, and I can sincerely recommend both to anyone who's remotely interested in making his car quicker and more nimble. Thanks also to Tone and Charles for their invaluable advice and help. - Now I'm looking forward to seeing how the Bilstein B16 Ride Control coilover which is just being installed will further change the driving characteristics of my car!