You asked for it, you got it. The suspension of our 2008 BMW 135i lays bare before you -- sort of.
BMW has proven that the simple strut front suspension doesn't have to be an instant ticket to mediocrity. A front engine, rear-drive powertrain layout helps a lot, of course. And so does a forward-mounted steering rack, hinted-at here by the backside caliper placement.
BMW is trying to cram 10 lbs of suspension in a 5-lb wheelwell, in a manner of speaking. You have to get creative when you put a big engine in a small car. And creative here means convoluted. Just look at that aluminum lower link! It twists and turns to make clearance for the steering tie-rod (green), the tire at full left lock and the stabilizer bar (red).
Econoboxes and budget-minded front-drive cars can't afford what it takes to maximize a strut layout. They usually have one-piece lower A- or L-arms, a relatively large positive scrub radius*, a propensity for torque steer and steering kickback, and lackluster steering feel.
*Scrub radius = the distance between the center of the tire's contact patch and the imaginary point where the steering pivot axis meets the road.
But the 135i has zero torque steer because it's rear-drive. And the high price they charge for BMWs means they can invest more to make the strut work better in the steering department.
You'll notice that there is no one-piece lower arm in sight. Instead, there are two aluminum lower links (yellow and blue) and two lower ball joints. When steering (green), you get an odd monkey-motion at the ball joints because steering pivot axis moves outboard to the imaginary point where the two lower links would intersect if they were extended out. This arrangement can result in a zero (or even negative) scrub radius if the engineer so desires.
All it takes is money.
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Speaking of money, check out these massive 6-piston Brembo calipers. They've got two solid bridges for extra rigidity (the word Brembo appears on one), so the pads aren't as easy to change as those on our Subaru STI. Instead, the caliper has to come off the car (yellow bolts) so the pads can be loaded from the other side.
Brembo adapts basic off-the shelf caliper designs to numerous applications, so it's no surprise that two large mass dampers (green) were necessary to tweak the resonant frequency of the caliper for this application.
Too bad the hub doesn't have some sort of paint or anti-corrosion coating. How about you guys spend a few extra cents here? Jeez. And we see zero road salt in California. I had to thump the tire a little bit to break the rim free.
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The rear is harder to see, and much less spectacular. For one, everything is made of steel. And the entire rear suspension attaches to a rubber-mounted subframe, a nod to comfort and noise isolation.
This is a multilink design with 5 links. As with the 370Z, the lower link (galvanized steel, indicated by yellow) has been spread wide to act as the lower spring mount. And here it serves as a lower shock mount, too. A lack of space results in an upper link (green) that is tightly packaged above it, undoubtedly making for some high loads. The lower end of the stabilizer bar link (red) attaches directly to the hub.
But that slender rear stab bar doesn't look too efficient to me, owing to a convoluted shape and a long separation between the support bushing (upper green) and the end link (lower green). This car does understeer more than we think it should. This bar looks like it does so little that I wonder if it would make much difference if I secretly unbolted it?
The upper links are indicated by yellow and red (OK, I'll stop recycling colors).
Here's the 5th and final link (sea-foam blue). The rear brakes are 2-piston fixed units, but check out the four mass dampers riveted straight to the brake pad ears (desert-sand mustardseed yellow). Someone had a brake squeal problem during prototype development. Don't expect your 135i brakes to remain quiet if you switch to aftermarket pads.
Kudos to the 135is obvious and pre-padded jack points. All four of these babies are easy to spot and use.
But I've never been a fan of lug bolts. I like hanging the wheel on the studs before I install lug nuts. And nuts are usually made to fail before studs, so the weak link is easily replaced. What say you?
Dan Edmunds, Director of Vehicle Testing @ 20,687 miles
jjacquot says:
10:54 AM, 04/23/09
OK, Mr. Technical. Since you seem to know your suspension jargon how about this: What's the name for the spot where the steering axis hits the ground?
audisport says:
11:00 AM, 04/23/09
Very nice little suspension lesson. Thanks!
arm51 says:
11:02 AM, 04/23/09
I love the posts explaining the technical underpinnings of cars that we rarely get a look at! As for lug bolts\nuts, I'd take the nuts any day. I prefer to hang the wheel on the stud as well and an easily replaceable single point of failure is good.
ampim says:
11:09 AM, 04/23/09
I'm thinking the rear roll bar was there to improve..um..marketing. It wouldn't be the first time a roll bar has made an appearance to appease the marketers. Then again, they probably designed one in there from the start just so they'd have one more knob to tweak when it comes time to tune. Then again, maybe it’s what keeps the axle from tramping. I say un bolt it and find out!
vvk says:
11:09 AM, 04/23/09
Thanks, Dan. It would be extremely interesting to see your analysis of E46 M3 suspension before you guys get rid of it. Please?
mihai says:
11:12 AM, 04/23/09
>> What's the name for the spot where the steering axis hits the ground?
The Dave point!
mentally says:
11:13 AM, 04/23/09
You do know your stuff don't you.
dougtheeng says:
11:28 AM, 04/23/09
The arrows were most helpful.
saxpride52 says:
11:31 AM, 04/23/09
Where's Dave Coleman :( Your garage is getting nice and big, maybe you should pick him up :)
So when are you guys going to try a new tire for the 135i? Sumitomos!
jjacquot says:
11:31 AM, 04/23/09
Mihai beats Dan to the punch:
http://www.modified.com/editors/technobabble/9909scc_technobabble/index.html
Josh
chavis10 says:
11:32 AM, 04/23/09
The higher the scrub radius that larger affect torque steer has on the helm. Audi's multilink front suspension had an ideal virtual steering axis connection point(with a scrub radius of zero) which is why they never have torque steer (even the FronTrak models).
Unfortunately, most fwd vehicles don't have a similar design because of the high cost.
joefrompa says:
11:47 AM, 04/23/09
I didn't realize the 135i used steel in the rear, that's dissapointing considering the price point of the car.
I like the clearly identified jacking points, but I don't like those padded locations. You need to use a jack adapter to use those properly. Correct me if I'm wrong about that.
These technical discussions are fantastic, because they answer the age old discussion about "Why is this car expensive compared to car Y? The interiors are the same, they have the same power, etc. etc."
If it wasn't for BMW's suspensions (and a few other bits), I'd just be an Infiniti G35 fan. But the BMW suspension just feels completely unique to me, when putting power down or cruising on bumpy roads.
Thanks Dan!
Joe
actualsize says:
11:54 AM, 04/23/09
Re: joefrompa's jack concern...
No trouble, no adapter. I've got a big 3-ton floor jack with a standard 4- or 5-inch dish shaped lifting pad. It slips right under, and the jack points are tall enough that my jack stays clear of the soft rocker panels without my usual 2x4 spacer.
subytrojan says:
11:55 AM, 04/23/09
Awesome blog entry, Dan! :o) :thumbsup:
I vote for a suspension/brake walkaround on an Evo next!
Lug bolts FTL. Lug nuts FTW! :o)
canadaphant says:
11:57 AM, 04/23/09
Again, excellent work-keep it up please!
chavis10 says:
12:21 PM, 04/23/09
"These technical discussions are fantastic, because they answer the age old discussion about "Why is this car expensive compared to car Y? The interiors are the same, they have the same power, etc. etc."
I believe Infiniti's FM (as well as GM's sigma) have aluminum rear suspension pieces and we know they cost more than their steel equivalent so I wouldn't say that a BMW chassis costs more than the competitors. They are probably just tuned better. Plus, we all know that an aluminum intensive front SLA/multilink (as in the FM and Sigma) set up is more expensive than BMW's strut with double-pivot low control links, so...
Also interesting is how the 135 gets 6 pot brakes when even the M cars do without. What gives? I remember an engineer from the M shop said multi piston brakes have little to do with actual stopping distances which is probably true but he failed to mention the disadvantages when subjected to continuous abuse as seen on a track.
desmolicious says:
01:00 PM, 04/23/09
Please do a hard parts walk around on the SMART. I want to know what makes it so awesome.
mentally says:
01:24 PM, 04/23/09
Should've done this on the X5!!!!!!!!!!!
joefrompa says:
01:31 PM, 04/23/09
Chavis - AFAIK, Stopping distance is mostly determined by tires, suspension setup, and then brake hardware....since most brake setups nowadays can easily handle a single 60-0 stop.
I'm very impressed that the Cadillac CTS managed 119 feet on Michelin all-seasons. Those tires suck, so it's all that much more impressive to me.
I'm not trying to say that BMW uses the most expensive materials, just that the overall design yields a great experience and alot of engineering obviously goes into it (as well as some costly parts).
I love me some 50/50 weight distribution too, but BMW has been slowly losing that on more models.
jguan says:
01:32 PM, 04/23/09
Nice write up, maybe you guys could make YouTube videos of these posts too!
subytrojan says:
02:11 PM, 04/23/09
LOL @ your comment, Huss (desmolicious)!
Right on regarding tire grip being a primary factor in stopping distances, Joe.
Repeated braking is where BBKs (big brake kits) and really "good" brake systems shine (or are exposed) such as on a road course.
kevinlch says:
02:31 PM, 04/23/09
Thanks a lot again for a great great write up.
Hope more will come.
jazzy14 says:
02:32 PM, 04/23/09
You may be interested to know that the E9x M3 suspension pieces are all bolt ons for the 1-series. They have much stiffer bushings and are made of aluminum (instead of steel in the 135i rear). The M3 subframe bushings can also be used to firm things up. Keep in mind that the 1-series has run flat tires while the //M cars run standard tires. The 1-series bushings are soft to compensate for the stiff sidewalls on the RFT. The stiff M3 pieces with the RFT make the ride a little choppy. Ditching the (crappy) RFT after adding the M3 suspension pieces will bring things back to the same comfort level on the road while providing a better handling car.
chavis10 says:
03:14 PM, 04/23/09
good points jazzy.
benson2175 says:
05:01 PM, 04/23/09
BMW could have saved a few bucks on those calliper resonance weights by sticking on some lead wheel weights like on the Camaro. Of course that would be hick.
revmatch says:
06:55 PM, 04/23/09
Excellent job. I have enjoyed both of the suspension write-ups and would love to see this sort of in-depth walkthrough as part of your standard review format. Please keep these highly-informative posts coming!
jaguar36 says:
06:13 AM, 04/24/09
Wooo, thanks for the arrows, they make this a truly excellent post!
jkavanagh says:
10:03 AM, 04/24/09
Here's a nice solution to the 135i's wheel bolt problem... a stud conversion!
http://www.vorshlag.com/index.php?cPath=1_78_167
PBR says:
06:06 PM, 04/24/09
wheel bolts vs studs+nuts:
The wheel should center on the part of the hub that sticks out of the center of the brake rotor, at least the Bimmers (4) and VWs (1) I've had did. They're a little more annoying than studs in that on the front there's nothing to hold the rotor/hub while you try to match up the bolt holes in the wheel with the ones in the rotor/hub, If you don't match them up there's not much to do but pull it off and try again.
As for durability, when I had a VW GTi MkI I stocked up on bolts, figuring they wouldn't last long. Wrong, I never did manage to wear one out in 6 years of swapping tires for performance driving events and using up leftover tires thrown away by others. The best metal in that car was the wheel bolts, I guar-on-tee!
howardroark says:
01:11 PM, 04/25/09
Your comment about disconnecting the rear anti-sway bar to DECREASE understeer is HORRIBLE advice. You should be careful making statements like that which are patently false. Reducing the rear anti-roll bar will INCREASE understeer. It always mystifies me that sources like Edmunds, which hold themselves out as automotive experts, are so ignorant and misinformed. How can any of us take anything you say seriously when you bash BMW engineering with pure ignorance and misinformation? If someone were to act on your implied suggestion to disconnect the rear anti-sway bar it would be DANGEROUS. You guys ought to focus on improving your reporting of the available paint colors (you need help in that department too).
actualsize says:
02:49 PM, 04/27/09
@ howardroark: You misunderstood my point. The rear stabilizer bar in this case looks so inefficient in execution that disconnecting it might NOT make understeer any worse that it already is, even though that would be the expected textbook result. I think this car needs a more effective/efficient rear stabilizer bar to make it understeer less -- or a larger one.
jkp1187 says:
09:04 PM, 05/ 3/09
I've stripped two bolts in about 15 years of driving. Zero nuts.
And I'd also like to hear your opinion on Sumitomos for the 135i.
morder says:
02:58 AM, 07/14/09
BEWARE!!!
You should NOT get off the caliper using the yellow bolts!!!
You have to un-screw the bolts connecting the caliper to the knuckle.
It is really important and something to pay attention...
Always with Brembo calipers having brackets connecting the aluminum body to the knuckle, you have to unscrew the bolts between bracket and knuckle
Bye and regards from italy
chikoo says:
12:58 PM, 09/16/11
So how hard is it for any other car company to replicate a BMW suspension and make their cars behave the same? It has been ages that "others" are trying to get as good as BMW. Maybe they need Dan and his jack stands in their reverse engineering department ....LOL