I can’t load a bunch of jhm’s pages at work for some reason. if someone wants to fill in the rest I’ll update the post.
b6 3.0 stock rotor weight:
front - 19.5lbs
rear - ?
B5S4 stock rotor weight:
front - 21lbs
rear - ?
b6/B7 s4 stock rotor weight:
front - 25lbs
rear - 13lbs
JHM lw rotor (321mm)
front - 16lbs
rear - ?
JHM lw rotor (330mm)
front - 19.5lbs
rear - ?
JHM lw rotor (345mm)
front - 19.5 lbs
rear - ?
A good comparison as a point of reference:
-Rotor OD: Stock = 312mm(12.3"), JHM = 330mm(13") - .7" larger
-Rotor Thickness: Stock = 25mm(1"), JHM = 32mm(1.26") - .26" thicker
-Rotor Weight: Stock = 19.5lbs(each), JHM = 19.5lbs(each) - same
-Rotor Cooling Air-gap: Stock = 8mm(.315"), JHM = 15.5mm(.610") - LARGER for better cooling
-Rotor Cooling Center Fin Design: Stock = Straight Fin, JHM = Airfoil - MORE EFFICIENT for better cooling
-Bolt pattern: - SAME (fits like an OEM rotor would)
Another factor with our brakes that an Audi tech pointed out to me, for the rears the stock ones aren’t vented, so while it wouldn’t do much for weight it would improve braking characteristics to switch to vented rears.
From what I gather a b7 a4 or b6/b7 s4 caliper with a 321mm rotor would be plenty for us. that caliper plus jhm’s 2 piece lw rotors for the b5 seem like they would be a good solution, both in performance and weight reduction, also more cost effective than a BBK or b6/b7 s4 alternative.
I am unsure of other details on the rear brakes, or how brake bias would be affected or what would need to be changed. Read too much misinformation online to comment on it, perhaps someone else can chime in on it and I’ll update this post accordingly
As far as rotors go, I don’t think we have any lightweight options available for us. The only ones I see readily available after a quick google search are JHM’s rotors. If we upgrade to an S4 brake set and use the l/w rotors we are probably breaking even as far as weight is concerned. Much more stopping power, but the same rotational mass.
I think a good idea would be for us to post up good wheels and tires to look into.
TSW Nurburings are cheap and light iirc
Forgestars are light/cheap
if you are willing to spend a bit more, Enkei, OZ, SSR and Volk are great options. I’ve had OZ Ultraleggeras and SSR Comps and they are both very very light.
For tires:
I love my Hankook RS3s but they are a bit heavier. I was willing to trade weight for grip/price. Normally Continental makes the lightest tires in my searching around.
yes, I read about those ones awhile ago but forgot the names. I like the forgestar f14’s, similar to the nurburgring’s. Either one would look awesome in a gloss black… mmmm… Hurry up JHM before I start buying wheels!
Well having to upgrade to the S4 caliper to get LW rotors isn’t really a bad thing at all. Our stock brakes aren’t anything to write home about and the S4/B7’s caliper is great. Also the caliper isn’t rotating, just the rotor, so really you’d be adding dead weight for the caliper itself. And I’d imagine the weight of the JHM LW rotor is probably still less than our stock 3.0 rotor.
I believe there is another variable of “unsprung weight”. I suppose this would be anything that does not add to the weight on the springs? I could be off base here though.
The weight on a string analogy is interesting to think about but that doesn’t take into consideration the friction/drag from the overall weight of the car. The motor still has to move all that weight forward.
I’m having a hard time trying to understand how it really works.
It would definitely be easier to turn the wheels if the car was floating, but it’s not. This is where I am sort of confused.
I SO BADLY want to believe that if I save 50 lbs in a rotor, wheel, and tire combo that it will feel like 200lbs! I’m just skeptical.
This is one of the posts from the Bimmer Forums. Does anyone care to try to decipher it? haha
Assumptions:
M3 Curb Weight (except wheels): 2950 lbs
M3 Peak Wheel Force in 1st Gear: 2830 lbs (actually doesn’t really matter, but just to make realistic numbers. I’m assuming stock wheel sizes and gearing, and 200 lb/ft pk torque after drivetrain losses)
All wheels same size, running 225/45-17" tires. Wheel+tire radius R=1.04ft.
No friction forces (wheel bearings, etc)
No wheel slip
Wheels+tires have evenly distributed mass (uniform disc).
Math:
Angular acceleration of wheel = aa.
Car acceleration = A = aa*R
F (tire-to-ground) = mass(M3) * A + I(wheels)aa
= mass(M3) * A + I(wheels)(A/R)
= (mass(M3) + I(wheels)/R) * A
Or otherwise
A = F / (mass(M3) + I(wheels)/R)
Moment of Inertia of 16lb wheel + 20 lb tire:
I = mr^2 = (36/3.215)*(1.04ft^2) = 12.111
Moment of Inertia of 24lb wheel + 25 lb tire:
I = mr^2 = (49/3.215)*(1.04ft^2) = 16.484
Mass of M3 = (curb weight) / G
With 16lb wheel combo: = (2950+ (364)) / 3.215 = 962.36
With 24lb wheel combo: = (2950+ (494)) / 3.215 = 978.54
With the assumptions used, lighter wheel+tire combo gives 3.1% acceleration improvement. Equivalent to dropping a 6.2s 0-60 to 6.0s.
If stock M3 is then 3146 lbs (24lb wheel+25 lb tire combo) then 3.1% weight reduction is 97.53 lbs. Actual weight reduction was 52 lbs. So in this case, 1lb of removed rotational weight is equivalent to removing 1.87 lbs from the body. That’s not even close to the 7lb number tossed around. Though the assumption of even distribution of wheel mass does skew this number down some.
I crunched the numbers yesterday (and didn’t save the document, doh!), I’m pretty sure our rotors are almost the exact same weight as the lightweight s4 ones. I actually don’t mind my brakes, heck of a lot better than the stock 1.8 ones… would be nice to have bigger rotors though.
I read through the bimmer forum post too… That long equation you posted is interesting, I don’t follow 100% as my math is a bit rusty, but it’s not quite applicable to how an actual wheel works. There’s a lot more variables that play into it. What I get from it in laymans terms is like Count said, they make a big difference in off the line and from a 30mph roll, which is the situations most of us will find ourselves in.
I’m basically going to go by whatever Count, Beemercer, and Justin say. They are much more knowledgeable about this than I am.
I agree that I will believe what those guys say. I have no doubt they know what they are talking about. I just wish there was a way to see or calculate the results of lighter wheels or rotors.
Thanks I put lots of testing when I first got my rotors. Countvohn and his explanation was a great example of exactly what’s happening.
Brsox I know you mentioned drag and car weight but those are som things that are not directly affected by the rotors so they are omitted from the example. That and its just a example
Math wise there isn’t one straight calculation the BMW calculation was nice but not thuro as the weight reduction will be more pronounced under certain situations
i tried posting this in AZ. i was watching trucks on spike tv and they were working on project rolling thunder. according to them aluminum driveshafts (http://www.dennysdriveshaft.com) where the best way to save and gain more power from the motor to the wheels. i’m sorry if that does not make sense lol. but thats how i understood it.
lightweight drive shaft = a better trasfer of power from motor to wheels.
that combined with JHM’s tune and the gears to make the car more of a RWD car i think would be a great combo. plus of course LW flywheel, and LW rotors.
CB hood and trunk ? taking out spare tire? and my monstrous SUB box?
The important factor is unsprung weight. This is why Audi’s control arms are all aluminum- to reduce unsprung weight.
This is a direct correlation to better handling.
The lightweight driveshafts help because of lighter rotating mass (just like lightweight wheels).
Carbon fiber driveshafts are available as well.
Lightweight wheels reduce rotating mass & unsprung weight.
I did notice that OEM Audi wheels are really heavy. I have aftermarket wheels and they are noticably lighter than the OEM ones- by a lot. Driving response & acceleration was noticably improved and I actually gained 1-2mpg’s on my stop-n-go commute.
1lb of rotating weight is much more severe than 1lb of sprung weight.
Reducing unsprung weight is much more noticable than weight reduction on the body. However, the body is still a big mass that the suspension has to handle and pull/stop and those weight reductions pay in slalom, braking and acceleration.
Heavier wheels tend to want to stay where they are pointed- so this may be good for a more luxurious, planted feel
I wish my remote control car was working. I would rig up a little science experiment. The damn control arm broke and its old so they don’t make the parts anymore. It wont drive straight. :’(
Its been said a few times but there is no direct mathematical calculation. The weight loss benefits on rotational components is going to be much larger then on just standard weight loss from random parts.
The benefit of the performance gain will be from the parts that extend further out from the central drive of the motor. Also what kind of sucks is that altho there is a difference in overall drive performance this isn’t something that can be measured by a dyno measurement tool. As a dyno is more one domential
I was wondering if we could get a list of all rotational mass parts that are available in a light weight version and the Price of each of them. And maybe also a list of other parts that are non- rotational as well. Maybe include how many lbs. each part drops and then maybe a scale of 1-10 or something on how much of a gain it will have and list them all from greatest to least in whatever category you guys want. Just brain storming here and throwing out ideas.
What do you guys think?
