i lost some weight - input on unsprung vs sprung (and rolling inertia)

**muffinman** · 10-23-2008 11:27 PM

i just recently "downgraded" from 19x8.5" RS4 reps wrapped in 235/35/19 Bridgestone Potenza RE960AS to 18x8.5" BBS CH's mounted on 225/40/18...

... one of the bigger reasons i made the switch was because i thought i could drop some significant rolling weight.. (the other was to reduce rolling inertia aka less resistance = faster acceleration, quicker braking)

so here's the specs:

19x8.5" RS4 Reps - 27lb
235/35/19 Bridgestone Pole Position AS - 26lb
total: 53lb

18x8.5" BBS CH's - 24.5lb
225/40/18 BFGoodrich Super Sports - 23lb
total: 47.5lb

so that's about a 5.5 lb of unsprung weight PER CORNER, for a total of 22lb of unsprung *rotational* weight savings

so my question is, how many pounds of SPRUNG WEIGHT is that equivalent too? i've heard people say 1 lb of unsprung weight = 10lb of sprung weight

my butt dyno tells me i'm accelerating noticeably quicker, braking distance has been decreased, and thus far, my gas mileage seems to have slightly improved.

thanks

**Sharkfin** · 10-24-2008 02:41 AM

haha I can't answer the numbers question for you but I can tell you my new 20" BBS CKs are heavy but I love the look lol

**positiv_rate** · 10-24-2008 03:17 AM

Hello!

As you can see in my sig, I'm running the exact same wheels (good choice btw!). I too was concerned about weight, even though I find 19" complements the B7 chassis much better, I went with the more conservative size. Also largely due to our bombed-out roads here in Montreal (winters are really harsh with our roads with constant freezing/thawing cycles), I felt it couldn't hurt to have a little more rubber between the road & the rim.

As far as unsprung weight goes, I've read the same info you've heard. Bottom line: lighter is better. But then I went & put bigger/heavier S4 rotors up front... hmmm... I guess it would be 11lbs worse if I was running 19"s!

Hope you're happy with your choice. Happy motoring!

**Audized** · 10-24-2008 03:24 AM

looking great! I did the exact same thing, going from 19" RS4 reps to 18" BBS CH in March and never regretted the move. Those 18's look like 19's with CO's. That makes me crave for some CO's on my car so my 18's don't look like 17's on stock sport suspension.

It would be tough to calculate the exact sprung weight equivalent to the decrease in unsprung weight as the wheel/tire designs can play a significant role in the calculation. PM sent!

**muffinman** · 10-24-2008 04:16 AM

thanks autumn - i was never really a big fan of bbs ch's until i saw them in person on your a4

according to that spreadsheet (if i'm analyzing this correctly), i'm saving around chassis weight equivalent of around 188~200lb

do the horsepower/torque figures matter? i put in 235/278, but i'm not sure if they're asking for crank or wheel

**muffinman** · 10-24-2008 04:20 AM

Originally Posted by positiv_rate

Hello!

As you can see in my sig, I'm running the exact same wheels (good choice btw!). I too was concerned about weight, even though I find 19" complements the B7 chassis much better, I went with the more conservative size. Also largely due to our bombed-out roads here in Montreal (winters are really harsh with our roads with constant freezing/thawing cycles), I felt it couldn't hurt to have a little more rubber between the road & the rim.

As far as unsprung weight goes, I've read the same info you've heard. Bottom line: lighter is better. But then I went & put bigger/heavier S4 rotors up front... hmmm... I guess it would be 11lbs worse if I was running 19"s!

Hope you're happy with your choice. Happy motoring!

ouch! 11lb... but those S4 brakes are so BALLER

i agree that 19's fill out our fenders alot better, but 18's with low-pro tires on a dropped A4 doesn't look bad at all. i'm actually running 225/40/18's but even that, in conjunction with the lighter weight 18" CH's, has drastically improved my ride quality. i'm loving the increase in acceleration and braking performance

i've been tracking my gas mileage via www.mymilemarker.com for the past year and a half - made a note to see if my gas mileage improves over the next few weeks.

**muffinman** · 10-24-2008 04:26 AM

Originally Posted by nsmith12

haha I can't answer the numbers question for you but I can tell you my new 20" BBS CKs are heavy but I love the look lol

CK's are hot - where are the pics??

haha, i was all set to buy a set of 19" BBS CK's in gunmetal... but the weight really turned me off

**Audized** · 10-24-2008 11:43 AM

Originally Posted by muffinman

do the horsepower/torque figures matter? i put in 235/278, but i'm not sure if they're asking for crank or wheel

They are asking engine HP and TQ, but since we have quattro the drivetrain loss is so significant that it can not be neglected. I would put in AWH and AWT for the best approximation. As a matter of fact, HP does not make a difference here as torque is what's used to rotate the wheels. BTW, I fixed a mistake found the original template this morning. Check out the link I sent you yesterday.

**Quattro** · 10-24-2008 11:50 AM

off topic: BBS FTW!

**TheHeggi** · 10-24-2008 12:51 PM

I've heard 1lb= 7lbs I think, it was either 7 or 10lbs. Either way, it will not make a "significant" difference. It might be slightly noticeable, but not a sole reason to make the switch. Should be just slightly more noticeable than a CAI on a Civic...you know it's there so you try to feel the gains, but it's less than you think haha

**Charlescrs6** · 10-24-2008 01:48 PM

Originally Posted by TheHeggi

I've heard 1lb= 7lbs I think, it was either 7 or 10lbs. Either way, it will not make a "significant" difference. It might be slightly noticeable, but not a sole reason to make the switch. Should be just slightly more noticeable than a CAI on a Civic...you know it's there so you try to feel the gains, but it's less than you think haha

haha nice comparison