How much slower??

[choppers_rule]

Thinking about going 19", I have 18" replica AMG now.
My question is....how much slower in accelaration from 18" to 19" considering they are not the light weight wheels?
Because from stock 15" to 18" ....big difference!! I don't want to suffer too much on the power.

[vbrock1137]

Well, rotational intertia increases exponentially with increasing diameter so... probably a noticable difference.

But, those 18" replicas are probably pretty heavy, so if you got relatively lightweight 19"s (if such a thing exists) who knows.

I'm sure someone who's upgraded from 18 to 19" can share their experience.

[c55m8o]

I did some calculations and found the tires have a much more dramatic effect on the moment of inertia (the rotational equivalent of "m" in F=ma, known as "I") then the rims, since anywhere from the low 20s to 30s pounds of rubber (depending on what tires you choose and the width) is spinning out beyond the diameter of the rims, and as vbrock1137 correctly states, the moment of inertia increases with the square of the distance from the axis of rotation.

I did my analysis between 25bl 18" rims and 20lb 17"rims, using the same 225mm wide Bridgestone tires (one of the only ones that publish weight) with a 25" diameter, that weighed 25 lbs. Analysis Here (http://www.mbworld.org/forums/showthread.php?p=1385188#post1385188).

Some of those findings were:
a) the moment of inertia of the 17" tires was some 7.5x larger then that of the 17" rims. [!]
b) the moment of inertia of the 18" tires was some 5x larger then that of the 18" rims.
c) the moment of inertia of the 17" set [rims + tires] is 39.96 lb-ft^2 ; I of the 18" set is 42.35 lb-ft^2. That's a 6% increase from the 17" set to the 18" set.
d) 87% of the moment of inertia of the rims comes from the outer drum [!], where I was using a 5 spoke rims that has a weight distribution of the rim estimated as 25% to the hub, 35% to the 5 spokes, and 40% to the drum. (I have to research how accurate that approximation is.) This again, is because the effect of the weight increases by the square of the distance from the axis. As a result, just 40% of the weight of the rim ends up producing 87% of the rim's rotational momentum.

So, for 19", let try and estimate... Starting with:
- 17" set: tire's I = 35.3401 ; rim's I = 4.620 ; total I = 39.96
- 18" set: tire's I = 35.1909 ; rim's I = 7.155 ; total I = 42.35

NOTE: you'll notice tires don't change in "I" as much as the rims do, as I calculated a bit over 1/2 of the weight, thus inertia, is in the tread, not the sidewalls .. however tho the sidewall are 1/2" less, I surmise manufacturers must be adding more rubber to stiffen them up since those Bridgestones are rated the same overall weight .. creating an "I" of the sidewalls very close between the 18" and the 17".

Now getting back to the approximation, let's say the 19" rims are 30 pounds... If I were to estimate what the values would be for the 19" set, I would think it would be something like:
- 19" set: tire's I = 35.05 ; rim's I = 12.7 ; total I = 47.75

(when I turn on my laptop that has the formulas in the spreadsheet I made to do this comparison above, I'll copy the 18" line to a 19" line with all the appropriate weights and lengths, and calculate an accurate figure)

That would be an increase in moment of inertia from the 17" set of 19.5% for the 19" set. [!] ... so that says where the rotational momentum increases a modest 6% going up 5 lbs to an 18" rim and tire set from an initial 17" set, it increases almost 20% going up 10lbs to the 19" set.

How that affects your 0-60mph time, I haven't sat down with the right formulas to figure yet. Let's just say, most likely noticable. ;)

[choppers_rule]

Wow....Very well put......I love the slammed look with 19" but affraid of the power loss. Maybe I should replace my replica with a light weight 18". I don;t know...I'm confuseddddd:rolleyes:

[vbrock1137]

Daaamn... you really put some time into that c55m8o, I'm impressed. That sounds like something I'd be doing for one of my mech. engineering classes. :cool:

Hmm, yeah, you could calculate the amount of energy needed to accelerate the car (knowing the mass) from 0-60 in the amount of time it takes with 18"s (this time would be measured), also taking into account the energy needed to spin the wheels up to the speed they'd be rotating at 60mph. You could then simply plug in the difference in rotational inertia for 19" and solve for the new time.

Damn, wish my final exams were this interesting back when I was learning this stuff.

[choppers_rule]

Indeed....very impresive report:D

[c55m8o]

...follow-up post after plugging the values into my spreadsheet instead of the approximation... I got some corrections, some good news, and some bad news...

Correction 1: The tire's sidewall is not close to 1/2 the weight of the tire, it is from 1/5th to 1/7th the weight of the tire's tread depending on the tire's profile (x2 for the two sidewalls tho).

Correction 2: I found a problem with how I was calculating the "I" of the rim's hub and corrected it ; however the hub's "I " is miniscule when compared to the rest of the rim, so it had a tiny effect. The values for "I" I listed become:
- 17" set: tire's I = 35.3401 ; rim's I = 5.462 ; total I = 40.8025
- 18" set: tire's I = 35.1909 ; rim's I = 7.655 ; total I = 42.8458

difference: 5% increase in moment of inertia going to 18" (25lb) from 17" (20lb)


Good News: I overestimated the "I" of the 19" rim and tire set, in regards to the Rim! ;) Entering the values into the formulas returned:
- 19" set: tire's I = 35.1447 ; rim's I = 10.298 ; total I = 45.4427 (using a 30lb rim)

difference: 11.4% increase in moment of inertia going to 30lb 19" from 20lb 17" (not the almost 20% increase I thought it would be! :o sorry)


Bad News: The tires I used are not real! :eek: The tires I've been using for this comparison had a 225mm width, outer diameter of 25" or 25.1", and weighed 25lbs. However, Bridgestone doesn't actually make that tire in a 19". Others manufacturers may, or Bridgestone may in another line other then the Potenza SO-3 tires ... however, if you were doing this for real, in going to 19" you'd have to go to a 245 wide tire ... using a profile height that trys to keep the speedo close to accurate (still 2mph slow @ 60mph), the true weight of the tire is 28lbs, not 25lbs. (the sidewall is only 3.1" of rubber tho ... :eek: )

Now as has been mentioned already, the tires are what actually contribute the lion's share of the moment of inertia of a tire + rim set. Going to a 28lb tire, the resulting moment of inertia of the realistic tire + rims set you could put together in 19" with that tire becomes:

- 19" set w/28lb tire: tire's I = 39.3621 ; rim's I = 10.298 ; total I = 49.66

difference: 21.7% increase in moment of inertia going to 19" 30lb rims with 28lb tires, from 17" 20lb rims with 25lb tires... So you're back up at about the same percentage for the set anyway...:rolleyes: ;) ...but ... BUT ... find light tires, and going to 19" doesn't hurt so much! Example:

- 19" set w/23lb tire: tire's I = 32.3332 ; rim's I = 10.298 ; total I = 42.6311

difference: 4.5% increase in moment of inertia going to 19" rims with 23lb tires, from 17" rims with 25lb tires...

That's back to Good News IMO... :D

EDIT: It occurs to me some may ask, so ... if starting with a 25lb tire, 17" 20lb rim, the weight of tire you want to use when going to a my example 5-spoke 19", 30lb rim to negate the increase in "I" of the bigger rim, is a 21.7lb tire... ;) That way the 19" set has the same rotational inertia as the 17" set. However, don't forget, the total unsprung weight of the 19" set still went up a good deal! I'm simply talking about how hard they are to spin-up to speed here.

[iced]

well to tell u the truth, although i graduated from a scientific clas in high school, being the top of my class (not lying) ...i hated physics.... no i c y! b-coz they didn't include cars for sure!!!

man ,c55, i have a q for u, (i wanna test u..lol)
i m currently running at 15" stocks on 205/60 or 65 i dt remember,
i wanna buy some new rims along with tyres, i know that i can gain a lil bit of performance, co ma current tyres r like HUGE....
but i'm just asking for ur confirmation.(+it's up to u to pick the best choice of tyre measurement) ....:rolleyes:

thnx man

[c55m8o]

LoL ... gotta write some analysis for work so can't get on that right now. However, I gotta ask for you to do something 1st in either case.

(can you?) Please take one wheel off the car and weigh it ... Rim+tire together. :eek: Not kidding.

I really don't have any frame of reference for what your current rims weigh, so couldn't readily approximate any gain or loss with different types or sizes. Then read off the manufacturer and brand/style of tires, and its size. Maybe I'll get lucky and that manufacturer will publish weights. Else I can approximet from a few tire brands that do publish weights. I can take it from there. Really can't help you without some basis to start from tho.

edit: additionally, it's not necessarily "speed" you're picking up with a bigger rim; usually you're losing speed as the tire+rim package is heaview. It's handling ; turning/turn-in responsiveness. However what I believe you're saying is you think it'll go lighter with bigger rims and hi-perf tires. Well, maybe. Tell me the starting point of what you have now and we'll see.

[iced]

man..lol thnx for ur concern...i'll try gatting them weigh asap....not kidding... and we'll c were is it going

[!!!!!NJ Drive]

Originally posted by choppers_rule
Wow....Very well put......I love the slammed look with 19" but affraid of the power loss. Maybe I should replace my replica with a light weight 18". I don;t know...I'm confuseddddd:rolleyes: easiest way to think of it is if you go 19s, go multi spoke or mesh to save weight

[Renn 208]

also in response to chopper's comment:

At the heart of performance modding (and even car design), everything's a compromise. Understanding how a mod will affect your car both positively and negatively helps you make the decision that's right for you. Even more important is to understand why you want to make a change. Change for change's sake often leads to nowhere. But with specific goals and a specific purpose in mind, modding's the process of finding what it takes to meet your needs.

[c55m8o]

Originally posted by !!!!!NJ Drive
easiest way to think of it is if you go 19s, go multi spoke or mesh to save weight Each rim is unique.

Yes, the Kinesis K58 rims (5 spoke) I have are like 24lbs; but the same sized mesh rims (which I wish I had, but they would have cost 4x more as I bought mine used) is something like 21 or 22lbs.

On the flip side there was one guy on here who posted pics of his new 18" mesh rims. Looked oh so hott! I looked into what rims they were and discovered they weighed about 28 or 29 lbs... @ 18" :eek: ugh...

So just wanted to say just 'cus it's mesh doesn't mean it's light.