Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 911 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
T77911S's Avatar
 
Join Date: Oct 2006
Location: MYR S.C.
Posts: 11,545
285-35 rear and 235-40 front.

the rear of the car sits really hi but the tires fit well in the fenders.

not trying to steel the thread.
__________________
86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 12-12-2018, 09:46 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #21 (permalink)
Registered User
 
1QuickS's Avatar
 
Join Date: Feb 2008
Posts: 1,210
Steve is right. Rotational moment of inertia relates to how much torque is required for rotational acceleration. A pair of wheels can weigh the same but one will have higher rotational inertia making it swallow more torque to accelerate in rotation. The maximum impact of rotational inertia comes at the maximum radius of the part. Rubber is heavy and is at the maximum radius so tire weight affects rotational moment of inertia the most.

For those interested: J=(.5*Pi*R^4)
Where:
  • J is rotational moment of inertia
  • Pi is 3.14159
  • R is radius from rotational axis to point of mass

It is obvious that the distance (Diameter and width of tire in this case) affects torque requirement to accelerate to the 4th power.

Last edited by 1QuickS; 12-12-2018 at 10:05 AM..
Old 12-12-2018, 10:02 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #22 (permalink)
Registered User
 
Bill Douglas's Avatar
 
Join Date: Jun 2000
Location: bottom left corner of the world
Posts: 12,611
Yep, rotating mass. High school physics.

I tried to weigh the wheels but the Fuchs were too light for the bathroom scales I was using. I could have done the me holding them and subtracted the me from the weight. But just wanted to get the swap done.

Probably the wrong term, but acceleration with the old wheels felt more linear, more even. Whereas the 16" wheels have a more lively "get up and go" feel about them.

The circumference was exactly the same. I chose the particular tire for it's circumference.
__________________
see www.lyallbaybeach.co.nz
Old 12-12-2018, 10:47 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #23 (permalink)
Registered User
 
T77911S's Avatar
 
Join Date: Oct 2006
Location: MYR S.C.
Posts: 11,545
I was just looking at the "gearing" change by going to a smaller wheel as far as acceleration.
__________________
86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 12-12-2018, 10:53 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #24 (permalink)
Registered User
 
GaryR's Avatar
 
Join Date: Mar 2005
Location: Valencia, Spain
Posts: 4,597
Garage
Send a message via ICQ to GaryR
Quote:
Originally Posted by T77911S View Post
I was just looking at the "gearing" change by going to a smaller wheel as far as acceleration.
it's the OD of the tire, not the wheel, look at that for comparison..
__________________
Gary R.
Old 12-12-2018, 11:45 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #25 (permalink)
Moderator
 
Bill Verburg's Avatar
 
Join Date: Dec 2000
Posts: 23,279
Garage
Quote:
Originally Posted by 1QuickS View Post
Steve is right. Rotational moment of inertia relates to how much torque is required for rotational acceleration. A pair of wheels can weigh the same but one will have higher rotational inertia making it swallow more torque to accelerate in rotation. The maximum impact of rotational inertia comes at the maximum radius of the part. Rubber is heavy and is at the maximum radius so tire weight affects rotational moment of inertia the most.

For those interested: J=(.5*Pi*R^4)
Where:
  • J is rotational moment of inertia
  • Pi is 3.14159
  • R is radius from rotational axis to point of mass

It is obvious that the distance (Diameter and width of tire in this case) affects torque requirement to accelerate to the 4th power.
It's actually way more complicated than that due to the geometry and where in the geometry the various mass is located.

There is no 4th power exponential, only exp 2

The rotating torque curves are 2nd degree and are also dependant on the linear acceleration of the vehicle as that determines the circular acceleration of the wheel/tire assembly. Inertial cost is is proportional to the mass, summed mass distribution radius and acceleration.

The gearing cost is separate and is linear in nature, it is ~12lb-ft per extra inch of OD
__________________
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
| Pelican Home |Rennlist Wheels |Rennlist Brakes |
Old 12-12-2018, 12:41 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #26 (permalink)
Moderator
 
Bill Verburg's Avatar
 
Join Date: Dec 2000
Posts: 23,279
Garage
Quote:
Originally Posted by T77911S View Post
I was just looking at the "gearing" change by going to a smaller wheel as far as acceleration.
Quote:
Originally Posted by GaryR View Post
it's the OD of the tire, not the wheel, look at that for comparison..
They both factor is but in somewhat different ways

just for comparison here 2 implementations of BS SO-4 tires in slightly different sizes

the front is 235/40 compared to 245/35 on the same wheel, the the tires weight the same but the inertial cost of the 245 is less because the mass distribution is in aggregate closer to the axis of rotation, there is no gearing cost here. the 245 would have a +4.3lb-ft axle gain available for acceleration rather than rotating the tires

In back 285/35 compared to 285/30, the 30 is both lighter and shorter than the 35, here there is an inertial cost to the 35 of ~11.5lb-ft for the axle, this is a maximum cost at max acceleration and less at more leisurely accelerations and an additional gearing cost that is always there of ~13lb-ft

Here the total benefit of the 245/35 & 285/30 at max accel is ~29lb-ft
__________________
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
| Pelican Home |Rennlist Wheels |Rennlist Brakes |
Old 12-12-2018, 12:59 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #27 (permalink)
Registered User
 
GaryR's Avatar
 
Join Date: Mar 2005
Location: Valencia, Spain
Posts: 4,597
Garage
Send a message via ICQ to GaryR
Quote:
Originally Posted by Bill Verburg View Post
They both factor is but in somewhat different ways
Bill, I was only referring to the gearing change (in my case) going from an 18" wheel to a 15" wheel on Hoosier R7s, which was substantial. I know you printed out the gearing and accel charts for me back then, I was amazed at the difference on paper and on track.

18" 275/35 R18 -25.5" diameter, 80.2" circumfrence
15" 275/35 R15 - 23" diameter, 72.2" circumfrence
__________________
Gary R.
Old 12-12-2018, 01:13 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #28 (permalink)
Moderator
 
Bill Verburg's Avatar
 
Join Date: Dec 2000
Posts: 23,279
Garage
Quote:
Originally Posted by GaryR View Post
Bill, I was only referring to the gearing change (in my case) going from an 18" wheel to a 15" wheel on Hoosier R7s, which was substantial. I know you printed out the gearing and accel charts for me back then, I was amazed at the difference on paper and on track.

18" 275/35 R18 -25.5" diameter, 80.2" circumfrence
15" 275/35 R15 - 23" diameter, 72.2" circumfrence
I know, but there is sooo much more involved I just have to interject when given an opening, Are you still in Europe?
__________________
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
| Pelican Home |Rennlist Wheels |Rennlist Brakes |
Old 12-12-2018, 01:20 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #29 (permalink)
Registered User
 
GaryR's Avatar
 
Join Date: Mar 2005
Location: Valencia, Spain
Posts: 4,597
Garage
Send a message via ICQ to GaryR
Quote:
Originally Posted by Bill Verburg View Post
I know, but there is sooo much more involved I just have to interject when given an opening, Are you still in Europe?
Yes sir, Spain is home now and for the forseeable future..
__________________
Gary R.
Old 12-12-2018, 02:19 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #30 (permalink)
911 Mania
 
Join Date: Jan 2014
Location: PDX
Posts: 174
Garage
Quote:
Originally Posted by Bill Verburg View Post
I know, but there is sooo much more involved I just have to interject when given an opening, Are you still in Europe?
Bill, Sure appreciate your detailed non confrontational input. It sure challenges me and I know when I see your name I better get a second cup of coffee and put on my thinking cap. Thanks for sharing your vast knowledge and the polite way you present it. Iíve saved so many of your posts to refer to when I start my project. Thanks also for including the visual aids or data, it helps me absorb the info.

BTW, also use your examples with my kids to illustrate why math is so important and provide them with a real life examples.

Good info!
__________________
Hemmingway said it best, ďThere are only three sports: bullfighting, motor racing, and mountaineering; all the rest are merely games.Ē
Old 12-12-2018, 03:06 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #31 (permalink)
914-6Werkshop
 
Join Date: Dec 2009
Location: Wilmington, DE
Posts: 2,184
Some of the fake cup wheels weigh 30lbs per wheel. Add in wider-heavier tires and it can really add up. Then, unless you go low profile, the 17’s make the car sit taller. It’s not always a good look on these cars.
__________________
914-6Werkshop.com
Old 12-12-2018, 03:35 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #32 (permalink)
Reiver
 
Reiver's Avatar
 
Join Date: Nov 2011
Posts: 27,461
Who makes forged alloy wheels the same size as Fuchs that weigh less?
__________________
De Oppresso Liber
Old 12-12-2018, 04:55 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #33 (permalink)
Registered User
 
Bill Douglas's Avatar
 
Join Date: Jun 2000
Location: bottom left corner of the world
Posts: 12,611
Just to keep this thread rolling

I weighed one of the fake Carrera Cup wheels on the bathroom scales. 23.4KG that is 51.6lb That's with a Bridgestone tire, so not a rubbish tire adding weight.

The 16" Fuchs with 225 Hankook tire felt waaay lighter when handling them both.
__________________
see www.lyallbaybeach.co.nz
Old 12-12-2018, 05:34 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #34 (permalink)
Registered User
 
Tori's Avatar
 
Join Date: Jul 2016
Location: Pismo Beach, CA.
Posts: 652
I believe it. I felt a difference when i switched wheels on my Ram truck. Went from the factory chromed steel wheels to the factory alloy wheel. I weighed each bare wheel, 17# difference ! 17 lbs EACH WHEEL !!! I did not expect to feel a difference, just assumed the truck would.... I could tell the very first time i touched the accelerator pedal.
__________________
1987 911 Carrera Coupe
1987 Buick Grand National
1971 Plymouth Roadrunner
Old 12-12-2018, 07:29 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #35 (permalink)
Registered User
 
Join Date: Oct 2016
Location: PNW
Posts: 517
Quote:
Originally Posted by Reiver View Post
Who makes forged alloy wheels the same size as Fuchs that weigh less?
What size Fuchs? Original Minilites maybe, Gas burners are probably the lightest. Fuchs will be hard to beat, I feel if there was something substantial lighter that didnít compromise on strength you would see them in motorsport, instead of Fuchs everywhere.
__________________
_______________________________
1982 911 SC 230,000mi and counting
Old 12-12-2018, 07:31 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #36 (permalink)
Reiver
 
Reiver's Avatar
 
Join Date: Nov 2011
Posts: 27,461
Quote:
Originally Posted by QueWhy View Post
What size Fuchs? Original Minilites maybe, Gas burners are probably the lightest. Fuchs will be hard to beat, I feel if there was something substantial lighter that didnít compromise on strength you would see them in motorsport, instead of Fuchs everywhere.
well whatever size as long as they and the Fuchs are similar.....what do the Magnus Walkers weigh? They are advertised as 'light' and they can't be referring to the cost.
__________________
De Oppresso Liber
Old 12-12-2018, 07:36 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #37 (permalink)
914-6Werkshop
 
Join Date: Dec 2009
Location: Wilmington, DE
Posts: 2,184
ATS cookies aren’t bad but there aren’t many wheels out there with the strength to weight of factory Fuchs. Old mag wheels are getting very brittle due to age. Fuchs look right on our cars and come in all the right sizes. Picking the right tire and not just throwing on the widest possible rubber also goes a long way to the “feel” of the car.
__________________
914-6Werkshop.com
Old 12-13-2018, 04:27 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #38 (permalink)
Registered User
 
sithot's Avatar
 
Join Date: Sep 1999
Posts: 779
Alois Ruf on Weight (in his '65 Coupe)

"Driving an early 911 is an experience on itís own:

Itís 1000 kilos. Today weíre driving 1,600(kg) and the 500 horsepower that we use today feel different to the 130 (hp) we drove at that time because the extra weight, the 60% more weight is not really compensated by the extra horsepower because there is no compensation for extra weight. There I nothing better than saving weight in an automobile."

Alois Ruf
https://www.youtube.com/watch?v=yfYHCbyo6CE
__________________
R Gruppe #111
Early S Registry #235
res ipsa loquitur
Old 12-13-2018, 01:37 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #39 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 04:17 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.