Parts Catalog Accessories Catalog Tech Info Tech Forums
 
  Search our site:    
 Cart  | Project List | Order Status | Help    
  Join us November 1st for Casino Night at the Pelican Parts Open House!
Benefiting LuMind - Research and Treatment for individuals with Down Syndrome
Go Back   Pelican Parts Technical BBS > 1- Porsche Technical Forums > Porsche 911 Technical Forum

Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Wheel, Spring, and Torsion Bar --rates?

I have been thinking about "Wheel Rates" v "Spring Rates" as they might relate to a 911 torsion bar suspention.

I can see that calulating wheel rates from spring rates could be a very complex calc.

However, I think we might be able to get close enough to use for comparisions if we apporach this in a bit more simplistic way.

Here is what I am thinking:

The front seems easy.
If we look at the front the wheel goes up and down at the same rate as the strut or shock bottom. Thus, the spring rate and wheel weight for the front should be bassicaly the same for a torsion bar of coil over set up.

On the front the length of the A arm on a coil strut is mostly irrelivant as all it dose is locate the strut. On a torsion bar suspention the A arm is the spring's lever and is used in the calc of its rate.

The rear is a little more complicated in that the shock is mounted a little over 4" past the centerline of the wheel. The wheel center rotates at about 18.5" around the center of the rear arm pivot or link. This should mean the rear shock point moves roughly 15% further than the wheel at any time. Thus, the rear wheel rate should be about 15% higher than the rated spring rate.

Elephant Racing now offeres coil over kits for 911's and has a spring rate calulator depending on use and weight which is cool.

See: http://www.elephantracing.com/suspension/coiloverkits/911coiloverkits.htm

It is interesting to use this to compair a couple of there recomendations.

First level of upgrade for a car like mine is 21/27 torsion bars or 175/200 rated springs.

This is about 173/191 for a torsion bar set up and / or 175/230 rated springs.

The full race recomendation for torsion bars is 23/31 and / or 400/550 rated springs.

That should be about 250/332 for torsion bars or a wheel rate of 400/630 for the coil overs.

24/33 TB's which is the stiffest available and used along with large sway bars by many full on raceers is about 290/427.

One of the big advantages of coil over is access to higher spring rates.

For the record, below is what I belive to be the wheel rates for different torsion bar sizes:

Front: (lbs/inch) (% of stock)
19 110# ---------(18.8mm)
21 173 157%
22 210 191%
23 250 227%
24 290 264%
Rear
24 122# ----------(24.1mm)
25 140 115%
26 165 135%
27 191 157%
28 221 181%
29 254 208%
30 294 241%
31 332 272%
33 427 350%

(Note that rear TB was 24.1mm till 86 then 25mm. 930 rear's were 26mm.)

Dose this look close to being valid and if not what am I missing.

Thx.
Old 06-26-2009, 12:29 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #1 (permalink)
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Source of torsion bar rates: http://tech.rennlist.com/911/pdf/torsionbars.PDF

Another TB rates source: http://instant-g.com/Data/911CoilConv.html

Old 06-26-2009, 01:18 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #2 (permalink)
Moderator
 
304065's Avatar
 
Join Date: Dec 2001
Posts: 9,275
Those TB rates topping out in the 400s make sense-- The Factory certainly knew that the biggest TBs were too small, hence coil overs in the early 1970s. It's impossible to get enough spring rate in the tail of a TB 911 for race purposes.
__________________
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 06-26-2009, 01:32 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #3 (permalink)
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
I was wondering about that! It seems to look like in most race pictures of TB 911's like they are twisting or rolling about the rear.

I have been suspect that the way to get the front to stick (less front camber loss) is to stiffen up the rear which is not what we are told to do.
Old 06-26-2009, 02:41 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #4 (permalink)
Registered User
 
Join Date: Dec 2006
Location: NC
Posts: 189
I have the 24/33 set up. I feel pretty good about making adjustments using sways bars and tire pressures. I'm sure more modern struts offer greater adjustment but, I wanted to keep it simple and work primarily on my driving ability. The variables offered by a triple or double adjustable (Moton type shocks) require some real technical analysis. I hope to work up to that when I grow up, for now TB's keep my car pretty well balanced as a race car. Better than many of the cars I see on track and in the zone with other more modern set ups. The higher rear spring rate bias seems to work just fine. My shocks are revalved per the folks at Smart Racing. My .02
__________________
Keith
82SC Track Car
Old 06-26-2009, 04:39 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #5 (permalink)
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Keith, Good data point, thx.


PS: I do not have accurate measurements and am not a geometry wiz. Thus, I am starting to believe that a rear coil over spring could run a wheel rate about 19% more that its rating. I still think a front spring rate will be the same wheel rate.

I am also now suspect of my Front - Torsion Bar ratings I used and that are floting around. It used a 12" arm length to compute them which is longer than the A arm and front spring rates 'might' be about 8% higher.

Thought I would post this in-case someone comes across this latter.
Old 06-29-2009, 03:20 PM
  Recommend this thread for the PelicanWiki    Reply With Quote #6 (permalink)
 
Now Available for Ordering:   101 Projects For Your BMW 3 Series 1982-2000  [more info]
Old 06-29-2009, 03:20 PM
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 



All times are GMT -8. The time now is 11:59 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2011 Pelican Parts - 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.