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Smart quod bastardus
 
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I agree and appreciate all the data 911ST.
My question perhaps was not as clear as I wanted. I just wondered why the recommendation of a significantly stiffer front sway bar than rear sway bar. I would suspect that since the torsion bars are matched to the 930 weight distribution 40/60 why not run equal sway bar rates or possibly bias a stiffer sway bar rearward?
I know the stiff front bar would bias towards understeer, but if the torsion bars are so well balanced wouldn't such a big stiffness difference for the front sway bar lead to excessive push?
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 02-23-2009, 09:35 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #41 (permalink)
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Hello 911st,
Does your information apply to the track only? This would include autox as well as road race.
I only ask because you state the need for larger front sway bars than the rear (i.e. 27/22's), for balance reasons.
I run 22's on both ends, and find for the STREET, setting the rears at 3/4 full stiff while having the fronts set and 1/4 from full soft, yields the best balance. Any stiffer on the fronts , or softer on the rears, causes too much front end push. During the rainy season, I run full soft on the front just to get some bite.
Simple casual questions for you, no debate needed.
Mark

P.S. Yes I know that slamming my car to the degree I did has made some minor compromises in ideal suspension geometry, so we do not need to go there.
Old 02-23-2009, 09:38 AM
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Quote:
Originally Posted by fredmeister View Post
I agree and appreciate all the data 911ST.
My question perhaps was not as clear as I wanted. I just wondered why the recommendation of a significantly stiffer front sway bar than rear sway bar. I would suspect that since the torsion bars are matched to the 930 weight distribution 40/60 why not run equal sway bar rates or possibly bias a stiffer sway bar rearward?
I know the stiff front bar would bias toward under-steer, but if the torsion bars are so well balanced wouldn't such a big stiffness difference for the front sway bar lead to excessive push?
I have spent some time thinking about it and I think it has to do with the geometry of how the sway bar levers interact with the front and rear suspension.

If you look closely the rear is a single lever that is hinged not to far from the torsion bar center line so in effect the TB is basically operating on apx an 18" lever (bar CL to axel).

The front however is two levers compounded or acting in each other. There is the short arm from the torsion bar CL to the end of the bar and it restest in about the midle of the second arm that runs between the TB center line and the wheel center. Thus, the front has compound levers require more force to exert less sway bar force at the wheel.

Also note that a car with wider track like a turbo increases one of these two levers which actually reduces the front spring rate over a normal width 911 with the same spring rates.

I know you have an engineer's mind. Please think this through or digram it and see if you come to the same conclusion?
Old 02-23-2009, 10:15 AM
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Quote:
Originally Posted by full quack View Post
Hello 911st,
Does your information apply to the track only? This would include autox as well as road race.
I only ask because you state the need for larger front sway bars than the rear (i.e. 27/22's), for balance reasons.
I run 22's on both ends, and find for the STREET, setting the rears at 3/4 full stiff while having the fronts set and 1/4 from full soft, yields the best balance. Any stiffer on the fronts , or softer on the rears, causes too much front end push. During the rainy season, I run full soft on the front just to get some bite.
Simple casual questions for you, no debate needed.
Mark

P.S. Yes I know that slamming my car to the degree I did has made some minor compromises in ideal suspension geometry, so we do not need to go there.
Please, I am not an expert. However, on a 930 that have a balanced torsion bar set up will likly work best with matching ft & rr. 22/22 are a great combination.

It is the normal 911 that can use a much larger front and then only under one condition where they use the 33mm rear torsion bars.

Something interesting. If you get your 930 dialed in for the track, try disconnecting the rear sway bar for autoX. (Or, just tighten the front and go full loose on the rear.) This will stiffen up the rear to let it rotate easier and let the front bit better so it will not push as much. I know of one instructor that ran 22/28's with 22/22 sways on a narrow body. He disconnected his rear sway when he autoz''d.
Old 02-23-2009, 10:26 AM
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Quote:
Originally Posted by 911st View Post
Something interesting. If you get your 930 dialed in for the track, try disconnecting the rear sway bar for autoX. (Or, just tighten the front and go full loose on the rear.) This will stiffen up the rear to let it rotate easier and let the front bit better so it will not push as much. I know of one instructor that ran 22/28's with 22/22 sways on a narrow body. He disconnected his rear sway when he autoz''d.
Sorry I got that backwards. No rear sway on the track makes the rear stick. A big or stiff rear setting is for autoX.
Old 02-23-2009, 10:30 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #45 (permalink)
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After lots of trying this... and that... I settled with 22 and 29 TBs with 22 front and rear SBs and the car is totally predictable (street and track) with minor SBs adjustments. keeping in mind that you need to consider tires/wheels and shocks; after all, the entire susp system must be in synch to work properly.
Old 02-23-2009, 10:38 AM
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Smart quod bastardus
 
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Quote:
Originally Posted by 911st View Post
I have spent some time thinking about it and I think it has to do with the geometry of how the sway bar levers interact with the front and rear suspension.

If you look closely the rear is a single lever that is hinged not to far from the torsion bar center line so in effect the TB is basically operating on apx an 18" lever (bar CL to axel).

The front however is two levers compounded or acting in each other. There is the short arm from the torsion bar CL to the end of the bar and it restest in about the midle of the second arm that runs between the TB center line and the wheel center. Thus, the front has compound levers require more force to exert less sway bar force at the wheel.

Also note that a car with wider track like a turbo increases one of these two levers which actually reduces the front spring rate over a normal width 911 with the same spring rates.

I know you have an engineer's mind. Please think this through or digram it and see if you come to the same conclusion?
In fact I am an engineer......am I that obvious?....joking.
You know you're right, what youre saying is that you need to consider wheel rates acting thru all the lever arms and distances. I am a little lazy to start measuring all the lever arm and other stuff and was looking for an easy way out based upon others experiences. To do this right on your own would require running alot of kinematic equations and stuff, but if only some of the club racers would tell us the specs on their rides we could avoid alot of experimentation.
I have got Smart racing bars on my car and found their site to have pretty good info on set-up options. Currently I have front 27 sways and 31 rear with the adjusters set more or less mid point on both ends. Running with stock torsion bars and bushings right now at US ride height. My thinking was that reducing roll via large sways is a good start till I can afford to add torsion bars and bushings to make the thing killer on track DE days.
I just don't get the results on lowered track times that I expected when adding the sway bar mods over running the car one season bone stock. My ego is still sore.......running out of excuses if the torsion bars don't make a big difference.
Another option is to switch the front and rear sway bars to get 31 front 27 rear since Smart bars you can do that....though don't want to do it until sure it is the right move.

Fred
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"

Last edited by fredmeister; 02-24-2009 at 11:01 AM..
Old 02-24-2009, 10:57 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #47 (permalink)
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Fred,

I am not an expert but those are some big bars.

I like big springs with smaller sways over the other way.

I want the springs to do as much work as possible and mostly use the sways to tune the front to rear balance. I suspect large sways complicate setting the shocks up as they have to deal with a big spring rate changes from crossing RxR tracks to putting one wheel in a hole. (Soft with stops, hard with a corner.) I would guess, huge sways are an attempt to overcome the limit in spring rates with using torsion bars.

Additionally, with large sways, a pot hole transfers a lot to the other side's suspension upsetting both sides.

As to the front requiring a larger sway. It may be much more simpler than I had said. The attachment point of the sway bar link is about 6" from the hinge up front. The rear is about 10" from the hinge. Both ft & rr are about 18" from the hinge to the wheel center on a normal wheel base car. The turbo has about a 20" front arm compared to a 18" rear on a turbo. Thus, it takes a bigger sway to make for the same spring rate at the wheel on the front.

I am still learning so all this is subject to verification.

Old 02-24-2009, 11:45 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #48 (permalink)
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Here's a shot of my car with 22/31 TB's and 22 Weltmeister SB's set full hard on front and full soft in the rear:




It's a little tail happy at high speed so if I was to keep the 22mm front TB, I'd probably go with a smaller 30mm rear or maybe even a 29. For the street or autocross, I think 22/29 or 22/30 would have too much understeer.

I'd like to stiffen it up a little more so I may try going to a 23mm front and keep the 31mm rear before I hit the track again.
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Old 02-24-2009, 12:18 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #49 (permalink)
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Just big sway bars aren't going to do much. To get the best track handling you need to do the whole thing as a package. Torsion bars, revalved shocks & sway bars. You will notice the biggest improvement on the track from re-valved shocks. I use to run the F&R 30 re-vavled shocks on our street/track car. They are pretty stiff for the street. If you want to retain a good street ride I would go with the F&R 10 valving. I also ran 27mm front & rear sway bars. The typical race set up is 31 front & 27 rear sway bars. This is what we run on our race car. Both bars are about in the middle.

You should talk to Smart Racing to get the appropriate package for your car.
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Old 02-24-2009, 12:22 PM
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Here's an old shot taken at Infineon (turn11). Not much body roll and the inside front tire is off the ground. This was with S/R f&r 40 valving and 27 front & rear sway bars and pretty soft coil overs 300 front & 450 rear.
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Old 02-24-2009, 12:32 PM
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