Sway Bar Rate Vs Spring Rate

[simpletool]

Anybody know the contribution that sway bars make compared to spring rates?

ie. What is the effective wheel rate of a whiteline 24mm adjustable sway bar?

I'm trying to do some basic calculations for front to rear balance. So far I am guessing that the stock sway bar is around 50% of the spring rates, and I'm working from that. I know the front bar adjustments are worth ~8% each and the rear ~12% each.

[blind_elk]

Sway bars aren't springs. They make no contribution to the overall front to rear spring rate balance.

[GTSBoy]

Anti-roll bars (there's no such thing as sway bars on a Skyline - or most other vehicles for that matter) are in fact springs, and they do in fact contribute to the wheel rate. If you hit a bump with both wheels on the same axle at teh same time, then they have effectively zero spring rate. But if you hit a bump with only one wheel, then the bar does add to the wheel rate, and transfers some of the load directly to the opposite side of the car as well (although that load transfer is out of phase, ie up for down).

[simpletool]

Sway bars aren't springs. They make no contribution to the overall front to rear spring rate balance.

That's "flat out" wrong.

[R32Abuser]

Anybody know the contribution that sway bars make compared to spring rates?

ie. What is the effective wheel rate of a whiteline 24mm adjustable sway bar?

I'm trying to do some basic calculations for front to rear balance. So far I am guessing that the stock sway bar is around 50% of the spring rates, and I'm working from that. I know the front bar adjustments are worth ~8% each and the rear ~12% each.

To calculate all of this, you need a decent amount of geometry information from the vehicle in question. Aspects such as motion ratio (Anti Roll bar MR and wheel:shock MR) all need to be known to calculate the overall contribution.

I've always planned to do this on my 32 GTST but have never found the time to do this...Sydneykid might be able to shed some light on this. However, he no longer operates on this forum...

Sway bars aren't springs. They make no contribution to the overall front to rear spring rate balance.

Firstly, sway bars ARE springs. In most cases (like in Skylines) they are torsion springs.

I think there is some confusion with the wording being used here.

Spring rate ~ The force input required to compress a spring a unit distance. I.e. N/mm = Netwons (force) required to compress that spring 1mm

Usually when people refer to the "spring rates" of a vehicle you talk about the stiffness of the springs which sit around the shock (aka ride springs). These items dictate the ride stiffness of the vehicle, but also add to the overall roll stiffness of said vehicle.

Wheel Rate ~ The force input required to displace the wheel a unit distance. I.e. N/mm = Newtons (force) required to displace the wheel upwards 1mm. This value is a combination of the vehicles roll stiffness and ride stiffness.

Sway bars are a common, incorrectly used replacement name for Anti Roll Bars (ARB's). ARB's affect the roll stiffness and consequently the overall wheel rate of a vehicle on a common axle during a roll event/single wheel bump. ARB's do not affect the "spring rate" or front/rear spring rate balance of the vehicle when both wheels on a common axle are displaced the same distance, in the same direction.

Anti-roll bars (there's no such thing as sway bars on a Skyline - or most other vehicles for that matter) are in fact springs, and they do in fact contribute to the wheel rate. If you hit a bump with both wheels on the same axle at teh same time, then they have effectively zero spring rate. But if you hit a bump with only one wheel, then the bar does add to the wheel rate, and transfers some of the load directly to the opposite side of the car as well (although that load transfer is out of phase, ie up for down).

As GTSBoy states, drive over a bump with both wheels - the ARB contributes zero stiffness to the wheel rate. Drive over the same bump with one wheel and the wheel rate is altered. Although ARB's will not affect the wheel rate in a perfect world situation (double bump/droop), any dips, bumps, crannies etc. will caus the ARB to affect the wheel rate.

This is why you don't want to go to stiff an ARB as the stiffer you go, the more the car will want to pull/follow these deviations in the road surface (aka "tramlining").

Edited July 23, 2012 by R32Abuser

[simpletool]

Well I've had quite a few incarnations of springs and way bar settings. I also remember what most of them felt like in some detail. So I've put together a simple excel sheet for myself with spring rates and sway bar settings and my feelings for each. I had to make an assumption of sway bar rates so I could add them together with spring rates. I've guessed a stock front sway bar rate of 1.25kg/mm and rear of 1.0kg/mm - I highly doubt this is going to be accurate but I had to start somewhere. ( I've guessed 50% front and 35% rear contribution from the sway bars).

As long as I am in the ballpark it helps to add some numbers to subjective feel (balance front to rear, entry oversteer/understeer, etc). From what I've got it seems to make some sense and my observations are somewhat in-line with the numbers.. When I get my 5kg/mm front springs this week then I'll be able to ad them to the list and adjust sway bars accordingly.

I'd also like to know the relative effect of roll couple and roll centres front vs rear. ie. Does lowering 10mm all round cause a greater increase in roll couple in the front or rear, or very similar? I realise there are only a couple of people on here who likely know this.

Edited July 23, 2012 by simpletool

[GTSBoy]

You can calculate the anti-roll bar rate. It is best treated as a simple torsion bar. The diameter is known, and the arm length is known or can be measured in minutes. Google the formula. You can even work out the motion ratio of the connection point of the anti-roll bar vs. the wheel in about 30 seconds from underneath.

[R32Abuser]

Agreed you can work out the bar rate that is installed on the car etc. Thats not hard as all you need is the geometrical properties of the bar as you mentioned. Also agreed that you can work out the ARB M.R. based off on vehicle measurements. I also like to validate the above in a suspension modelling program Susprog3D or Optimum G.

Has proved reasonably accurate so far on race cars... Just need to get round to doing it on my R32

Edited July 23, 2012 by R32Abuser

[simpletool]

You can calculate the anti-roll bar rate. It is best treated as a simple torsion bar. The diameter is known, and the arm length is known or can be measured in minutes. Google the formula. You can even work out the motion ratio of the connection point of the anti-roll bar vs. the wheel in about 30 seconds from underneath.

Pretty sure from memory it's connected to the strut and the Skyline is pretty damn close to 1:1. So the rate is just additive to the springs.

The calculators although guestimates also do tend to agree with my assumption that the stock bar is 1.25 and the rear 1.0 The white-line front I estimate goes from 1.5kg/mm up to 2.1kg/mm. The rear from 1.25kg/mm up to 1.6kg/mm. These figures could be a fair way off but they provide some reference point to work with at least. Hard to estimate as the bar is not straight or symmetrical, it is made of a material that I don't accurately know the modulus and it has irregular bends before the arms.

[GTSBoy]

The irregularities in the bar can be ignored. Compared to the basic calc, they do not add or subtract a large % of the total. Simiarly, the details of the "arm" section don't really matter either. The arms are short relative to the length of the torsion section, so the strength/spring rate contribution of the arm loaded in simple beam/bending is so much higher than the torsion section that it doesn't figure. It just acts as a lever. As expected/desired.

Because the stiffness varies with the 4th power of the diameter, a 24mm bar is about 3.2x the stiffness of an 18mm bar. That's a somewhat bigger ratio than the differences between "stock" and "Whiteline" mentioned above. Similarly, I would not have expected that the small change in lever arm length caused by moving from one hole to another in a Whiteline bar would be enough to change the rate by 25% (of the higher rate). I would not have expected much more than 10%. Will have to go under the car and measure the arm length change on mine. Should have read these posts before this time tonight. Car was only on the hoist 30 minutes ago!!

cheers

[simpletool]

Is the stock bar 18mm? I don't have it any more so I have no idea. There is no way the white line bar is 320% of the stock bar, that would be crazy.

The front has quite a few holes. I'll take some measurements when I can. Here are the pics of the 2 items which shows the leverage on the rear bar is much less than the front (and so each hole has a higher % effect). I think SK said each front adjustment is approx 8% and each rear is 12%.

It'd be good to get some reference values for the contribution of the sway bar to spring rates.

Rear

Front

[simpletool]

OK well maybe I should have remember this chart from Whiteline. Perhaps this is as accurate as I'm going to get?

http://www.whiteline.com.au/docs/bulletins/BL-281.pdf

[GTSBoy]

Yeah, so the Whiteline chart shows what I said. The % increase from 18mm to 24mm is 216%. That means the total stiffness is 100+216=316%. No arguing with D^4.

Rear R32 auto bar is only 18mm. Cannot speak for other types.

My front R32 bar is 22mm. I don't think that that is stock. When the 18mm was on the rear, it was a fairly committed understeerer.

[simpletool]

The stock rear is 18mm on the R33 GTST, from a few internet searches. The front bar is unknown still.

So for the rear I have a choice of 85%, 123% or 167% increase. These whilteline bars are obviously stiffer and with a larger spread than I thought ~

[GTSBoy]

Yuh, consider the 27mm bar that everyone was oozing over a while back. How bloody stiff must that make the car?!!??

[simpletool]

OK so here is my guestimate comparisons and thoughts. Might not be worth anything to people, but it's free. The revised harder sway bar rates makes the ratio correlate better to seat of the pants feel.

Caution - rates are entirely guestimates.

R33 wheel rates.xlsx

Edited August 7, 2012 by simpletool