Suspension modeling

[xXDavidCXx]

Spent the day home on quarantine after getting tested, so I had some time to crunch numbers and settle on the sway bar details.

Called Hotchkis, they won't tell me, and the sales team does not even know, the inner wall thickness. But they did regurgitate the 95%/125% stiffness from their website.

Using that info and the chart posted by 00Zero settled on 312/360 for the Hotchkis front bar. Reverse engineering those numbers I get a wall thickness about 28.7 which comes out close enough to 312/360 that I'll let it pass as good enough.

Using the posted numbers for the 034 bar as well, I ran a few scenarios of my past 4 setups of different combos of my OEM/Ohlins/Hypercoil springs and the OEM/Hotchkis/034 bars.


This is a typical forum member who puts on a large rear bar and normally experience more rotation. As the role couple decreases the amount of weight transferred to the front axle decreases. You will see that 50.5% is very low, even as I continued to modify the suspension.

Setup

% front roll couple

Stock springs and sway bars	67.4​
Stock springs and front bar, 034 rear bar full stiff	50.5​
Stock springs and front bar, 034 rear bar full soft	52.1​

Then I tried upgraded swaybars with stock springs and shocks. I ran this setup at Solo Nationals, I think I ran stiff on both bars @ 64.8% which is a small improvement on the stock setup. Understeerd at the limit, but I thought grip was pretty good, but lots of body roll.

Stock springs Hotchkis front bar soft, 034 bar soft	63.7​
Stock springs Hotchkis front bar stiff, 034 bar soft	66.3​
Stock springs Hotchkis front bar stiff, 034 bar stiff	64.8​
Stock springs Hotchkis front bar soft, 034 bar stiff	62.2​

Then I put on the Ohlins setup. The best was only 64.7%, not really better than stock springs and bars, and understeered a lot!

400F 400R springs Hotchkis soft 034 soft	66.0​
400F 400R springs Hotchkis stiff 034 soft	67.9​
400F 400R springs Hotchkis soft 034 stiff	64.7​
400F 400R springs Hotchkis stiff 034 stiff	66.7​

Then I started experimenting with ride frequency calculations and decided that 425/750 would be a good starting point.

I ended the season on the OEM front bar and the rear bar on stiff for a 51.3% roll couple. With 1/8 in total rear toe out, this was the best setup so far and where I will pick up in 2021, depending on the rest of the findings.

425F 750R springs, OEM front, 034 soft	52.4​
425F 750R springs, OEM front, 034 stiff	51.3​
425F 750R springs, Hotchkis soft, 034 soft	60.7​
425F 750R springs, Hotchkis soft, 034 stiff	59.7​
425F 750R springs, Hotchkis stiff, 034 soft	62.8​
425F 750R springs, Hotchkis stiff, 034 stiff	61.8​

What will be interesting to know, is if the Hotchkis front bar @ full stiff is more useful to reduce roll (61.3%), or is the OEM bar better for the loose setup I want (50.4%), or something in-between.

 

[Oversteermybagel]

I hope you are well.

This is very interesting, this only tells half the story though. It explains how the car handles when it breaks loose but tells nothing about when it breaks loose. Clearly your 2020 best setup is going to be better than forum member for going around high G corners, but how much better. For each of these setups it would be most interesting to see what your camber change is at 2Gs of lateral grip for each of these setups. It be interesting to see how much static camber is needed in order to keep the tire at -0.5 or whatever ideal might be for each depending on setup

 

[xXDavidCXx]

I hope you are well.

This is very interesting, this only tells half the story though. It explains how the car handles when it breaks loose but tells nothing about when it breaks loose. Clearly your 2020 best setup is going to be better than forum member for going around high G corners, but how much better. For each of these setups it would be most interesting to see what your camber change is at 2Gs of lateral grip for each of these setups. It be interesting to see how much static camber is needed in order to keep the tire at -0.5 or whatever ideal might be for each depending on setup

That is exactly what is next. I'll probably run @ 1.5G but might as well run @ 2G too. And using the modeling find out what angle the car will be at to find the optimum balance between static camber and roll resistance.

 

[xXDavidCXx]

Here is the same list by organized understeer to oversteer, top to bottom.

Added some hypothetical choices and my current daily winter setup.

Setup	% front roll couple
400F 400R springs Hotchkis stiff 034 soft	67.9
Stock springs and sway bars	67.4
400F 400R springs Hotchkis stiff 034 stiff	66.7
Stock springs Hotchkis front bar stiff, 034 bar soft	66.3
400F 400R springs Hotchkis soft 034 soft	66.0
Stock springs Hotchkis front bar stiff, 034 bar stiff	64.8
400F 400R springs Hotchkis soft 034 stiff	64.7
Stock springs Hotchkis front bar soft, 034 bar soft	63.7
425F 750R springs, Hotchkis stiff, 034 soft	62.8
Stock springs Hotchkis front bar soft, 034 bar stiff	62.2
425F 750R springs, Hotchkis stiff, 034 stiff	61.8
425F 750R springs, Hotchkis soft, 034 soft	60.7
425F 750R springs, Hotchkis soft, 034 stiff	59.7
425F 750R springs, H&R F, 034 R soft	59.3
425F 750R springs, H&R F, 034 R med	58.8
425F 750R springs, H&R F, 034 R stiff	58.3
425F 750R springs, Hotchkis F stiff, H&R R	57.7
425F 750R springs, Hotchkis F stiff, Neuspeed R	56.8
425F 750R springs, OEM front, 034 soft	52.4
Stock springs and front bar, 034 rear bar full soft	52.1
425F 750R springs, OEM front, 034 stiff (end of 2020 setup)	51.3
425F 1250R springs, Hotchkis F stiff, Neuspeed R	50.6
Stock springs and front bar, 034 rear bar full stiff	50.5
Stock spring F, 400 rear; OEM bar F, 034 R soft (winter setup)	48.4

 

[xXDavidCXx]

Added the Hotchkis front bar, which btw is not longer in production.

Front
Mfg.	OD	Thickness	Stiffness	Increase over GTI	Solid dim	Stiffness
	[mm]	[mm]	[mm^4]	[%]	[mm]	[lb/in]
	23.2	3.5	220828	-15%	21.7	136
	22		234256	-10%		144
vw	24	3.8	259437	0%	22.6	160
e-golf	25	4.0	307104	18%	23.5	189
	24		331776	28%	24.0	205
whiteline	24.5		360300	39%	24.5	222
	25		390625	51%	25.0	241
h&r	26		456976	76%	26.0	282
eibach EU	28	4	454656	75%	26.0	280
			505901	95%	26.7	312
			557789	115%	27.3	344
eibach US	29	6.35	636690	145%	28.2	393
	28		614656	137%	28.0	379
Hotchkis	31.75	3.05		95%		312
				125%		360

 

[Oversteermybagel]

That is exactly what is next. I'll probably run @ 1.5G but might as well run @ 2G too. And using the modeling find out what angle the car will be at to find the optimum balance between static camber and roll resistance.

And obviously its not clear how important maximum sustainable Gs are (how many sweepers do you REALLY see) against transients or other more intangibles but I anxiously await the results

 

[jmblur]

Sure. I worry that you will not be able to get those bolts tight enough to avoid slipping your alignment at high traction loads. I do not know this for sure though, but you don't see hardware like this in purpose built McStrut race cars. For comparison consider the adjustable ball joints on the front LCA in the mk7 GTi TCR/LMS cars

http://imgur.com/a/1cHkXe2

You can see its adjusted via threaded rod to prevent slippage.

The friction from the bolts is going to far exceed any holding strength from the threaded rod. The threaded rod is there to allow micro-adjustments without "guess and check". Loosen the four bolts, turn the threaded rod 1/2 a turn or whatever you need, retighten.

Still a lovely design though!

 

[burgerkong]

I pinged Eurocode so they can divulge the wall thickness of their RSB, OD is 25mm, and this is what they claim for stiffness:

 

[00Zero]

@xXDavidCXx, in your roll couple calcs what motion ratios are you using for the bars?

In trying to decide what to do with my own car, I have been comparing the ratio of the combined (bar+spring) ride frequency front to rear. I know frc is widely used but in my head it seem weird to not account for the weight as well. Thoughts?

An example (From memory, I'm on my phone right now) I came up with a stock gti being about 8% stiffer (combined ride frequency) ) in the front, a clubsport s, mk8 or 22mm rear bar being about even, a 24mm rsb stiffer.0% stiffer in the rear, a 26mm or your last season stx setup about 20% stiffer in the rear, and your winter setup about 30% stiffer. Im not trying to pick on your setups but I have been using your experiences as a baseline for comparisons and calculations.

With that I have been targeting between 10-20% additional stiffness in the rear, but it will be interesting to see what the modeling says would work best.

Ps, best of luck waiting on the test result, we did the same with my son a few weeks ago and it is nerve racking.

 

[xXDavidCXx]

@xXDavidCXx, in your roll couple calcs what motion ratios are you using for the bars?

In trying to decide what to do with my own car, I have been comparing the ratio of the combined (bar+spring) ride frequency front to rear. I know frc is widely used but in my head it seem weird to not account for the weight as well. Thoughts?

An example (From memory, I'm on my phone right now) I came up with a stock gti being about 8% stiffer (combined ride frequency) ) in the front, a clubsport s, mk8 or 22mm rear bar being about even, a 24mm rsb stiffer.0% stiffer in the rear, a 26mm or your last season stx setup about 20% stiffer in the rear, and your winter setup about 30% stiffer. Im not trying to pick on your setups but I have been using your experiences as a baseline for comparisons and calculations.

With that I have been targeting between 10-20% additional stiffness in the rear, but it will be interesting to see what the modeling says would work best.

Ps, best of luck waiting on the test result, we did the same with my son a few weeks ago and it is nerve racking.

Front 0.98 (spring and bar), rear spring 0.64, bar 0.45 (not verified). The rear numbers could be wrong though, I need to re-measure the entire car.

I need to remeasure the front as well. I didn't follow the correct procedure. I just assumed B=A because I assumed A would be the lower ball joint, not the center of the tire.

 

[burgerkong]

Literally useless answers from Eurocode as well.

 

[00Zero]

Wow, that is the least helpful response ever.

Both bars solid 20mm to 25mm would be a 144% increase in stiffness so close to the 2.5x clamed and possible with a change in arm length.

They say it is tubular so it would probably be safe to assume it has a stiffness between 24-25mm so in the neighborhood of 690- 820 lbs./in up from the stock 333lbs./in

See the lines for eibach eu and us on the rsb table I posted for examples of the wall thickness change effect on stiffness.

 

[xXDavidCXx]

Changed the front page to reflect my move to using Suspension Analyzer.

 

[xXDavidCXx]

Updated the front page with more data from the racing aspiration's model, and a camber vs body roll analysis.

 

[xXDavidCXx]

Front 0.98 (spring and bar), rear spring 0.64, bar 0.45 (not verified). The rear numbers could be wrong though, I need to re-measure the entire car.

I need to remeasure the front as well. I didn't follow the correct procedure. I just assumed B=A because I assumed A would be the lower ball joint, not the center of the tire.

View attachment 194818

Based on this diagram, with my winter tire/wheel setup:
B=13.75in
A=19in
MR @ wheel=0.72
Strut angle=10deg
MR of strut=0.973

For the rear I calculated:
B=15in
A=27.25in
MR=0.55

This puts my winter setup ride frequencies at:
F=0.93
R=1.35

Adding my race springs, but not accounting for the wider wheels:
F=1.45
R=1.85

@JackRabbitSLIM

Guess what spring rates you need for a true 2.0/2.2hz split?
800/1050

I've been schooled.

The dude abides.