Spring rates, motion ratios, wheel frequency

What are yours?

My contribution:
07 Open class STI:

250 lbs/in front
200 lbs/in rear

1:1 motion ratio f/r (right?)

Sprung Corner Weight:
820 front
580 rear

Wheel Frequency:
104 CPM front
110 CPM rear

Hey Carl I get it except the wheel frequency part and CPM (what? Counts per minute?) It's new material for me.

I'm really interested in vehicle dynamics but what I know is self taught from what I've read in books and picked up here or there. I've never had formal training on the matter. But I find this stuff extremely interesting. Always interested in learning more.

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Michael LeCompte

Cycles per minute.
From alan staniforth's race and rally car source book:
Wheel Frequency = 187.8 x SQRT(Wheel Rate / Sprung Weight)
(square root symbol wouldnt work, so SQRT= square root )
I'm new to it too, which is why I want to gather more data points, but to me it seems that wheel frequency is a good way of comparing suspension rates from car to car because it accounts for suspension leverage, vehicle weight, etc. Though one thing that it doesnt account for is sway bars.
Heres a snippet of my excel sheet:


What I wanted was to get recommendations on spring rates for my volvo 240 that I'm wanting to build (someone buy my golf so I can start!) but I figured why limit my data source to just volvo 240's. This way there can be more discussion and more learning for everyone!



Edited 2 time(s). Last edit at 04/24/2009 09:24PM by Carl S.

Carl S Wrote:

> figured why limit my data source to just volvo
> 240's. This way there can be more discussion and
> more learning for everyone!
>
>

Hey I can appreciate that! I just ordered up a copy of the book.
A friend of mine has the scales for his circle track car. I'm going to try to get over to his shop this week.

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Michael LeCompte

What seems to be missing from this type of analysis is the tuning and optimizing of springs rates to account for different suspension types. E.g., I would think one might decide different spring settings for the same weights, for a live axle versus an IRS, to help the live axle to track the road better. But, maybe with enough data, you will find different; could be interesting.

Data for you, Carl:
_______________________________

What seemed optimum on our old 71 Opel Ascona:
240 lbs/in front - Twin control arm with about 1.5:1 leverage ratio on the lower control arm (where the spring sets)
200 lbs/in rear - Live rear axle with torque tube; about 1.15 leverage ratio

Sprung corner wieghts (approx) with driver & co-driver
F 650 lbs
R 575 lbs

Comments: This was with older Bilstien shocks, 'sport settings' from the early 80's so have not idea how these would compare to modern strut or shock valve settings.
Spring rates were probably not been optimum, as these were springs in the stock perches, and with significantly higher rates at usable ride heights, the springs would leave the perches on the front, so we were near the practical limit. The rear springs WOULD leave the eprches by about 3/4" with the 200 lb.in rate.


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What we have been running on the Starion:
F 375 lb/in - McPherson strut
R 250 lb/in - McPherson strut

Srpung corner weights (approx) with driver and co-driver
Front 910
Rear 690

COMMENTS: Front strut inserts were GAB rally setting inserts; am changing to the Bilstein 41mm from JVL with around 325 lb/in springs. The original, stiffer spring settings were good for limiting front motion and not bashing the underside too much, and handled well enough, but may not be optimium (a smidgen on stiff side).
The rear struts were originally some "road race secret" setting KYB struts (Stock PN's but not stock valving! For TOTALLY legal Showroom Stock racing by the original owner, you know...). Current rears are Bilstein from JVL with the 250 lbs rate, works nice.

Regards,
Mark B.

Carl S Wrote:
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> 1:1 motion ratio f/r (right?)

Everybody just approximates struts as 1:1 but if you actually calculate it from the geometry on the car I think you will find that it's not quite there. My car is around 0.97:1.0

Do you get that by the cosine of the sai?

cos(15 degrees) = .966

So I suspect so that is the cal Tim is referring to.

Cool. I was trying to find that in race car vehicle dynamics, but it basically just says measure it.

SAI on 240s is 12 degrees.
Or actually, that's the angle between spindle axis and a plane perpendicular to the strut tube (ball joint mounting face).

The illustrious JVL is recommending 325 F and 175 R for 240s.

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Monika Hawkinson
Seattle, WA

Sofa King Wrote:
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>
> The illustrious JVL is recommending 325 F and 175
> R for 240s.
>

Is that with the stock rear suspension? Not the turret stuff?

That's with appropriately valved dampers.
I think by "the turret stuff" you mean the long-travel 50mm coilover shocks, which makes it easier to spec valving, spring rate, travel, etc. However, if you can do the math and figure the motion ratio on the rear, and find appropriate length and damping rate to fit in the stock location, then the same spring rate still applies.

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Monika Hawkinson
Seattle, WA

Yeah I already measured the motion ratio on the rear, like 1.22 or something for the spring and .74ish for the shock. I was just wondering if that 175 is what John recommends for the stock spring location, or if thats what the guys going with the coilovers in the rear use.

That's for coilovers.

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Monika Hawkinson
Seattle, WA

Gotcha.