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suspension maths?

740atl

740atl

Gear acquisition syndrome
Joined
Apr 17, 2010
Location
USA
The limit of my suspension knowledge is 'get bilsteins and use bigger swaybars'.

How do you go about choosing a spring rate rate for coilovers (240 with a v8)? Is there a mathematical way to go about making an educated choice?

In a related note, how do you best determine what valving is required for the dampers to go along with your choice of springs? I see you guys quoting valving numbers for certain springs and I see people talking about certain spring rates not working well with a certain shock. I really don't understand what you're talking about.

If you could offer some advice or perhaps reference texts I'd appreciate it.


My current setup uses 325lb springs with bilstein hd struts up front (not shortened) along with 250lb rear springs and chevy 4x4 bilstein hd's.
 
You'll need corner weights, unsprung weights and a target suspension frequency as well as motion ratios. All of this information is sitting on this site in various threads.

325# springs can't be controlled by stock HD's. Not sure about the rears. I just called up Bilstein and had them revalve a new set of shocks and struts for my set up. Cost was super cheap and they work great. Know what your car does for when you get a call from them - they need some real information.
 
Canuck said:
325# springs can't be controlled by stock HD's.
Click to expand...

see... that there is what I'm talking about. Where did you get that information? How do you know that the setup won't work? Are you talking from experience or is there something concrete which made you come to that understanding? Oh... and why?
 
If you have a strong math back ground, a book that will give you the necessary equations is Dynamics: Theory and Applications by Kane and Levinson.. You are also going to have to take your car apart and weigh all the suspension components and measure their lever arm lengths and attachment points. You will also need a set of accurate scales to get the corner weights of the car. Do you even have it figured out what your goal is? That is why Canuck said you want to know the frequency you are trying to achieve. No single suspension set up works perfectly in every situation. Good luck.
 
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740atl said:
see... that there is what I'm talking about. Where did you get that information? How do you know that the setup won't work? Are you talking from experience or is there something concrete which made you come to that understanding? Oh... and why?
Click to expand...

I ran 300's on HD's last year and while it didn't feel terrible, the ride was not very well controlled. In racing, the transitions were terrible - the car flopped around a lot. With a set of 400# fronts (250# rears) to set the ride frequency at around 2.2/2.5 Hz front/rear (I'm not going to run and look this up) and the work done with corner weights, motion ratios and some reading on farnorthracing.com I contacted Bilstein and they made me up some shocks. They wanted to talk to me first - so be ready with real information about what your car is currently set like and what it does that your not liking - these are race car guys, but internet jocks - they don't have time for weak descriptions and shrugs.

Look, on the street, most people will not notice that your shock is under damped - they won't feel the oscillation...riding in a car with 300# fronts on HD's would drive me nuts. Do you feel it bob up and down after every bump? When I was going hard into a corner, the car would flop, wouldn't "take a set" and wouldn't generate grip. This was making driving it at the limit pretty harry. You had to be very careful or you'd spin it or plough through a corner. No amount of explaining this to people would register as it would oversteer or understeer depending a lot too much on me and not enough on other factors.

People said, "Trail brake into the corner to settle the front end." and I'd try and that didn't work. Or the old, "slow in fast out." - my favourite was "You have to get that thing rockin to get it through a slalom." And you did. It would flop it's way around the track and just stay off the throttle or bad things would happen. Get my point?

So I read what guys had done on this board WAY back in the day and worked through their threads and occasionally bothered them. They had worked out empirically what worked and they ended up with ride frequencies like the above mentioned. I then had the chance to get my car corner weighted at a race, and bought some necessary things from Kaplhenke Racing (note I didn't say optional).

The car now corners FLAT and hard - I still need to unfix a few things I changed so I can get some caster back (long story...about longer LCA's). I've had SCCA National Champion drivers drive the car and all comment on the crispness of the handling. One of the best changes was a set of custom valved HD's. Adjustable Koni's are better - no doubt, but I don't race on multiple surfaces and I'm happy with what I've got. The online calculator on Far North Racing is all you really need to do this - you will need to scour the site for numbers or just whip out your ruler and get this all measured up. And by ruler I mean level surface, plumb bob, callipers, tape measure, tape, and sharpies (you'll loose at least one of these - use for marking the floor and marking bolt centres).

Here's a little help.

 
^^
Let's make a serious attempt at having a discussion that leads to information sharing and educating everyone that wants to learn from others. That is what the Performance and Maintenance forums are for.

Nice start from Canuck.
 
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I ran HD's out of the box in my 244, 400lb front, 225 rear, 28mm front sway, no rear, poly or spherical in every location.
On smooth surfaces the HD's worked great with the standard valving. It could have used a 25mm up front, and 450lb springs to fine tune it a bit. The car worked great, and several people on here who have driven or ridden in it will agree.
Zero math was required for this build, just using known ratios for springs and sway bars.
That was on a 240.

On the 780 I had 500lb on the front with revalved HD's, don't recall the rear for the IRS off the top of my head, and a small sway bar back there. Good bushings and proper LSD. With the 500lb springs I had to send the HD's to Bilstein to have them valved. Fairly simple process, I gave them my information, discussed the usage, and they did the deal.

There's no need to understand the whole of the science behind it as long as you can apply the knowledge gained from the science appropriately.
 
Define "worked great" please. We need the context. If you're driving your car at 60 MPH, and quickly flick it, what happens? Are you seriously racing or just street driving? Any shock will work on a smooth surface - without suspension travel, there is no need for a damper. What happens on an uneven surface and driven at the limit?
 
Canuck said:
I ran 300's on HD's last year and while it didn't feel terrible, the ride was not very well controlled. In racing, the transitions were terrible - the car flopped around a lot. With a set of 400# fronts (250# rears) to set the ride frequency at around 2.2/2.5 Hz front/rear (I'm not going to run and look this up) and the work done with corner weights, motion ratios and some reading on farnorthracing.com I contacted Bilstein and they made me up some shocks. They wanted to talk to me first - so be ready with real information about what your car is currently set like and what it does that your not liking - these are race car guys, but internet jocks - they don't have time for weak descriptions and shrugs.

Look, on the street, most people will not notice that your shock is under damped - they won't feel the oscillation...riding in a car with 300# fronts on HD's would drive me nuts. Do you feel it bob up and down after every bump? When I was going hard into a corner, the car would flop, wouldn't "take a set" and wouldn't generate grip. This was making driving it at the limit pretty harry. You had to be very careful or you'd spin it or plough through a corner. No amount of explaining this to people would register as it would oversteer or understeer depending a lot too much on me and not enough on other factors.

People said, "Trail brake into the corner to settle the front end." and I'd try and that didn't work. Or the old, "slow in fast out." - my favourite was "You have to get that thing rockin to get it through a slalom." And you did. It would flop it's way around the track and just stay off the throttle or bad things would happen. Get my point?

So I read what guys had done on this board WAY back in the day and worked through their threads and occasionally bothered them. They had worked out empirically what worked and they ended up with ride frequencies like the above mentioned. I then had the chance to get my car corner weighted at a race, and bought some necessary things from Kaplhenke Racing (note I didn't say optional).

The car now corners FLAT and hard - I still need to unfix a few things I changed so I can get some caster back (long story...about longer LCA's). I've had SCCA National Champion drivers drive the car and all comment on the crispness of the handling. One of the best changes was a set of custom valved HD's. Adjustable Koni's are better - no doubt, but I don't race on multiple surfaces and I'm happy with what I've got. The online calculator on Far North Racing is all you really need to do this - you will need to scour the site for numbers or just whip out your ruler and get this all measured up. And by ruler I mean level surface, plumb bob, callipers, tape measure, tape, and sharpies (you'll loose at least one of these - use for marking the floor and marking bolt centres).

Here's a little help.

Click to expand...

Thanks Canuck!

I'm glad you mentioned control.. the next question I was going to ask you was what would it feel like driving it?

I haven't driven the car yet... the coilovers were something I got in in a kit from someone here selling it years ago and it finally got on the car. (And I'm not kidding when I'm completely clueless in this endeavor. There wasn't any planning or thought given to what worked or didn't.)

I'm looking at valving numbers for the bilstein hd from Dave Barton's site 129 compression/75 rebound... I'm not sure what the numbers represent... the only thing I can find is from eshocks.. http://www.eshocks.com/bil_ORgd.asp?Manf=All where they say
Bilstein valvings for Off-Road are measured in Newtons at a velocity of 0.52 meters/seconds (approximately 20 inches/second). The ratings shown correspond to those measurements; rebound force is the first number, followed by compression force (rebound / compression). Conventionally, the ratings are written as one tenth the damping force in Newtons.
Click to expand...
.

If that's the case, the 129 is 1290 Newtons which is 290 pounds (and 168lbsf for the rear respectively).

I guess what I'm asking is aside from your experience, how do you know that that number represents an underdamped situation for the 300lb springs?

I found a bit of information online about ride frequencies (LINK)... In the article I get the idea of ride frequency represents and they mention a ride frequency for a passenger car 0-1.5Hz and 1.5 - 2.0 Hz for "sedan racecars and moderate downforce formula cars "

You said you chose 2.2/2.5 Hz front/rear... I am assuming your vehicle is a track car? I don't do any autocrossing or racing... onramps and offramps are my racetrack I guess... I'm not ruling out autocrossing for the future, but would you set the ride frequency that high for conditions more like I'm referring to?

I found a bit of information on motion ratio (LINK) and found a link on Tb (link) and (link). The magic number is 0.74(repeating). I assume this number is the same for all 240's?

Thanks again!

Mike
 
A huge amount will depend on the intended usage of the car, both spring rate wise, ride height wise, and damper valving wise. A pure track set up will differ from a tarmac rally set up. to a gravel rally set up, to a drag set and to a road set up. Commonest mistakes are not enough droop travel, especially at the rear. too much anti roll bar making an independent front suspension effectively a beam axle, and inappropriate damper valving for a heavy live rear axle car. The subject is huge and can be subjective. Either get a pro to do some analysis or just increase front spring rate percentage by the same percentage of additional front end weight gain, get some Bilsteins that Bilstein have valved as well as possible given what info you can muster, and start experimenting from there if it's a road car for normal road usage. Add perhaps 25 to 35% spring rate to the front for fast road usage, maybe 15% to 20% to the rear. Expect to spend money on various springs. Don't fiddle about with 10 lbs ins rate changes, go in at LEAST 50 lbs ins to get in the ball park. If you go too low without addressing roll centre height changes and steering geo expect to have a dog o n your hands. Double adjustable dampers will speed things up, and small diameter none progressive "race" type springs will aid the budget. Having custom stock diameter progressive springs wound will cost a fortune. Use progressive rate bump stops effectively to keep sane linear spring rates with a progressive rate in hard cornering, they are much cheaper than progressive rate springs. Computer programmes are only as good as the accuracy in which data is added. Getting a pro shop to draw all the geo up and work it all out is beyond all but the most well financed amateur racers. If you get a good pliable set up that does nothing sudden or surprising, with good traction, and no weird tyre wear you are doing OK...

75% of road cars that people have made home handling mods to, that come to me because they have given their drivers a fright turn out to be too low, too stiff, with bugger all suspension droop, and with nasty front bump steer due to incorrect steering rack arm inclination.
 
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Chris, I appreciate your response. Lots to work with there. I have read about the influence of the swaybar on handling and how when people did get actualy suspension/damper improvements, less swaybar was necessary.

I'm afraid this is a street car that may someday see an autocross course. I unfortunately will not be able to seek out a professional for this as this is severely out of my price range. I was primarily looking for advice that would allow me to make an educated guess on what might work.
 
Ok... I plugged away at this a bit.

Using the equation here to calculate spring rate... it's the simple harmonic motion spring frequency equation reworked to solve for spring constant.

Ksp (spring rate in N/m) =4(pi^2)(ridefrequency^2)(sprung mass in kg)(motion ration^2)

Given a motion ratio of 0.744 (from turbobricks)
a ride frequency of 1.5 (passenger car)
corner weights for v8 240 from Dave Barton's site (convert to kg for the equation).
LEFT RIGHT LEFT PCT RIGHT PCT

F: 896# 890# 27.7 27.6

R: 702# 742# 21.7 23.0

TOTAL 3229# F/R PERCENT = 55.3 / 44.7

I calculate a spring rate of 114lb/in.... which seems shockingly low... and is probably that of a spring rate of a stock volvo 240 coilspring.... seeing that Dave Barton's site says the ipd coilspring is 152/129 F/R.

ride frequency of 2Hz = 203lb/in
ride frequency of 2.5Hz = 317lb/in


Ok... so someone check me on this and see if I am getting this.... the choice spring rate is dictated by your corner weights and how stiff you want the ride to be. How'd I do?


Here's another question.... how harsh is a ride frequency of about 2.5?


EDIT... and btw... please someone check my maths.
 
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"How to make your car handle" by Fred Puhn, and every Carroll Smith book you can afford to buy will set you in good stead. Most aftermarket dampers are inferior to Bilstein Sports unless you get into expensive high end stuff. Entry level, in my opinion, for double adjustables are the UK built Nitrons. I do a lot on high end Jap cars, and the likes of Tein, HKS, and other brightly packaged and tastelessly anodized stuff is plain junk, internally. Koni and Bilstein actually have good valving and decent pistons, seals, rod chrome and top bushings. A decen damper is one you can buy spares for. ask the question, if you can't get bits, different valving etcetera it's not what you need to be buying. Bilstein is my go to manufacturer for a decent sport damper unless my customer can afford exotical like Proflex, Penske etcetera. Their prices will make most people wince, although it's lovely gear :) If you are going to lower the car much you NEED the lower wishbone to strut spacers, Kalphenke or whatever they are called seem to do a nice forged unit. For autocross you need some compliance, and a GOOD LSD. Don't forget cheap tyres on the best suspension you can create will never allow the car to work to its ability. If you can run special road legal track tyres, do so, they are totally different to a road tyre, in construction, compound and design criteria. my favourite is the expensive, but brilliant, Michelin Pilot Sport Cup. CUP being the operative word. If I can help with specifics I will, I am certainly not an F1 suspension designer, but I do specialise in suspension work for amateur track racers and people wanting a fast road car that still works on an average road surface.
 
Canuck said:
Define "worked great" please. We need the context. If you're driving your car at 60 MPH, and quickly flick it, what happens? Are you seriously racing or just street driving? Any shock will work on a smooth surface - without suspension travel, there is no need for a damper. What happens on an uneven surface and driven at the limit?
Click to expand...

At 60 it would change direction, that simple. Add steering input and it went there.
Biased towards oversteer, but controllable easily.

I didn't encounter too many uneven surfaces when driving at the limit, the limit was too high to safely reach on the street. The uneven surfaces I did encounter were handled without drama. It was stiff enough where it was felt, but the car never felt unstable.

There is suspension travel even on smooth surfaces, proper damping is needed there just as much as on rough surfaces.
 
Following. My 240 has the usual suspension mods and drives lovely on the street but never liked it on a (bumpy) track of course driving way faster than on the street. Even with real tires.

Can?t and won?t comment on anything since I am still reading up on the theory and testing what I can cheaply change.

I might have some good Ebooks about suspension theory that are worth it to share.
 
Easy stuff first - driving the car is fine on smooth roads...crossing railway tracks is another story - pot holes, dodge that stuff regardless of your shock settings. This car is driven (including winter), though not my DD. I autocross it regularly. I wouldn't change anything much for track driving. Most people have no idea what the limit of their car/suspension is (hey, if you haven't spun it - you don't know and don't try to figure this out on the street please). So, I've driven it thousands of kilometers in a single day on the highway and it's no issue at all. I'm also not 30 years old anymore (or even 40) - so if it was nonsense it would get changed right away.

I also struggled to figure out Dave's dyno sheet. The ones I'm about to post are coming out of the same facility, but the units are given and are NOT what the R-Sport ones are. These are in both N/Sec and #/sec for velocity measurements and force in N and #.

The standard for Bilstein is to report shock force (that's the compression and rebound numbers) at 0.5246 m/s in # - so my shocks are WAY stiffer than what you've seen. I've included the typed report as well in the interest of data.

For the use of the car and the springs, this is what feels right. How the shock gets to these ultimate numbers is also important. This is what's being controlled by the shim stack.

Have you included bars in your calculations? What are you using for sprung mass?

Anyway - here are the charts and data.















740atl said:
Thanks Canuck!

I'm glad you mentioned control.. the next question I was going to ask you was what would it feel like driving it?

I haven't driven the car yet... the coilovers were something I got in in a kit from someone here selling it years ago and it finally got on the car. (And I'm not kidding when I'm completely clueless in this endeavor. There wasn't any planning or thought given to what worked or didn't.)

I'm looking at valving numbers for the bilstein hd from Dave Barton's site 129 compression/75 rebound... I'm not sure what the numbers represent... the only thing I can find is from eshocks.. http://www.eshocks.com/bil_ORgd.asp?Manf=All where they say .

If that's the case, the 129 is 1290 Newtons which is 290 pounds (and 168lbsf for the rear respectively).

I guess what I'm asking is aside from your experience, how do you know that that number represents an underdamped situation for the 300lb springs?

I found a bit of information online about ride frequencies (LINK)... In the article I get the idea of ride frequency represents and they mention a ride frequency for a passenger car 0-1.5Hz and 1.5 - 2.0 Hz for "sedan racecars and moderate downforce formula cars "

You said you chose 2.2/2.5 Hz front/rear... I am assuming your vehicle is a track car? I don't do any autocrossing or racing... onramps and offramps are my racetrack I guess... I'm not ruling out autocrossing for the future, but would you set the ride frequency that high for conditions more like I'm referring to?

I found a bit of information on motion ratio (LINK) and found a link on Tb (link) and (link). The magic number is 0.74(repeating). I assume this number is the same for all 240's?

Thanks again!

Mike
Click to expand...
 
Canuck... quick question... it took me a minute to figure out that both graphs were the same but with different units :) .... In the compression side (top) of the graph, the two separate curves represent front (top) and rear (bottom)?

Canuck said:
Click to expand...


Also, In the data tables, the values reported at 524mm/s (.524m/s), there are two sets... one at 210/260 Comp/rebound, the other at 323/599... I'm assuming this is these are rear and front respectively?

Easy stuff first - driving the car is fine on smooth roads...crossing railway tracks is another story - pot holes, dodge that stuff regardless of your shock settings. This car is driven (including winter), though not my DD. I autocross it regularly. I wouldn't change anything much for track driving. Most people have no idea what the limit of their car/suspension is (hey, if you haven't spun it - you don't know and don't try to figure this out on the street please). So, I've driven it thousands of kilometers in a single day on the highway and it's no issue at all. I'm also not 30 years old anymore (or even 40) - so if it was nonsense it would get changed right away.
Click to expand...

Great! Exactly what I was looking for.

To answer your question, no, I haven't thought about swaybars yet... I am busily reading about it though.
 
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