Strut question for 74 super

[Eatoniashoprat]

That's a great thread, I'd forgetten about it. It should almost be linked in the suspension sticky.

I think I might just go with some 100lb springs for the front, see how it works, and then upgrade to 944 TB when I actually find some. Just hopefully with 100lb up front and stock out back it doesn't do anything weird.

Humble how do the 200lbs/inch ride on the street? There are a bunch of sources telling me that 100lbs/inch would be about right for my car seeing mostly street and some track but you have a lighter car with double the spring rate. Could this be due to a different rating system? I'll probably go with the QA1's as you have but I don't want to order the 100lbs/inch QA1's and have them too soft.

Mike

[evilC]

Originally Posted by Eatoniashoprat

Warning, I'm just learning this stuff!

I came across this page below:

http://farnorthracing.com/autocross_secrets5.html

By 80-100Hz did you perhaps mean 0.8 - 1.0 Hz? as suggested by this page or is there something I'm missing?

Mike

Hi Mike, there is either an error in the text or they have automatically factored in a different constant. The equation that I use is:
WR{wheel rate} = (SF{spring frequency}/187.8)^2 x SW{sprung weight}. You can check this out on the Eibach Spring site for convenience:
http://performance-suspension.eibach...sion_worksheet as an example - just rearrange the equation or alternatively plug in the figures. You will see that the Cycles Per Minute (or Hz) are 80+. Remember on the front Macpherson struts the spring rate = the wheel rate for all intents and purposes and I have not deducted the unsprung weight from the corner weight as this is negligible.

Originally Posted by Eatoniashoprat

That's a great thread, I'd forgetten about it. It should almost be linked in the suspension sticky.

I think I might just go with some 100lb springs for the front, see how it works, and then upgrade to 944 TB when I actually find some. Just hopefully with 100lb up front and stock out back it doesn't do anything weird.

Humble how do the 200lbs/inch ride on the street? There are a bunch of sources telling me that 100lbs/inch would be about right for my car seeing mostly street and some track but you have a lighter car with double the spring rate. Could this be due to a different rating system? I'll probably go with the QA1's as you have but I don't want to order the 100lbs/inch QA1's and have them too soft.

Mike

If the standard spring rate on the front is 70-80lb/in then conventional wisdom suggests a 30% increase in rate for fast road that comes out to 91-104lb/in so that the 100lb/in is about right but the final value very much depends on your corner weight with all the extras you are carrying. On the rear where you suggest retaining the standard torsion bar the spring frequency is 95CPM on Humble's corner weights that will be too close to the front CPM of around 93CPM - that will induce uncomfortable pitching for and aft as the two frequencies are too alike, there needs to be at least a 10% difference. Why not add some coilovers to the rear with some light springs to increase the wheel rate?. The damper top mount will have some load capacity for light springing and if you get stiffer springs you could always add a 5 bar KC brace.

Clive

[Eatoniashoprat]

Originally Posted by evilC

Hi Mike, there is either an error in the text or they have automatically factored in a different constant. The equation that I use is:
WR{wheel rate} = (SF{spring frequency}/187.8)^2 x SW{sprung weight}. You can check this out on the Eibach Spring site for convenience:
http://performance-suspension.eibach...sion_worksheet as an example - just rearrange the equation or alternatively plug in the figures. You will see that the Cycles Per Minute (or Hz) are 80+. Remember on the front Macpherson struts the spring rate = the wheel rate for all intents and purposes and I have not deducted the unsprung weight from the corner weight as this is negligible.


If the standard spring rate on the front is 70-80lb/in then conventional wisdom suggests a 30% increase in rate for fast road that comes out to 91-104lb/in so that the 100lb/in is about right but the final value very much depends on your corner weight with all the extras you are carrying. On the rear where you suggest retaining the standard torsion bar the spring frequency is 95CPM on Humble's corner weights that will be too close to the front CPM of around 93CPM - that will induce uncomfortable pitching for and aft as the two frequencies are too alike, there needs to be at least a 10% difference. Why not add some coilovers to the rear with some light springs to increase the wheel rate?. The damper top mount will have some load capacity for light springing and if you get stiffer springs you could always add a 5 bar KC brace.


Clive

Thanks Clive, The wheel frequency in these equations is the natural frequency of the spring as installed with the given corner weight right? If this is the natural frequency then intuitively if you push down on the bumper and let go the spring should try to oscillate at its natural frequency (only with the shock trying to dampen it). The reason I started looking it it is because 80Hz didn't seem right but I can definitely see the car bouncing up and down at 1Hz. Wait! I just realized that you are talking in CPM (cycles per minute), and the other page is in Hz (cycles per second) and if you convert they're in the same ballpark. . Those darn units!


The calculations you did I believe were for Humbles car at 1900lbs, whereas my car is about 2200-2300lb with me in it. And actually the factory springs were measured by topline to be around 63 lbs, and the maxx springs I have right now are 71.5lbs. Without punching the numbers in *guess* tells me I should be in the 105-125lb/inch spring range.

For the rear it's either weld in the 2 extra bars to make mine a 5-bar (do these with the motor/tranny installed?) and go coilovers OR, go the cheap way and put in 944 bars, although with 125lbs/inch springs this is probably going to have a similar CPM to the 125lbs/inch and will need to be even stiffer. Or go 100lbs/inch springs to be on the low end of the spectrum and do 944 bars. Time to crunch some numbers! *grabs calculator*

not to mention the fact that I don't have a rear sway bar (yet?)

[Eatoniashoprat]

on a macpherson front end shouldn't the motion ratio be measured to the center of the tire? So the lever arm would now be something like 1:1.1 or so?

Also, using a lever arm of 1:1.277 in the back is giving me goofy numbers. I can't seem to replicate the ones you produced in the other thread. I'm getting CPM values that are huge (150+). This is using the spring rate values you gave for the stock and 944 TB at the shock position (186 and 216, respectively).

This is fun


Good info here at this link also

http://www.swayaway.com/TechRoom.php#MOTIONRATIO

[evilC]

Originally Posted by Eatoniashoprat

on a macpherson front end shouldn't the motion ratio be measured to the center of the tire? So the lever arm would now be something like 1:1.1 or so?

Also, using a lever arm of 1:1.277 in the back is giving me goofy numbers. I can't seem to replicate the ones you produced in the other thread. I'm getting CPM values that are huge (150+). This is using the spring rate values you gave for the stock and 944 TB at the shock position (186 and 216, respectively).

This is fun


Good info here at this link also

http://www.swayaway.com/TechRoom.php#MOTIONRATIO

In a true macpherson strut the king pin inclination goes down through the centre of the strut and through to the centre of the tyre tread. Therefore, the spring acts directly in line with the contact point so the ratio must be 1:1 I appreciate that on negative scrub suspensions that inclination point actually strikes inboard of the centre of the tyre contact point so the motion ratio is marginally larger but not much.

In all this appreciate that the calculated spring rates are in fact only the starting point to achieve the desired handling and ride parameters so dwelling on CPMs in anything other than as an indication of the rate will be counter-productive. It might be of interest that in the early '80s after a great deal of testing the very competitive Talbot Sunbeam Tarmac Rally Car was using CPMs of 128 front and 133 rear and that was much less than the circuit racers.

It's Friday and a long w/e beckons so I intend to cool the brain down with some cold beers - Bye Bye!

Clive

[Eatoniashoprat]

Thanks Clive this has really helped a lot.

If I input my weight into the calculations this is what I get:

100lb/in front
CPM 85

170lb/in rear wheel rate (23.5mm)
CPM 95

~10% difference.

If I go higher rate in the front I'd have to go coilovers or even bigger TB otherwise the CPM's become very close.

Have a good weekend! Have a cold one for the germanlook forum (I may have several)

Mike

[Eatoniashoprat]

I think I just found an error on the Eibach springs worksheet page.

In step 2 it shows WR = C/MR^2 (assuming no ACF)

But I'm pretty sure it should be WR = C*MR^2

Otherwise your wheel rates would be higher than your spring rates since the motion ration is less than 1.

'Tip2' on that page also contradicts 'step 2'.

[evilC]

Originally Posted by Eatoniashoprat

Thanks Clive, The wheel frequency in these equations is the natural frequency of the spring as installed with the given corner weight right? If this is the natural frequency then intuitively if you push down on the bumper and let go the spring should try to oscillate at its natural frequency (only with the shock trying to dampen it). The reason I started looking it it is because 80Hz didn't seem right but I can definitely see the car bouncing up and down at 1Hz. Wait! I just realized that you are talking in CPM (cycles per minute), and the other page is in Hz (cycles per second) and if you convert they're in the same ballpark. . Those darn units!


The calculations you did I believe were for Humbles car at 1900lbs, whereas my car is about 2200-2300lb with me in it. And actually the factory springs were measured by topline to be around 63 lbs, and the maxx springs I have right now are 71.5lbs. Without punching the numbers in *guess* tells me I should be in the 105-125lb/inch spring range.

For the rear it's either weld in the 2 extra bars to make mine a 5-bar (do these with the motor/tranny installed?) and go coilovers OR, go the cheap way and put in 944 bars, although with 125lbs/inch springs this is probably going to have a similar CPM to the 125lbs/inch and will need to be even stiffer. Or go 100lbs/inch springs to be on the low end of the spectrum and do 944 bars. Time to crunch some numbers! *grabs calculator*

not to mention the fact that I don't have a rear sway bar (yet?)

Well Spotted! The CPM rates and Hz rates should be a factor of 60 apart. If you multiply the Hz rate by 60 then the CPM rate is 48 - 60, which will give very soft suspension. Traditionally, spring rates in the States have been softer than European rates so maybe that accounts for the difference.

At the rear I was working on 1.27 as the motion ratio and using one of the webs torsion bar calculators the standard 22mm TB gave a notional spring rate of 186lb/in with a wheel rate of 145.5lb/in (95CPM). The 23.5mm bars gave 216lb/in, 170lb/in wheel rate and a CPM of 103.

Staying with Humbles figures if you use 100lb/in springs at the front the CPM is 93 and with standard 944 TBs the CPM is 103 that is the 10% variation we are looking for. You could also retain the standard TBs and add 30lb/in springs as coil overs.

Clive