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Spring Rates

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EuropaTC:
Hi folks,

We were straying into spring rates on the Race Car Collection thread so rather than take that completely off topic, I've opened a specific thread. 

I'll start off by saying I have little knowledge in this area but as mentioned on Stefan's thread, I'm very interested and have been browsing through the Yahoo knowledgebase to see what people have used.  It's quite a minefield and so I've pulled some numbers out for what people have used in the past and attached them as a pdf file to this post.

Credit for all of this info is down to the Yahoo group members and most comes from people like Jay Mitchell, Phil Ethier, Tim Engel and lots of others that I've forgotten.....

The outline is;

OEM S2, 100lbs/ins Fr, 72lbs/ins R
OEM TC 116lbs/ins Fr, 75lbs/ins R

The ranges then seem to go between 140/150lbs Fr with 115/120lbs R through 200-250lbs Fr coupled with 100-150R which seem to be about the limits for road use.

After that some racers are up to 500/750lbs on the front but I couldn't find what rear rates were used.

The PDF puts all this in a neater and more digestible format.  One thing that does come out to me as a newbie is how significant the leverage differences are between the Fr & R suspension geometries and consequently how that can lead to what looks to be completely "wrong" differences in spring rates between front & rear with the fronts looking way too hard until you feed in the geometry leverages.

The PDF also includes a chart created by Phil Ethier and posted in the Yahoo files which I've included in case anyone on this forums isn't signed up with the Yahoo group (you should be  :) ). One really good feature is how he's listed the differences in ride heights for the spring combinations, very helpful for those of us with road rather than race cars.

I'm still trying to get my head around it, any comments/guidance/theories are more than welcome !

Brian

Serge:

Thank you Brian,


that's a nice summary of all of the discussions.

I'll add these links and files below:
- OptimumG tech tips (especially section on springs & dampers)
- Suspension dynamics calculator
- Spring & damper rate calculator from the files section on the Yahoo-group

http://www.optimumg.com/technical/technical-papers/

http://farnorthracing.com/autocross_secrets16.html

EDIT: I tried to attach to spring rate excell file, hower it was too large to add. You can find it in the files section in the Yahoo-group with the name: "Copy of SuspensionCalc"

I'll have to read up on all of the above, because a lot of the discussion between Jay and Phil deals with suspension frequencies etc and that's a little bit much for me at the moment, so I'll study up and try to dig a little bit deeper to try and understand all of the choises to try and find the optimum.

Also, a lot of it depends what kind of tyres you are using. Very sticky (semi-)slicks will give better results with stiffer springs than vintage racing rubber (threaded crossplies: Avon CR6ZZ/Dunlop CR65) that work a lot better with lower springrates.


Kind regards,

Serge
http://www.youtube.com/sleursmotorsport

EuropaTC:
Hi Serge,

 :)    I'm glad someone else is interested in all this talk about suspension frequencies, etc, and was hoping someone would come back.   I recognize some of the formula and principles, but I'm still trying to get my head around the frequency ranges that Jay talks about and what they mean in real life.   A couple of other points came out during the research, which I think makes this topic relevant to us all;


* It's easy to get recommendations but these may or may not work in your particular case/driving style
* what works on an S2 probably won't transfer exactly to the TC due to chassis, weight and wt. distributions
* front geometry is different wrt ride heights on the S2 & TC and it's probably not a good idea to translate ride heights between the two without checking camber changes.
* the leverage & angle on the front suspension of the S1,S2 & TC makes a hard spring appear softer in practice
* as you go higher in spring rates the ARB makes less contribution
And finally the main reason for my interest (which you've also mentioned) is that tire compounds have changed. Even if you buy vintage patterns the compounds are unlikely to be exactly the same as the 60s, and back then tires slid at lower speeds.  But now, even in the same sizes and profiles they grip more and so it's worth checking if what was excellent suspension in the 60s is still relevant today, especially if you change wheel/tyre profiles.  This year on the advice of Pat Thomas (long term Lotus racer) I more than doubled the spring rates on my Elan  and the difference on the exactly same tires is staggering.  I'd love to see a similar transformation with the Europa.

So that's my interest in the topic.

I've got a copy of suspensioncalc but rather oddly I'd sooner work through the maths myself because it will give me a better understanding of what's going on.   I hadn't seen the OptimumG pages which I've printed out to read properly so thank you for that link, it looks very useful.   FarNorthRacing looks a step too far for me at the moment  ;)

Brian
 

EuropaTC:
For anyone else intrigued by spring rates - has anyone measured (or know) the angle of the front spring/dampers ?   I'm currently doing some sums and it looks as if the mounting angle is significant in determining the actual rates. I tried to measure my car with a digital angle finder but it's really too big to fit alongside the spring.

My best guess is 60deg to the horizontal; a couple of degrees either way don't make much difference but I've seen references in the knowledgebase to a 75deg angle, which does make a difference to the sums. Anyone have any better info or thoughts ?

Brian

erioshi:
When I finally get a chance to start working on my car, I'm planning to try and capture the motion ratios and other details of the suspension to help verify the data that's already been collected.  It does look like the front suspension's spring may be leaned over far enough to have a falling spring-rate as they compress, but I haven't had a chance to get in and take any measurements.

What follow is a bit of explanation, for those who know please just make sure I haven't messed it up, otherwise it's here to help those who may not have any information on what we are talking about.  The whole frequency thing is fairly straight forward.  It is essentially a form of short-hand that allows for comparisons of spring rates between different cars.  One hz is equal to 60 cycles per minute, or one cycle per second.  Or a  suspension with a frequency of 1 hz would have a natural oscillation of one cycle per second (assuming the springs were bounced and then left to move the suspension up and down on its own without any interference from the dampers or ground).

As a tool, comparing frequencies can be fairly useful when comparing the data from different cars using the same chassis.  However when you start comparing between cars and chassis types that are dissimilar, the usefulness of the data can go down somewhat.  Spring frequency does not directly include the data needed to determine side to side roll resistance, but obviously higher spring rates (and the resulting higher frequencies) will be more resistant to body roll.

Another limiting factor in cornering performance and the effectiveness of high spring rates and stiff roll bars is chassis rigidity.  If a car's chassis is not sufficiently more resistant to flex than the effective spring rates in use, really stiff springs and bars can introduce chassis flex and turn the chassis itself into a lager, un-dampened main spring that can make trying to fine tune and fix handling problems almost impossible.  I'm not sure exactly what the stiffness of a Europa chassis is, but I've hear rumor (and read somewhere) that the backbone chassis is fairly flexible, and that is one of the reasons (along with the car's low weight and low un-sprung mass at each corner) that Lotus usually fitted the cars with soft springs.  I'm sure the typical roads of the day didn't invite rock hard spring rates either.  Measuring the stiffness of my car's chassis is also one of the things I'm hoping I can manage when I pull it all apart and restore it.

There is quite a bit of good reading in the links provided.  They are a great place to start for those who want to learn more about all of this stuff.

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