Author Topic: Spring Rates  (Read 4369 times)

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Offline EuropaTC

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Spring Rates
« on: Sunday,December 01, 2013, 09:36:41 AM »
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


Offline Serge

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Re: Spring Rates
« Reply #1 on: Sunday,December 01, 2013, 10:33:17 AM »

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
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Offline EuropaTC

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Re: Spring Rates
« Reply #2 on: Sunday,December 01, 2013, 11:33:46 PM »
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
 


Offline EuropaTC

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Re: Spring Rates
« Reply #3 on: Friday,December 06, 2013, 12:09:38 AM »
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

Offline erioshi

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Re: Spring Rates
« Reply #4 on: Monday,January 13, 2014, 12:50:54 AM »
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.

Offline EuropaTC

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Re: Spring Rates
« Reply #5 on: Tuesday,August 12, 2014, 10:15:45 PM »
Well, after more than 8 months of head scratching, I've decided to take a punt on this modification and ordered some adjustable dampers and higher rate springs.  They came this week and look far too nice to put on a car and get them dirty  ::)

For anyone interested in the rates, I've decided on suspension frequencies of around 1.8 which on my car/geometry translates to spring rates of 250lbs/ins & 140lbs/ins.  It's slightly higher at the front due to the roll bar contribution and of course they aren't exactly the same rates front & rear but I think it's good enough for a starting point.  I've read so much about differing theories of having higher rates at the front or balancing rates Fr/R that I've decided the only way to find out what suits me is to experiment myself.

Yesterday I installed the new kit and have yet to decide on ride heights, at the moment everything is on the lowest settings and might be too low for a road car.  I will have to drive it and settle everything down I think, but at the moment it looks "cool" to the little boy in me  ;)

Finally, a question for anyone else who has lowered their car and that is "have you found problems with bump steer and if so, what have you done ?" 

The second image shows the front of the car and the steering rod is at a noticeable angle to the rack, so I'm wondering if it will cause problems. I'd be interested if anyone else has encountered similar things.

Brian

Offline 4129R

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Re: Spring Rates
« Reply #6 on: Tuesday,August 12, 2014, 11:55:34 PM »
I have fitted the same shox.

Be careful not to hit anything with the anti-roll bar. I hit the garage concrete floor when driving in off the gravel, and smashed the A/R bar mounting pin in the bottom of the shox. I had to re-tap the hole in the ally, and re-fit the pin deeper into the shox. The pin is just a screw fit into the ally, with the hole drilled right through into the bigger hole where the rubber bush is situated.

Offline Serge

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Re: Spring Rates
« Reply #7 on: Wednesday,August 13, 2014, 12:53:40 AM »
They look nice! Did you buy them from ProTech or from SJ Sportscars? I've read that there were some issues with the bush sizes when they first sold them, and they weren't very helpful (but fortunately ProTech is).

What are the open and closed lengths of those shocks? And what size tyres are you using? Can you put up some photos of the wheel gap?

Hitting the ARB studs are a real problem, a fellow Europa owner here in belgium has had to re-weld his studs on his AVO's at least three times. I've decided that I don't want the studs to be part of the shock and I'll build a stud separately from the shocks. If I'm spending quite a lot of money on shocks, I don't want to ruin them by hitting the stud on our horrible Belgian roads.

Kind regards,

Serge

Offline EuropaTC

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Re: Spring Rates
« Reply #8 on: Wednesday,August 13, 2014, 12:55:27 AM »
Thanks for the tip, it's good to know that should the worst happen it's still fixable.  There's a slight ramp into my workshop which I do occasionally catch with the Elan's exhaust but the Europa was ok yesterday on it's maiden voyage up and down the drive  :) 

Are yours the ones from SJS Lotus with their front springs,  and if so where about have you got your ride height set ? On the lowest setting it looks just a bit too low/boy racerish at the moment.  But of course that might all change when I look at it again today.....

Brian 

Offline EuropaTC

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Re: Spring Rates
« Reply #9 on: Wednesday,August 13, 2014, 01:09:16 AM »
Hi folks,

They're SJS Lotus ones. I didn't notice any problems with the bushes at either end when fitting them, in fact on the original dampers I had washers centralising the top front mount which I didn't need to use with these. Everything else just slotted in place. I'll keep an eye on the rubber in the bushes though, the last ones I had seemed to collapse very early in life and I replaced them with bushes used in the front wishbones.

Serge, I didn't actually measure the damper dimensions just the springs in case I decide on another rate, but I did lay the rear old/new assemblies together and they looked about the same.  I will be playing with the set-up over the next few days so I'll take some more measurements and any part numbers that's on them. Presumably there will be full dimensions on the Protech website ?

My tyres are 195/50 x15 & 205/50 x15, very similar to the diameter of the previous 13" tyres. I only took photos of the assembly yesterday so I'll drive it round the block and get a few more today.

Brian

Offline 4129R

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Re: Spring Rates
« Reply #10 on: Wednesday,August 13, 2014, 10:42:53 AM »
Thanks for the tip, it's good to know that should the worst happen it's still fixable.  There's a slight ramp into my workshop which I do occasionally catch with the Elan's exhaust but the Europa was ok yesterday on it's maiden voyage up and down the drive  :) 

Are yours the ones from SJS Lotus with their front springs,  and if so where about have you got your ride height set ? On the lowest setting it looks just a bit too low/boy racerish at the moment.  But of course that might all change when I look at it again today.....

Brian

Mine are from SJS. I have left them on the factory low ride height.

If you do have a problem with the A/R bar stud pulling all the thread out of the shocker casing, you can re-drill and tap the ally casing for a larger diameter stud.

I hit my concrete garage floor with the whole A/R bar. I have now leveled the driveway with 5 bags of ballast from Wickes. Cost less than £10, and the only problem now is the gravel gets caught between the garage door and the concrete. If I cut the door too high, the mice will get it ! All the fun of living in the countryside.

Offline EuropaTC

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Re: Spring Rates
« Reply #11 on: Wednesday,August 13, 2014, 10:26:25 PM »
Thanks for the info. I spent some time yesterday with it and decided that a lot of my worries about ride height were just in my head.  It is lower than before certainly but after all it's got adjustable platforms and so can go anywhere it wants to, so I'd better just get on with it. 

What did surprise me was that there's very little increase in negative camber so unless I find other problems I'll leave it as it is for now and maybe make some shorter roll bar mounts to keep the relationship the same. The photo shows before and after shots taken on roughly the same part of our drive, you can see a difference at the front, the rear not so much but it is there.

And for trivia fans - when doing my calcs I'd taken the OEM spring data from the manual and for the rear it says 19.6 coils. The new ones are 19 but looked markedly different when I fitted them so out of interest I counted the old ones and found just over 22.  Either that's another "Manual Misprint" or perhaps some of the "Genuine Lotus" parts aren't that genuine ?

Brian

Offline pboedker

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Re: Spring Rates
« Reply #12 on: Wednesday,August 13, 2014, 11:53:45 PM »
They look great, Brian! :trophy:

I have the same on my rear wheels, from SJS and with the original spring rates. I set the dampening to the middle setting and has been happy with this for road use the last 6 years. I'm not bright enough to change it to another setting, even for the occasional club track days ;)

But they developed a squeek, which is mostly heard at slow speed over speed bumps and so. It looks like it is the inside of the spring rubbing on the height adjustment threads. I heard that the emptied tubes from 'silicone guns' fit perfectly in between so I will try to change this during the vinter hibernation. Of course... Now you've already fitted yours to the car, just thought I'd mention it.  :)

Re. the ride height, I set the rears to a height that looks OK compared to the slightly lowered front height (which was done by the PO). I've never found a clear opinion on what the front/rear ride heights should be - other than the manual mentioning something about 5" to the front crossmember and setting the rears so the car is level. I think this is a very thin explanation, especially considering the other wheel and tire sizes we now run on. So, I'm waiting for you to find the perfect solutions, after which it will be a small task to adjust to the same bumper heights plus the offset from the wheel/tire combo. In theory...  ::)
Peter Boedker
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Offline Serge

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Re: Spring Rates
« Reply #13 on: Thursday,August 14, 2014, 05:02:15 AM »
Thanks for the photos, Brian, it really looks great!

Do you feel that you made the right choice regarding the spring rates? Not too stiff for the road?


Kind regards,

Serge

Offline Bainford

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Re: Spring Rates
« Reply #14 on: Thursday,August 14, 2014, 08:43:18 AM »
The car looks great, Brian. The lowered stance is perfect. If it poses no issues with steet driving, I would certainly go with the current low setting.
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