13 replies to this topic

[andygo]

I wonder if anyone could explain the difference between two springs as detailed below.

 

Fitted to , for instance, the rear of a single seat race car. Ride height controlled by pushrod adjustment.

 

1/. A 250lb spring with significant preload wound on to achieve the target amount of rear droop of 20mm.

 

2/. A 300lb spring with less preload to achieve 20mm of droop.

 

I'm trying to run a softer set of rear springs with preload in order to quickly adjust with less preload to make the car softer in the event of rain. For instance the car would run approx 5mm more droop in the wet. However I can easily get the required droop by softening off the 300 lb springs.  On the other hand will the 250lb springs wound up give the car the same handling in the dry as the 300's?

If you see what I mean... 

 

I realise that testing would show one way or the other, but it's tricky to book a wet test day, even in the uk! 

[murpia]

This explains:

 

http://lmgtfy.com/?q...rate vs preload

 

Regards, Ian

[andygo]

Thanks Ian.    

 

I was just hoping someone on here could give an explanation related to single seaters. Whilst I appreciate the theory is the same, there's nothing like a voice of experience. 

[gruntguru]

No. If you want a softer rate you need a different spring rate (or a different motion ratio).

 

Aim for fully-sprung operation everywhere on the circuit (i.e. avoid hitting bump-stops, droop limiters or damper travel limit). This will mean adjusting the spring seats not the pushrods, when you change springs.

 

I say this because it sounds like the droop limit is coming in to play in your handling setup - loosening the car when the inside rear hits the droop limit. If this is the case, the car will be less likely to reach that point in the wet anyway because of the much lower lateral acceleration available and so will be tighter in the wet anyway.

[Joe Bosworth]

Andygo:

 

On the one hand, until you advise as to what your corner weights are AND what effective verticle bump rates your suspensions give at the wheel you will never get a fully sensible answer to your question.

 

On the other hand, once you give that information the answer to your question will likely be self evident.  

 

Regards 

[Lee Nicolle]

A 200lb spring is a spring that when compressed 1" has 200lb of energy. IF you 'preload' the spring you will lift the ride height nothing else. You have to get a different spring or rocker ratio to change the wheel rate. All springs will get heavier the more they are compressed. Variable rate included. A variable rate spring generally [if the right spring is used] will be softer at ride height and get  a lot stiffer when they compress.eg, ride height are 125lb, compressed a little more become 200 lb and then the normal linear rate increase like a normal spring. Not sure of their value on a open wheeler, can be very usefull [and annoying too] on a tintop or Sports Car. They make the car more compliant on bumps, making the shocks work easier but as soon as loaded compress to be a more normal race type spring rate.

Most modern  road cars have them. I have never used them  personally as I like to keep things simple but there can be more speed if you can get your head around them.

[murpia]

Hi Andy,

 

Sorry for the Google thing but you need to realise that unless you start using the correct terminology no-one can help.

 

Examples from your original post:

 

'softening off the 300lb springs' could mean changing the springs to a softer rate, reducing preload, lowering ride height or literally machining away some spring so it's no longer a 300lb spring...

'to make the car softer in the event of rain' could mean reducing wheel rate, reducing rear roll stiffness percentage or literally reducing the Rockwell hardness of the whole car...

 

So, if you rephrase your query to something technically explicit, then maybe we can help.

 

But as a short answer, achieving the same static ride height with two different combinations of spring rate and preload, will never result in the same car behaviour for each setup.

 

Regards, Ian

[rms]

The ONLY way you can have "preload" is to have zero droop and you "preload" the spring to a higher load than what the car weight exerts.

You can not run zero droop on the rear.

 

Do you mean spring abutment height to change ride height and then a change in pushrod length to reset ride height ?

 

Why do you want more droop on the rear ? Are you unloading the inside wheel ?

[andygo]

I understand you can't run zero droop in the rear. What I am trying to do is get the last little bit of performance out of the car in the wet.  The car is fine, competitive in the dry, but in the event of changeable or wet conditions it's better to soften off the rear end to help with traction. What I was trying to understand was would it be a feasible proposition to run a softer rear spring, say a 250lb instead of a 300lb and by winding up the spring seats on the dampers increase the spring  rate to a 300lb, but with the opportunity to do a quick adjustment, say in the assembly area, to a softer spring rate by unwinding the spring platforms and correcting the ride height by adjusting the pushrods.

 

Now with the pointers from various people on here, I realise that a 250lb spring is always going to be just that - possibly too soft for the dry as the preload has no effect on spring rate.. As I said in my OP, it would be fairly easy to test the car in the wet, but it's not possible to book a wet test, so hence trying to research the theory behind it all.

 

(Sorry if I'm not using the correct terminology and confusing people, but if I knew the correct terms I wouldn't be asking!)

[carlt]

have you tried softening the dampers in the wet ?

[andygo]

Yes, of course we soften off the dampers, but my original question was regarding the black art of springs and preloads. Thanks to the helpful inputs its become a lot clearer on the way forward. Only looking for small improvements as the car/driver/team package is doing ok, just looking for a tiny bit in the wet..

Edited by andygo, 14 September 2014 - 21:07.

[andygo]

Hi andygo,

 

you wont be able to achieve what you would like to do, by just preloading a softer (linear) spring - other have already explained the reasons.

 

If you use a progressive spring, on a droop limited damper/suspension then adding preload would change the effective springrate around the point of operation, but this may becomes slightly confusing in practice, and would require that you could get a "perfect" progressive spring made for the application at hand ( a spring with the desired spring rate transition) --> this may be not possible or if, could be expensive (custom made spring in low quantities)

 

What you could consider as a practical, workable and economical solution - IMHO - is using the softer spring ( the 250 lbs you mentioned, if this is a good setup for the wet), and then go and fit a "spring rubber" or whatever these things are called in your part of the world.

 

 

The underlying idea is, that you reduce the number of active coils [N] in your given spring and thereby increasing the springrate. Following from this relation.

 

 

So in the dry, you would run with them, and in case it starts to rain, you just pull them out on the grid or before going to the grid, and adjust the ride height back to where you want it by xx turns/flats (or whatever you use) at your push/pull rod.

 

Depending on the type of open wheeler you run, and the series, you could even attempt to to this (pull them out) during a pitstop, while changing onto your wet tyres.

( if this is feasable or not, you will know for yourself).

 

You will need to experiment a little bit around, to find the "correct" spring rubber ( they come in different shore hardness) and will need to see, how many coils you need to "block out" with them, to get to the higher rate you want. You can use the above given formula as an indication, as you know the springrate of your spring and can measure the ID/OD and count the number of coils.

 

It's a quite cheap way to "tune" your spring stiffness with only one given spring, and in small increments which are otherwise maybe not covered by "off the shelve" springs.

For most applications, this should work out o.k., you have to be a bit mindful about the maximum permissable stress levels in your spring(steel).

If you happen to run some "light weight" race springs (with steep pitch and a low number of coils --> less material --> less weight), you may end up breaking a spring, when trying to reduce the number of active coils, because these springs are allready designed at the material limit.

But for most "normal" off the shelve racing springs, this shouldn't be too much of an issue.

 

********

 

There are other options, using a in series combination of two springs ( the combined springrate if both springs are "active" will be lower then the lowest single spring rate) and then "block" one to get to the higher rate.

With this "technique" you could have 3 different spring rates from just two springs.

 

a.) combined springrate --> lowest = 1/ (1/springrate a + 1/springrate b) -->[Example combine a 900lbs and a 300 lbs spring = 1/ (1/900 + 1/300) = 225 lbs]

b.) use only spring rate a (block out spring b) = 900 lbs

c.) use only spring rate b (block out spring a) = 300 lbs

 

Therefore, you would run the second spring blocked out in the dry --> only running on the "dry" spring, and when it starts to rain, you would remove the "blocking" from the second spring and run on the compound spring rate.

This "version" requires some "space" (to fit the second spring), and it adds some weight and extra costs (for the second spring).

In the range you want to vary, the second spring would be quite stiff ( 1500 lbs if you want to go from 300 to 250), but would only require very little travel, so you maybe want to look a a bellville (spring)stack instaed of a coil spring.

 

Anyway, I think you get the idea.

IMHO the "spring rubber" in combination with the "rain setup spring" to bring the springrate up to "dry setup spec" is the easiest, lighest and probably most economical (cheapest) solution, for the problem at hand.

But you will know better if this works for you or not.

May need to spend a afternoon on a springtester with a "handfull" of springrubbers, to find the combination which works for you.

How many coils to block, with which type of "spring rubber".etc.,

But afterwards it's a quite straight forward process, pull out the springrubber & adjust the RH at the push/pull rod --> done.

 

There are/where some applications in the past, which made use of this concept (reducing the number of "active coils" on a given spring) to achieve a range of springrates from a single given spring.

Porsche used to have this on their older spec RS models and IIRC ( and I may don't) I think Radical/Intrax uses ( used to use) a similar concept on their club racing cars.

 

It requires a special "open end" spring and a specific spring seat design [see below].

If designed correctly, the ride height doesn't change when adjusting the spring seat to increase the stiffness.

Most people have thrown the factory setup out of their Porsche's and run different linear springs instead, because they couldn't get their "head around" the concept.

 

It looks something like this. ( here the rate is adjustable between 150-190 N/mm)

 

 

Alternatively, seeing that this seems to be a car which uses a pushrod/rocker layout you could consider changing the motion ratio of the suspension.

You will have to check if your current rocker has provision to use different positions (holes) to connect either the pushrod &/or the damper/spring unit or both.

If not, you could consider if producing a new/different rocker or modifying your current one, if there is enough room/material is a option for you.

see here as an example, the two options to connect the pushrod onto the rocker.

 

 

 

 

good luck

Hi TC3000,

 

Thats a very helpful guide you have kindly put together. Just need to find a UK supplier of the rubbers, sounds just the job. Had a look around yesterday on the net and couldn't find any at all, save for caravan tow vehicles! The guy at Eibach had never been asked about them before, so not a usual solution in the uk.

[Lee Nicolle]

Spring rubbers can be useful for a few extra inch pound. Just be carefull though as some springs seem to either break or sag next to where they are. To be fair mostly on dirt but the comment is valid on any car. Obviously a longer spring should be more forgiving but do not seem to be. This on a well known brand of spring that does not come down appreciably normally.

My only use of them was to evaluate if a stiffer spring would be usefull. About 6 laps. Then I changed to stiffer spring for the next session.

And like too many uerathane items the next time I went to use them one broke trying to fit it!  That a couple of years later. Some uerathane items are rubbish and have a very limited life . That includes bushes.

[andygo]

It's all a bit academic if I can't source them in the uk.   Its just something to try though. They would be on new Eibachs, so probably ok on smooth track.