Ivan
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| posted on 10/3/11 at 04:55 PM |
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Preload on front spring - what impact???
I feel really stupid asking this but on my Cobra (and in fact all Jag suspensioned Cobra's that I know of in SA) the front springs are 3"
longer than the extended shock so have about 900 lbs. of preload when installed.
(300lb/inch spring)
My question is - what impact does this preload have other than putting unnecessary strain on the shock when fully extended. If I had new springs made
of the same strength (300 lbs/inch) but 2" shorter so there was only 300lbs of preload when installed, would it make any difference at all to
the suspension rate, ride, handling or anything.
My guts tell me no difference but the little devil of inexperience and forgotten education or whatever says maybe I'm wrong so before spending
money on new springs pse help with advice.
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MikeCapon
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| posted on 10/3/11 at 05:12 PM |
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You are going to need a certain amount of preload to get your car to the right ride height. How much, I can't tell you but it could easily be
75mm depending on the weight of your car, the wheel to shock ratio and the spring rate.
Your best bet is to assemble the shocks and fit them, then see what ride height that gives you. More preload = higher ride height. If you ride height
is too high and the springs are the right rate then eventually you may need shorter springs.
As said though, 75mm is not excessive in terms of prelaod.
HTH
Mike
ETA Sorry I didn't actually answer your question. Different length springs of the same rate will not affect ride or handling. They just need to
be the right length to achieve the appropriate preload and also have enough stroke (total of gaps between the coils) to accomodate the compression
from the preload and the shock closing to it's minimum length.
If anything the shorter spring is lighter and cheaper
[Edited on 10/3/11 by MikeCapon]
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Ivan
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| posted on 11/3/11 at 06:51 AM |
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Mike - thanks for the reply - regrettably I don't understand what you are saying. The shocks have fixed spring platforms so there is no way of
adjusting ride height. What I am asking is that with the existing springs (300 lbs/inch) whose uncompressed length is 3" longer than the length
between the spring platforms when the shock is fully extended the ride height is fine at 5", If I had new springs that are say 2" shorter
but have the same rate would the ride height be the same.
My thoughts are "yes it would" because no matter how compressed the spring is once you put it on the shock, as soon as you put the shock
on the car and load it it will still compress by 1" for each 300 lbs of load and won't wait for 900 lbs of load before starting to
compress in the original case or 300 lbs of load before starting to compress in the case of the second spring - am I right or wrong.
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BenTyreman
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| posted on 11/3/11 at 08:09 AM |
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If you have 900 lbs of preload, then the suspension will not move until the car has put 900 lbs of weight on the spring. If the corner weight at the
spring is 1200 lbs, then the spring will compress a further 1".
If you have a shorter spring with 600 lbs of preload, the same 1200 lbs weight will compress the spring 2", lowering the ride height by 1"
over the longer spring.
If your car doesn't have 900 lbs of corner weight, then you will have a non-linear relationship between spring corner weight and spring
compression. If your car has only 600 lbs of corner weight, it would need an additional 300 lbs of weight generated from cornering/travelling over
bumps/etc before you will see any spring compression.
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britishtrident
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| posted on 11/3/11 at 08:10 AM |
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You are right -- in other words wheel rate and suspension frequency won't change.
A cheap way of lowering springs without altering the spring rate is to copy a technique used by builders of saloon racers for the old Group1
racers called "Scragging".
Scragg the springs over night in a very hot domestic oven ---- make a plate spring compressor using steel plates and a length of steel studding.
With this tool compress the spring fully and put the whole lot in a hot domestic oven overnight in the morning switch the oven off and allow the
spring to cool.
The free length of the spring should be reduced due to the high loading at elevated temperature accelerating the settling of the spring (engineers
call it high temperature creep).
The free length of the spring will be reduced but the spring rate will be unaltered, however the life of the spring will be reduced.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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britishtrident
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| posted on 11/3/11 at 08:20 AM |
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quote:
Originally posted by BenTyreman
If you have 900 lbs of preload, then the suspension will not move until the car has put 900 lbs of weight on the spring. If the corner weight at the
spring is 1200 lbs, then the spring will compress a further 1".
If you have a shorter spring with 600 lbs of preload, the same 1200 lbs weight will compress the spring 2", lowering the ride height by 1"
over the longer spring.
If your car doesn't have 900 lbs of corner weight, then you will have a non-linear relationship between spring corner weight and spring
compression. If your car has only 600 lbs of corner weight, it would need an additional 300 lbs of weight generated from cornering/travelling over
bumps/etc before you will see any spring compression.
Suspension leverage ratio comes into it or more correctly the square of the leverage ratio ----- on Jag suspension the spring is usually well inboard
of the outer ball joint.
If for a simple example if the leverage is 2:1 then 1" suspension movement will only compress the spring 0.5".
With a 300lb/in spring this means 150lbs change at the spring but only 75lbs change in load at the outer ball joint.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Ivan
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| posted on 11/3/11 at 08:41 AM |
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The reason why I asked is that currently the setup gives a wheel frequency of about 70 CPM which equates fairly well with most luxury limo's of
yonkers years ago and not with a proper sportscar, and also does nothing at all for the handling. I am in the process of sourcing shocks with
adjustable platforms and thought I might well have new springs made up to up the wheel rate to closer to 100. To have the springs made up I need to
decide on what free length I require and to do this I need to properly understand what impact that decision has - hence my question.
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MikeCapon
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| posted on 11/3/11 at 08:46 AM |
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Morning Ivan,
I’ll try to be clearer. I know that people who work every day with something are notoriously difficult to follow when they explain aspects of it.
Also, I’d assumed (wrongly) that you had adjustable spring platforms.
First off the ride height. When the car is at the correct ride height there should be a certain amount of suspension movement left to absorb bump
forces (the shock closes) and there must also be movement left to allow the wheel to drop (droop, the shock opens).
To illustrate this let’s assume that the bump movement left is 70% of the shock travel and the droop is 30% of the travel. In other words if you have
4 inches (100mm) of wheel movement, you should be able to lift the car 1.2” (30mm) before the wheel lifts. Under no circumstances (apart from the very
peculiar set ups on single seaters with aero) should the droop be zero. Reading your post you say “when the shock is fully extended the ride height is
fine at 5 inches”. That needs fixing. It would suggest to me that either the 5 inch ride height is too high or, if not, your shocks are too short, or
the shock fixings are incorrectly positioned.
The more you preload a spring, the higher the ride height. The car (when static) always pushes downwards with the same force but as you increase the
preload the spring pushes back upwards with a greater force and lifts the car. As you add more preload the ride height will continue to rise until you
get to the situation where you have no droop and the shock is fully open.
To answer the question in your second paragraph, your assumption is wrong. A 300lb spring already compressed by 3 inches is effectively already
pushing back with a force of 900 lbs. Before you can begin to compress the spring further you need to apply 900 lbs force against it. From there on it
will require 300 lbs force to compress each subsequent inch. To use your terminology the spring will “wait” to see 900 lbs before it will start to
compress further.
To fix your problem I would proceed as follows.
If the 5 inch ride height is achieved with the shock fully open you need to try and drop the ride height. I know pretty much zero about Cobras but if,
for example you can run a 4 inch ride height then you will have one inch of droop which is pretty much a minimum.
If you absolutely must have 5 inches of ride height then you either need longer shocks or the shock mounts repositioning so that you have a reasonable
amount of droop available.
Once the issue of the correct ride height is resolved only then can you begin to choose the right spring.
I hope that this helps you. Please keep us posted.
Cheers,
Mike
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MikeCapon
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| posted on 11/3/11 at 08:51 AM |
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quote:
Originally posted by Ivan
The reason why I asked is that currently the setup gives a wheel frequency of about 70 CPM which equates fairly well with most luxury limo's of
yonkers years ago and not with a proper sportscar, and also does nothing at all for the handling. I am in the process of sourcing shocks with
adjustable platforms and thought I might well have new springs made up to up the wheel rate to closer to 100. To have the springs made up I need to
decide on what free length I require and to do this I need to properly understand what impact that decision has - hence my question.
Spring length does not influence wheel frequency. Spring rate is the determining factor.
Unfortunately we no longer seem to have the facility of attaching files as I have an Excel spreadsheet which will help you.
Please mail me if you want a copy.
Cheers,
Mike
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Ivan
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| posted on 11/3/11 at 09:09 AM |
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Mike and others thanks for that clear explanation - you have resolved my concern - sorry, I had left out a full-stop in my post that caused your
concern Mike - the ride height is 5" with the car at its normal situation with the shock about 30% compressed - and not fully extended.
Re the spreadsheet - I have already created one to suit my needs that calculates spring rate together with a Cad drawing of the setup to enable me to
calculate suspension frequency, but thanks for the offer.
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Liam
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| posted on 11/3/11 at 09:42 AM |
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From your description - that the car's weight overcomes the preload and your shocks run around 30% compression at normal ride height - all
sounds good, i.e. your springs are the right length. That's either some serious leverage in your suspension or just a heavy car if 900 lbs
preload is 'right' and the result is 70 CPM, but whatever.
If you want to use harder springs to raise the wheel rate, if they are the same length as the current springs (and therefore require the same 3"
preload to fit them on) you will start running out of droop and raising your ride height. This is when adjustable spring platforms come in handy. If
you dont have them you'll have to carefully calculate the required length of spring (it will need to be shorter if you're going harder
such that the preload is the same 900 lbs as currently) and have them custom made (unless the result is an off the shelf length). You've got
some wiggle room to help you stick to off the shelf springs if you dont mind your ride height changing (or have an independant means of adjustment),
but you dont really want to upset the 30% static compression you currently have too much or you could run out of bump or droop (unless you currently
have way more travel than you need). Ideally, get some shocks with adjustable spring platforms and you wont have quite the same headaches.
Liam
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Ivan
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| posted on 11/3/11 at 11:49 AM |
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Lliam - thanks for that - the figures I quoted, except for frequency and spring rate where not actual as I haven't measured everything with the
spring off the shock yet and were used as an example only so shouldn't be taken as gospel, but you have added to my understanding anyway.
And yes - the car weighs 1200 kg or 300 kg per corner with a fair amount of leverage on the spring/shock so is not a lightweight but fairly average
for a Cobra.
[Edited on 11/3/11 by Ivan]
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britishtrident
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| posted on 11/3/11 at 01:59 PM |
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Normally if you fit harder/higher rate springs the free length of the harder spring is shorter even if the same ride height is to be maintained.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Frosty
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| posted on 11/3/11 at 02:00 PM |
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quote:
Originally posted by BenTyreman
If you have 900 lbs of preload, then the suspension will not move until the car has put 900 lbs of weight on the spring. If the corner weight at the
spring is 1200 lbs, then the spring will compress a further 1".
If you have a shorter spring with 600 lbs of preload, the same 1200 lbs weight will compress the spring 2", lowering the ride height by 1"
over the longer spring.
If your car doesn't have 900 lbs of corner weight, then you will have a non-linear relationship between spring corner weight and spring
compression. If your car has only 600 lbs of corner weight, it would need an additional 300 lbs of weight generated from cornering/travelling over
bumps/etc before you will see any spring compression.
I don't agree with this.
Let's assume a spring with no preload is fitted vertically for simplicity with a 1:1 ratio. If it's a 300lb spring, and the weight of the
car on that corner is 300lb, the spring will compress an inch due to the weight on it.
Now if you wind an inch of preload onto that spring first, when you put the car back on the floor, the spring will compress an inch still, just as it
did before. The only difference is that the corner would sit an inch higher, because the platform is higher. You have not changed anything except the
ride height. The difference in spring compression before and after putting the car back on the floor will be exactly the same in both cases.
Are you saying that with an inch of preload, the car weight would have to put 600lb of force on the spring? If so this is wrong. There is 300lb from
the spring collars which gives 1 inch, and 300lb from the car weight which gives the other inch. Total 2 inches compression, but the way the spring
behaves is identical to if it had no preload.
If you have 0lb preload, or 500lb preload on the spring, that preload is all on the spring platforms. If you have 0lb or 500lb preload, the amount the
car weight will compress the spring by will always be the same, regardless of preload.
The only time it's not the same is if you are using a progressive spring.
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Fred W B
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| posted on 11/3/11 at 02:22 PM |
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quote:
Now if you wind an inch of preload onto that spring first, when you put the car back on the floor, the spring will compress an inch still, just as it
did before.
No. In this case the spring will not move at all, as the car needs just 300 to support it. To move another inch the load needs to go to 600.
Cheers
Fred W B
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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BenTyreman
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| posted on 11/3/11 at 02:47 PM |
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quote:
Originally posted by Frosty
If you have 0lb preload, or 500lb preload on the spring, that preload is all on the spring platforms. If you have 0lb or 500lb preload, the amount the
car weight will compress the spring by will always be the same, regardless of preload.
I did say the corner weight on the spring, i.e. the effective corner weight seen by the spring. Next time you have a coil over unit off a car, try
compressing the spring. You can see the effect of preload on spring compression. You have to exceed the spring preload before the spring starts the
compress.
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matt_gsxr
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| posted on 11/3/11 at 02:54 PM |
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To answer the original question.
Spring rate will be the same.
Preload has no impact in your case (except in droop), as the weight of the car overcomes the preload.
Once the weight from the car is pushing on the spring then the shock might as well be 36inches long, there is no preload in your case.
If you want someone to workout what stiffness/length you need for a particular frequency then it is easy but you will need to post some measurements.
I am still surprised you've got ~1350lbs on each spring (i.e. 900lbs plus 1/3 of the travel)
Matt
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Frosty
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| posted on 11/3/11 at 03:38 PM |
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Ah sorry guys, you're quite right. I've sat and thought about it for a while and I can see what you mean. It makes sense now. Thanks for
explaining it.
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Liam
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| posted on 11/3/11 at 04:59 PM |
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quote:
Originally posted by matt_gsxr
I am still surprised you've got ~1350lbs on each spring (i.e. 900lbs plus 1/3 of the travel)
Yeah i was surprised too. It is a lardy Cobra though  . But even so sounds high - must be pretty high leverage in the system and therefore some
extremely highly loaded mounting points/pivots - hope it's all up to the task. Still that's nowt compared to <200kg bikes using ~700lb
springs on the rear wheel. That's leverage!
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britishtrident
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| posted on 11/3/11 at 05:51 PM |
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Leverage on Jag suspension is pretty high.
Some suspension springs on old tin tops were massive.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Ivan
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| posted on 11/3/11 at 08:22 PM |
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quote:
Originally posted by Liam
quote:
Originally posted by matt_gsxr
I am still surprised you've got ~1350lbs on each spring (i.e. 900lbs plus 1/3 of the travel)
Yeah i was surprised too. It is a lardy Cobra though . But even so sounds high - must be pretty high leverage in the system and therefore some
extremely highly loaded mounting points/pivots - hope it's all up to the task. Still that's nowt compared to <200kg bikes using ~700lb
springs on the rear wheel. That's leverage!
See my previous post - the 900 lbs of preload is fictitious although I suspect not that far off as I haven't actually measured the spring length
before and after installation yet - about to do it next week. At this stage just trying to understand what's needed to look at and why so I can
be sure to specify he right spring.
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