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Author: Subject: what suspension rockers do you use?
CNHSS1

posted on 27/1/10 at 10:27 AM Reply With Quote
what suspension rockers do you use?

im looking for some suspension rockers as used on some kit cars and single seater race cars. Ive seen some nice billet ally versions, some cast and some fabricated steel. These are to be used with pushrods from the wishbones to remotely mount the shocks.
What are you using and where from/which kit, as im in the market and need some inspiration from you clever types
cheers
CNH





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andylancaster3000

posted on 27/1/10 at 11:42 AM Reply With Quote
One example from my archive. Constructed from an aluminium hub containing the bearing/s to which two steel plates are screwed to it:

http://locostbuilders.co.uk/photos.php?action=showphoto&photo=2DSCF0036.JPG

Another, similar constuction method but much lighter weight, all aluminium, very light weight as only suited to a 200kg car!




Both these methods are pretty simple to construct and fairly robust but if it was for a heavier car or a road car and you had more cash you may want to consider machined come solid items or a cheaper alternative; fabricate them from thin sheet steel as found on many high end competition cars.

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Alan B

posted on 27/1/10 at 12:50 PM Reply With Quote
Fabbed steel ones are pretty simple to do..


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Theshed

posted on 27/1/10 at 02:37 PM Reply With Quote
I made mine in the same way - 1/4" 7075T6 2 x 38 x 7mm bearings. If the ratio is wrong then the outer plates can be changed fairly easily.
Front rocker and mount
Front rocker and mount
[/img]

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CNHSS1

posted on 27/1/10 at 11:14 PM Reply With Quote
cheers Chaps, just the sort of info i was looking for.
theshed,
have you used a proprietry bearing housing between those two plates?





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procomp

posted on 28/1/10 at 09:26 AM Reply With Quote
Hi

Before you go getting into the desighn too much there are a few things to consider.

1st make sure you do not make the same mistake as most of the kit manufacturers have done. And that is designing a system that once complete then requires a double adjustable damper with very specific valving. Ie you need 1000 worth of dampers for the front end. Unless of course you are wanting to spend that sort of budget.

2nd dont also do as some have and end up with a setup that places so much load into the pivots that it can literally rip mounting brackets of the chassis.

Cheers Matt






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CNHSS1

posted on 28/1/10 at 10:18 AM Reply With Quote
Hi Matt,
cheers for the tips and reply.
im no geometry expert, but have read all the usual books and played with the susp desgin on the car for a few years. Eessentially i want to remove the weight of coilover from unsprung weight, be able to induce rising rate into the setup (car does have working aero pack that produces actual DF F&R) and effectively move the weight closer to centre line of the car. Its for a hillclimber BTW.
have you any suggestions or thoughts as to other options? cheers, CNH





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MikeRJ

posted on 28/1/10 at 11:06 AM Reply With Quote
You might get a small saving in unsprung mass, but of course it will be at the cost of more total weight.

As for rising rate, most locost front suspensions have a small amount of rising rate built in, so long as the angle between the damper body and the bottom wishbone increases as the suspension is compressed.

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CNHSS1

posted on 28/1/10 at 12:19 PM Reply With Quote
its unfortunately not a locost at all, but ive always enjoyed the 'can-do' attitude and knowledge on here, more practical and hands on than some of the theory-only keyboard warrior on some forums!
i know what i want to achieve, just trying to get a feel for the best methods, as normally i create an engineering marvel (in my opinion ) that was then ten times more complex to build and implement than the cleverer guy that comes up with the simple, elegant solution, sure you know the score.
The inboard rocker layout will help with packaging of all the parts too as it allows a bit of flexibilty and choice in where all the gubbins go.

Matt
i shall probably end up with v.expensive dampers but want to create it all and then cough up for those at the last minute





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Frosty

posted on 28/1/10 at 04:16 PM Reply With Quote
I have to say I've never been convinced by the application on inboard suspension on kit cars. Don't get me wrong it certainly has a reason for existing, but I think the disadvantages quickly outweigh the advantages on a kit car.

I think the recent manufacturers that have included it in their kits have done it as a selling point instead of a technical reason.

IMO it makes initial spring selection extremely difficult unless you are a geometry genius, and the unsprung weight saving is absolutely minimal when you use a good quality damper.

There is a mild aero advantage in using push rods I'm sure, but the setup just looks like a nightmare on most kits. Inboard looks so over-complicated that it would put most people off from experimenting with the adjustment. But it's this individual adjustment that which makes cars much better at handling.

I'm struggling to think of real world reasons why someone would go for push rod over a standard setup on a 7, but I may be missing the point completely, so I'm keen to hear more about it.

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CNHSS1

posted on 28/1/10 at 04:28 PM Reply With Quote
not a 7 type, a hillclimber sports car running 9&10inch hillclimb super soft slicks, 40mm ride height, 50mm wheel travel, full aero package etc
Im happy that the maths for geometry & spring selection is covered to at least give a close enough point to start testing at, also its a competition-only car, so has no road compromises, so im hoping it'll be worth the ball ache!
could well be wrong of course...





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JB
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posted on 28/1/10 at 05:35 PM Reply With Quote
One of the advantages of inboard suspension is you can potentially run cheaper dampers.

Because you are using levers you can increase the motion ratio. ie the damper can move more than the wheel. As dampers work on passing oil through valves the more oil you pass the better the control. So fit the biggest dampers you can and then play with ratios.

Also consider that the loads on the rocker pivot point are double that of the ends where the pushrod and damper attach. So in theory the pivot bolt should be twice the strength of the others.

I made my own rockers out of ally plated bonded together with roller bearings for the pivot.

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Frosty

posted on 28/1/10 at 05:36 PM Reply With Quote
quote:
Originally posted by CNHSS1
not a 7 type, a hillclimber sports car running 9&10inch hillclimb super soft slicks, 40mm ride height, 50mm wheel travel, full aero package etc
Im happy that the maths for geometry & spring selection is covered to at least give a close enough point to start testing at, also its a competition-only car, so has no road compromises, so im hoping it'll be worth the ball ache!
could well be wrong of course...

Well I'll read your updates with great interest

[Edited on 28/1/10 by Frosty]

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CNHSS1

posted on 29/1/10 at 05:06 PM Reply With Quote
JB
ive had a look at your site, some great ideas there!
i seem to remember being next slot to you in the paddock a few years ago at Three Sisters when you were running Phil Shorts Mini Miglia for him, might be wrong person altogether though





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JB
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posted on 1/2/10 at 05:21 PM Reply With Quote
That was me. I used to help Phil.
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Brommers

posted on 2/2/10 at 10:39 AM Reply With Quote
quote:
Originally posted by Frosty
There is a mild aero advantage in using push rods I'm sure, but the setup just looks like a nightmare on most kits. Inboard looks so over-complicated that it would put most people off from experimenting with the adjustment. But it's this individual adjustment that which makes cars much better at handling.

I'm struggling to think of real world reasons why someone would go for push rod over a standard setup on a 7, but I may be missing the point completely, so I'm keen to hear more about it.


If anything, adjusting an inboard suspension system with bellcranks is easier since to alter the ride height, you just need to adjust the push/pull rod length - no need to fanny about with the damper.

The main operating advantage of inboard over outboard dampers (assuming you're not going fast enough for aero to be a significant effect) is that an outboard damper system is inherently falling rate. That is, the more the suspension compresses, the softer the suspension gets.

This is because the more the suspension compresses, the more horizontal the damper (and therefore the spring) gets, which reduces the effective spring and damper rates. The link between the angle of the dampers and the effective spring rate is obvious if you compare the spring rates on most se7enesque cars (other than a Striker). Generally you'll find the front spring rates are about double the rear spring rates. This is because the front dampers/springs are inclined and so have a less than 1:1 motion ratio wheel travel.

Falling rate suspension is Not A Good Thing - if anything you want rising rate. Rising rate suspension is easy to get with a bellcrank inboard system. This is A Good Thing. It also, as JB mentioned, allows you to alter the wheel to damper travel at will.

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MikeRJ

posted on 2/2/10 at 12:07 PM Reply With Quote
quote:
Originally posted by Brommers
The main operating advantage of inboard over outboard dampers (assuming you're not going fast enough for aero to be a significant effect) is that an outboard damper system is inherently falling rate. That is, the more the suspension compresses, the softer the suspension gets.



This is not the case for the majority of locosts which actually give a very small rising rate since the angle between the damper and the lower wishbone slightly increases as the suspension is compressed.

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Brommers

posted on 2/2/10 at 12:40 PM Reply With Quote
quote:
Originally posted by MikeRJ
quote:
Originally posted by Brommers
The main operating advantage of inboard over outboard dampers (assuming you're not going fast enough for aero to be a significant effect) is that an outboard damper system is inherently falling rate. That is, the more the suspension compresses, the softer the suspension gets.



This is not the case for the majority of locosts which actually give a very small rising rate since the angle between the damper and the lower wishbone slightly increases as the suspension is compressed.


Hmmm, been thinking about this. Allan Staniforth in 'Competition Car Suspension' (3rd Ed., page 52) says '...it is well to remember at the design stage that every suspension with a coil spring/damper unit leaning inwards from the bottom wishbone has built-in falling rate with all its handicaps'.

However, I've been thinking about it and I'm not sure he's right that this is necessarily the case. I think it's certainly possible to have a rising rate suspension design featuring inclined dampers, altough I'm not sure that most Locost designs would have this feature.

I'm assuming that most Locost designs are intended to have the lower wishbone flat (i.e. parallel with the ground) at rest? I'll dig out my graph paper, a protractor and a pair of compasses and do some sketches...

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JB
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posted on 3/2/10 at 09:51 AM Reply With Quote
Rising Rate Suspension:

I would avoid this and falling rate for the following reason. (in my opinion!)

I want my cars to be balanced in an oversteer understeer way. This makes them easy to drive and gets the most out of the tyres. Sideways is fun and looks good but it isnt fast.


If a car has rising rate suspension then as it rolls the stiffness increases, increasing the weight transfer and reducing the grip. This can upset the balance.

This suspension game is complicated enough without having to worry about the roll resistance altering during suspension travel.

So I prefer linear rate suspension.

Also note that my vehicles tend to have plenty of travel and soft springs and no aero. If you have short travel then what motion ratio you have will probably make very little difference as the suspension is so stiff anyway.

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hughpinder

posted on 4/2/10 at 11:43 AM Reply With Quote
For outboard suspension:

The lower arm rotates about its chassis mount. The lower damper mount on that arm will 'draw' a circle if you raise or lower the arm. If the upper damper mount is on the circle, the suspension rate will be linear. If the upper mount is inside the circle the suspension will be falling rate. If the upper mount is outside the circle the rate will be rising. If the shock absorber to arm angle increase to > 90 degrees at any point in its travel, the rate will be falling again.

One of the advantages of an inboard damper set up not mentioned yet is the facility to make a simple/small/light adjustable roll bar as shown in various pictures in Staniforths book.

One disadvantage is that the damper is in the airflow from the radiator, which can cause overheating of the damper (but shouldn't be a problem for a hill climb car as it won't be running all that long).

Hugh

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