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Leafspring characteristics.


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#1 jatwarks

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Posted 31 December 2011 - 12:56

Informed opinion required please.

How might the lengthening of a cars wheelbase, by moving the front axle forward on semi-eliptical leafsprings, affect the characteristics of the springs?

Thoughts that come to mind are;

Under braking, would the shorter front quarter of the springs cause the axle to rotate less? Or more?

Would the front of the car 'dive' more under braking, due to the assymetrical springs?

What changes might be expected in the performance of the springs?

Please bear in mind that the car is an 'historic' vehicle and that the basic design cannot be changed beyond 'adjusting' the existing set-up.

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#2 24gerrard

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Posted 31 December 2011 - 15:02

The springs would be more likely to break.

#3 carlt

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Posted 31 December 2011 - 15:16

i have a lot of experience with rear springs on a driven axle
the spring frequency is altered by moving the axle mounting point
presumably your leaves are chassis mounted at the front , shackles at their rear ?

#4 jatwarks

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Posted 31 December 2011 - 16:39

The springs would be more likely to break.


I realise that the loadings on the leaf elements would become less balanced, and therefore, potentially more damaging.

presumably your leaves are chassis mounted at the front , shackles at their rear ?


Correct.

I believe that the assymetrical set-up would disrupt the natural frequency and resonance of the springs. Would the shorter, stiffer lengths dominate, raising the natural frequency?

Would the longer rear half of the springs allow more axle rotation, more movement at the shackles? Would the axle be more likely to wind up during braking?

The change in wheelbase would be a matter of 2 - 3 inches.

#5 Bloggsworth

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Posted 31 December 2011 - 17:04

Not a problem to be considered in isolation. The steering would have to be moved forward, dampers and mountings, the centre of rotation would be changed; centre of pitch etc. As a matter of idle curiosity, why?

#6 jatwarks

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Posted 31 December 2011 - 18:21

As a matter of idle curiosity, why?


Just that really. A question of would a longer wheelbase add stability, and would that be compromised by the assymetrical spring arrangement.?

#7 cheapracer

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Posted 31 December 2011 - 18:23

Not a problem to be considered in isolation.


Agreed, you may stiffen the spring rate and roll stiffness a fraction and decrease bump steer a tad but overall I don't think you will notice it much other than the wheelbase increase.

Wider leaves will increase roll stiffness as well if that's legal.

As Bloggs says, be wary of what effect you have on the peripheral systems such as steering.


#8 carlt

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Posted 31 December 2011 - 19:04

Correct.

1. Would the shorter, stiffer lengths dominate, raising the natural frequency?

2. Would the longer rear half of the springs allow more axle rotation, more movement at the shackles? Would the axle be more likely to wind up during braking?

The change in wheelbase would be a matter of 2 - 3 inches.



I am assuming your axle is originally mounted centrally on the spring , then moving it assymetrical :
I have found in practice the opposite of 1.
yes for 2. but you can easily counter it , or use it to your benefit [ This will also be determined by the amount of grip you can generate with your historic tyres !? ]

You will only break the springs if they are crap/ready to break anyway , many axles are assymetrically mounted on leaf springs as standard


#9 Magoo

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Posted 31 December 2011 - 21:06

On the typical vehicle with semi-elliptic front leaf springs and beam axle, extending the wheelbase forward in such a manner will, among other things, reduce longitudinal pitch and shift the static weight distribution rearward. Whether it reduces spring windup is another matter. Altering the wheelbase enough to have a significant effect will produce a blatant change in the vehicle's appearance. However, it's not necessary to alter the axle location to achieve the spring properties you suggest.

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Edited by Magoo, 31 December 2011 - 21:08.


#10 fredeuce

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Posted 31 December 2011 - 21:20

many axles are assymetrically mounted on leaf springs as standard


I have to agree also with that observation.

I have played around with quite a few of these over the years and noticed that the greater percentage of them are assymetric.This has been mostly in relation to rear axles fitted on parallel semi-elliptic springs.

I can only assume the thinking is to shorten the distance between the centre bolt and the front mount to reduce the torque acting on the spring under accelaration to minimise axle wind up. The longer portion enables ride characteristics to be maintained which would otherwise be compromised if a shorter over all spring was used where the rear section was equal to the front.

#11 Greg Locock

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Posted 31 December 2011 - 21:42

It depends how serious you want to get about this. The SAE have a design manual for leafsprings, not surprisingly it is rather old. It includes equations and nomograms for all aspects of leafspring design.

SAE also promulgated a 3 rigid link model of a leafspring, with two torsional bushes in it to represent the compliance. This model, although crude, is perfectly adequate for a handling mode of a vehicle (the alternative is a non linear finite element model), and is also simple enough that you can figure out what is going on by hand.

Yes, off centre loading of a beam increases the spring rate. The effect on braking depends on whether it is front shackle or rear shackle. I've only worked on rear suspensions, where the priorities are different.

Does your spring have a second stage? I'm guessing not. That simplifies things no end.


Here's a couple of leafsprings i worked on

http://www.exploroz....a_Feedback.aspx

Bit overloaded I reckon

That has a crane simplex linkage (track rods to you and I)

http://www.themotorr...a3c44172423.jpg

well you know what a leafspring looks like!

For some reason lost in history I also built an FEA model of the 1 tonne heavy duty leafspring, which has 13 leaves (off the top of my head) and a second stage. Getting that bugger to run took weeks.


#12 24gerrard

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Posted 31 December 2011 - 22:06

Posted Image

This is the rear of our 1908 Panhard et Levasser.
Interesting springs.
Can anyone tell me how the four rear spring dampers work?

#13 carlt

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Posted 31 December 2011 - 22:45

Posted Image

This is the rear of our 1908 Panhard et Levasser.
Interesting springs.
Can anyone tell me how the four rear spring dampers work?


said in a French accent :-

"Monsieur , votre Levasser , ee is well hung "



are they the first design for Renaults revolutionary F1 Mass Dampers ?


Happy New Year :)

#14 24gerrard

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Posted 31 December 2011 - 23:03

said in a French accent :-

"Monsieur , votre Levasser , ee is well hung "



are they the first design for Renaults revolutionary F1 Mass Dampers ?


Happy New Year :)


Its all been done before.
Can someone tell me how they work?
Oh and Happy New Year to all :clap: :drunk:

Edited by 24gerrard, 31 December 2011 - 23:04.


#15 Greg Locock

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Posted 31 December 2011 - 23:44

are they the first design for Renaults revolutionary F1 Mass Dampers ?


I don't know if you think you are joking, but yes, that was my guess. Maybe there's a piston inside, and a very light spring. As the car bounces up and down the mass of the damper's body tries not to, and then oil or air tries to get past the piston and creates heat, damping the motion. Also used on the 2CV's rear suspension, early on. If you don't give a stuff about handling and just want the tire to remain in contact with the road and want a light body it is a reasonable solution. Given the rather orrible nature of dampers at the time it probably gave a better ride than most alternatives.

Edited by Greg Locock, 31 December 2011 - 23:49.


#16 carlt

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Posted 01 January 2012 - 10:54

Posted Image


Can anyone tell me how the four rear spring dampers work?



This is the birth of the Modern Art installation .
They represent the rear of a pair of oxen in harness .

They work , as any teamster/driver of that era would tell you , because oxen represent handling stability


#17 24gerrard

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Posted 01 January 2012 - 14:32

Posted Image

Definitely more art in this car than all your modern junk put together.

Edited by 24gerrard, 01 January 2012 - 14:35.


#18 24gerrard

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Posted 01 January 2012 - 14:59

Posted Image

Notice the front end, beam and long track rod.
Hanging rear shackles.
Air cooled flywheel and cone clutch are also interesting, as is the original Mulliner kick strip.
Engine is the usual license build Daimler T head of the period.
This engine design set the seal for future development and is very important in car history.

http://en.wikipedia.org/wiki/Panhard

http://www.teampanhard.org/

Just down the road from us is the car once owned and raced by Rolls.

Edited by 24gerrard, 01 January 2012 - 15:14.


#19 Lee Nicolle

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Posted 04 January 2012 - 09:46

Informed opinion required please.

How might the lengthening of a cars wheelbase, by moving the front axle forward on semi-eliptical leafsprings, affect the characteristics of the springs?

Thoughts that come to mind are;

Under braking, would the shorter front quarter of the springs cause the axle to rotate less? Or more?

Would the front of the car 'dive' more under braking, due to the assymetrical springs?

What changes might be expected in the performance of the springs?

Please bear in mind that the car is an 'historic' vehicle and that the basic design cannot be changed beyond 'adjusting' the existing set-up.

My very limited observation of front leaf springs has the centre bolt in or very near the centre of the spring. This is on old cars, most trucks and a lot of 4wds.The centre bolt will always be the spring centre. To move it will mean replacing the entire spring .And I suspect more bump steer too.And the caster will change too if you move the axle forward without reengineering the entire spring

Rear leaf springs generally have the centre bolt towards the front by 10-15%. But the centre of the spring is still the centre bolt as theoretically the spring has the same rate before and behind the centr bolt. The forward centre is mostly to do with torque reaction on the spring. At times not a pretty site!

A leaf spring generally beaks first at the centre bolt, failing that above the end of the next leaf down. I have replaced a lot over the years on cars and commercials

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#20 carlt

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Posted 04 January 2012 - 13:12

My very limited observation of front leaf springs has the centre bolt in or very near the centre of the spring. This is on old cars, most trucks and a lot of 4wds.The centre bolt will always be the spring centre. To move it will mean replacing the entire spring .And I suspect more bump steer too.And the caster will change too if you move the axle forward without reengineering the entire spring



why will moving the axle alter caster and induce bump steer ?
and what will reengineering the spring have to do with it ?

caster on a beam axle is governed by the king pin inclination and/or stub axle offset , so only by tilting the axle will this change .
bump steer is a product of steering geometry not the spring , so the steering geometry will need reengineering ; assuming a steering box and links ,then the steering box moved the same distance forward should restore steering geometry ?

#21 cheapracer

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Posted 04 January 2012 - 15:37

why will moving the axle alter caster

caster on a beam axle is governed by the king pin inclination and/or stub axle offset , so only by tilting the axle will this change .


Most springs are curved so moving the front axle forward will tilt the axle backwards increasing caster.


#22 carlt

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Posted 04 January 2012 - 16:48

Most springs are curved so moving the front axle forward will tilt the axle backwards increasing caster.

the axle plate flattens the spring when bolted up
I recon you will have to move the axle more than the suggested amount , at least beyond the short base leaf , to change the caster by any significant degree


this is not a modern racing car suspension where 1 or 2 degrees are going to significantly influence handling characteristics ?

#23 cheapracer

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Posted 04 January 2012 - 17:17

the axle plate flattens the spring when bolted up
I recon you will have to move the axle more than the suggested amount , at least beyond the short base leaf , to change the caster by any significant degree


this is not a modern racing car suspension where 1 or 2 degrees are going to significantly influence handling characteristics ?


I have no idea of the curvature of the leaf so sure, can't comment.

Depends on the tyres, might make it worse.


#24 Greg Locock

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Posted 04 January 2012 - 21:59

Kinematically a normal rear leaf spring operates very much like a trailing arm. If the eye is higher than the mounting to the axle then in bump you get kinematic recession in jounce, castor reduces, zero camber change, zero bump steer. In roll you get practically zero caster change, toe out in jounce and the obvious amount of camber change.

Toe out in jounce is oversteer, that's why you push the front eye down as far as possible.



#25 Lee Nicolle

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Posted 07 January 2012 - 00:57

why will moving the axle alter caster and induce bump steer ?
and what will reengineering the spring have to do with it ?

caster on a beam axle is governed by the king pin inclination and/or stub axle offset , so only by tilting the axle will this change .
bump steer is a product of steering geometry not the spring , so the steering geometry will need reengineering ; assuming a steering box and links ,then the steering box moved the same distance forward should restore steering geometry ?

Think about it. Put the centre forward or rearward and the spring rate is softer or stiffer.The spring winds up under bump and steers. [it does anyway but it has been increased by the alteration]
Caster on a beam is usually a wedge shim between the axle and spring, and changes as the spring goes down. [saggy springs generally require reshimming]

If you move the axle forward you will have to drill and new centrebolt hole, and the existing one will then be a place for the spring to break. Which they do regularly at the centre bolt already. An unsupported hole is like puttin a grind mark across the spring.Secondary leaves break where the keepers are riveted on too, again regularly.

I used to drive tip trucks decades ago and on construction sites we often broke springs, front and rear. And it still happens even now on modern trucks. The metalurgy is little changed on a leaf spring.
I have often seen broken leaves on older passenger cars and commercials. And 4wds too ofcourse.
The poor old leaf spring winds up under accelaration and braking, bounces up and down on a bumpy road like you would not believe. It twists and flexs sideways.And by nature self damps. So extra holes and stress points really would be stupid.

#26 carlt

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Posted 07 January 2012 - 22:10

1. Think about it. Put the centre forward or rearward and the spring rate is softer or stiffer.The spring winds up under bump and steers. [it does anyway but it has been increased by the alteration]
2. Caster on a beam is usually a wedge shim between the axle and spring, and changes as the spring goes down. [saggy springs generally require reshimming]

3. If you move the axle forward you will have to drill and new centrebolt hole, and the existing one will then be a place for the spring to break. Which they do regularly at the centre bolt already. An unsupported hole is like puttin a grind mark across the spring.Secondary leaves break where the keepers are riveted on too, again regularly.

4. So extra holes and stress points really would be stupid.


1. miniscule change with the op's suggested change
2. if used the shim is for fine adjustment - caster is king pin inclination and /or stub axle off set
3. make new main leaf or new set leaves

4 , don't be stupid then ! - make new springs

#27 Lee Nicolle

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Posted 07 January 2012 - 23:47

1. miniscule change with the op's suggested change
2. if used the shim is for fine adjustment - caster is king pin inclination and /or stub axle off set
3. make new main leaf or new set leaves

4 , don't be stupid then ! - make new springs

On eg a 30" long spring a 4" move is over 10% so very LARGE.
The wedge is generally used for the minor amounts of caster these require.And to fine tune it. Large caster changes will be more than a wedge. Move the axle forward on the spring and the caster will change a lot. Unless a brand new redesigned spring is made.

#28 carlt

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Posted 08 January 2012 - 12:33

On eg a 30" long spring a 4" move is over 10% so very LARGE.
The wedge is generally used for the minor amounts of caster these require.And to fine tune it. Large caster changes will be more than a wedge. Move the axle forward on the spring and the caster will change a lot. Unless a brand new redesigned spring is made.


dont want to start an argument ;-]
have a look at post no4 2-3" increase is being considered