swaybar blade force estimate

[touge_ae101]

so i'm trying to think of ways to make my rear swaybar easy to adjust, modify and look pretty sweet all at the same time.

have been thinking/tinkering on solidworks with this type of design. the space lets me run a straight bar and there is enough movement to have 100mm adjustment on the swaybar link.

also wanting to use a slot design with a tightish fit between the blade and swaybar so i can go to a smaller bar easily if it gets too much for the road. currently looking at 19mm.
this is what i'm looking at:



i'm looking at using 10mm alloy for the blades as once you start getting much thicker it becomes more expensive and you have to look at getting it machined from a solid block rather than a bar, which adds up to more $$ at the end of the day.

so what i'm wanting to know is peoples knowledge on how much force the blade of the swaybar would need to stand up to?

the estimates i made were for 300N (this seems a bit small ). that is based on 6kg/cm rear springs with 50mm of travel under hard cornering. is this along the right lines? my logic went like this--> the force on one blade will be equal and opposite to the force the opposing strut is putting on the other blade. is this correct?

any thoughts/advice appreciated!

wanting to completely model the stress distribution and flex in solidworks before i make it to ensure it is going to work!

[KinLoud]

Probably way easier to work out swaybar rate in lbs/inch or kg/mm rather than nm.

For example -
Corolla 4afe swaybar 13mm thick (solid), bar length 600mm, arm length 219mm, effective arm 200mm
This gives a rate of 45lbs/inch or 0.8kg/mm

Corolla 4age swaybar 15.7mm thick (solid) all other dimensions same as above
This gives a rate of 95lbs/inch or 1.7kg/mm

So you have 100mm adjustment - from your diagram it looks like minimum will be 175mm arm, max 275mm arm.
Presuming 600mm bar length.
With a 19mm diameter
With 175mm arm rate will be 284lbs/inch or 5kg/mm
With 275mm arm rate will be 107lbs/inch or 1.9kg/mm
- Pretty stiff really!

Maybe start with 17mm
With 175mm arm rate will be 182lbs/inch or 3.2kg/mm
With 275mm arm rate will be 68lbs/inch or 1.2kg/mm

ALSO
I would make the arm out of steel - aluminium alloy doesn't have a good resistance to fatigue of the type a swaybar will have!

Ken

[Truenotch]

Have been looking into similar things lately and the conclusion was - buy a Sprintcar swaybar (it's just a straight bar with splined ends) from ProParts in palmy and make ends to suit.


Nick at Mag and Turbo will be keen to buy a set / share his ideas - he's building one for the integra atm.

[touge_ae101]

Probably way easier to work out swaybar rate in lbs/inch or kg/mm rather than nm.

For example -
Corolla 4afe swaybar 13mm thick (solid), bar length 600mm, arm length 219mm, effective arm 200mm
This gives a rate of 45lbs/inch or 0.8kg/mm

Corolla 4age swaybar 15.7mm thick (solid) all other dimensions same as above
This gives a rate of 95lbs/inch or 1.7kg/mm

So you have 100mm adjustment - from your diagram it looks like minimum will be 175mm arm, max 275mm arm.
Presuming 600mm bar length.
With a 19mm diameter
With 175mm arm rate will be 284lbs/inch or 5kg/mm
With 275mm arm rate will be 107lbs/inch or 1.9kg/mm
- Pretty stiff really!

Maybe start with 17mm
With 175mm arm rate will be 182lbs/inch or 3.2kg/mm
With 275mm arm rate will be 68lbs/inch or 1.2kg/mm

ALSO
I would make the arm out of steel - aluminium alloy doesn't have a good resistance to fatigue of the type a swaybar will have!

Ken

thanks for the input ken. could you elaborate on where you got those figures/calculations from?

adjustment on the arm is between 200-300mm

yeah i am wodnering if 19mm will be too stiff so thats why i want to make it so i can interchange to a 17mm bar by simply undoing some circlips and slipping the blades off the end and onto another bar.

am quite aware of the weight of such an item made out of steel weighs though both in unsprung weight and in overall adding to the weight of the car.

[touge_ae101]

Have been looking into similar things lately and the conclusion was - buy a Sprintcar swaybar (it's just a straight bar with splined ends) from ProParts in palmy and make ends to suit.


Nick at Mag and Turbo will be keen to buy a set / share his ideas - he's building one for the integra atm.

yeah swaybars don't really seem that hard to make but the guys who are out there making them (that i know of) just go off a 'oh yeah that size will prob do' measurement which isn't really what i'm after.

there must be a simpler solution out there! would be interesting to see what other guys have come up with. have already got some chromoly bar which i'll just machine a flat on each end to slot on.

[KinLoud]

The corolla 4afe and 4age measurements came from swaybars I have.

For comfort on the road springs tend to be softer and roll resistance is mainly provided by swaybar.
But on the track you can get better results by using higher (stiffer) rate springs to reduce/control body roll as it improves the independance of the suspension over one wheel bumps

I wrote my own spreadsheet swaybar rate calculator but there are several good online calculators
try this one
http://www.fromsteve.net/tech/Sway-Bar-Rate-Calculator

Ken

[RomanV]

What I think could be interesting, would be to have a variable rate swaybar, by having very short arms on it which work through an arc.

If the bar starts out parallel and then works through the arc up towards vertical, it will apply a non linear amount of increasing swaybar spring rate with every mm of suspension travel. So you get the benefit of increased roll resistance, while minimising the effective spring rate over one wheel bumps.

[soopachargen]

dare you to do it in cabin adjustable

[touge_ae101]

cheers ken i will look into it a bit further. wanting to keep it a street orientated setup so no stiffer than 5-6kg rear springs and stiffen it up more with swaybar. its already very light in the back and don't want to lose bump control when fanging on some backroads.

dare you to do it in cabin adjustable

don't say things like that, thats when i start thinking 'yeah i could probably do that'. and then my pocket ends up paying for it! it is possible but steel wire/pulleys are too expensive.

make it out of handbrake cables? they must be cheap enough!

[RomanV]

You'd need a cable that could both pull and push though I guess?

How about gear shifter cables from a front wheel drive, or an MR2.



Hey Kinloud, I was just thinking about something you said today...

So you have 100mm adjustment - from your diagram it looks like minimum will be 175mm arm, max 275mm arm.
Presuming 600mm bar length.
With a 19mm diameter
With 175mm arm rate will be 284lbs/inch or 5kg/mm
With 275mm arm rate will be 107lbs/inch or 1.9kg/mm
- Pretty stiff really!

IS that the spring rate per side, or the spring rate in total of the whole bar?

Because a swaybar effectively adds spring rate to one side, and removes it from the other. So is that adding 5kg/mm to one side and removing from the other, or adding 2.5kg/mm and removing from the other side?

As 2.5kgmm as a swaybar rate doesnt sound completely outrageous, if you've got say 10kgmm springs or something I guess?

[soopachargen]

I was told that pushbike gear lever cables were really good for the job

I have a CAD drawing for the evo 5 wrc rear swaybar setup from ralli-art, but its printed and i dont have a scanner

[RomanV]

21 speed swaybar

[soopachargen]

would you go for gripshift??

[metal_sean_head]

Do it!!

[RobertC]

bicycle shifters only pull or release in set increments. So you would just need the return to be spring loaded.

I'm sure a handbrake lever, a SS mountain bike brake cable and an old trampoline spring would do the trick.

[RomanV]

I quickly sketched up something in autocad to show what I mean about a variable rate swaybar, by making the arm so short that it moves through a large arc.



If the blue on the right hand side is the % of vertical suspension travel, and red is the arc that the swaybar arm moves through.

You can see that if it's moving through an arc of that range, the swaybar rate more almost triples by the time the suspension compression nears 100%, as each 10% of vertical suspension travel makes the swaybar rotate by a non linear increasing amount.

Although I guess since the drop links move across as well as up when it's moving through an arc, it might change things a bit.

Edit: I just realised why I think this idea wont work.

When the swaybar rate increases on one side, it decreases on the other side by the same amount? Making it the same as having a linear swaybar rate anyway

TL;DR: My idea doesnt work, please ignore.

P.S. Next idea: Having swaybar arm length adjusted by steering angle, more steering angle = stiffer swaybar.

[touge_ae101]

P.S. Next idea: Having swaybar arm length adjusted by steering angle, more steering angle = stiffer swaybar.

what about when you are in a 4 wheel drift and have almost neutral steering angle? surely thats when you would want the swaybar to be really stiff?

aha g-sensor to tell the swaybar when it needs to stiffen up

[gasman]

Probably way easier to work out swaybar rate in lbs/inch or kg/mm rather than nm.

For example -
Corolla 4afe swaybar 13mm thick (solid), bar length 600mm, arm length 219mm, effective arm 200mm
This gives a rate of 45lbs/inch or 0.8kg/mm

Corolla 4age swaybar 15.7mm thick (solid) all other dimensions same as above
This gives a rate of 95lbs/inch or 1.7kg/mm

So you have 100mm adjustment - from your diagram it looks like minimum will be 175mm arm, max 275mm arm.
Presuming 600mm bar length.
With a 19mm diameter
With 175mm arm rate will be 284lbs/inch or 5kg/mm
With 275mm arm rate will be 107lbs/inch or 1.9kg/mm
- Pretty stiff really!

Maybe start with 17mm
With 175mm arm rate will be 182lbs/inch or 3.2kg/mm
With 275mm arm rate will be 68lbs/inch or 1.2kg/mm

ALSO
I would make the arm out of steel - aluminium alloy doesn't have a good resistance to fatigue of the type a swaybar will have!

Ken

thanks for the input ken. could you elaborate on where you got those figures/calculations from?

adjustment on the arm is between 200-300mm

yeah i am wodnering if 19mm will be too stiff so thats why i want to make it so i can interchange to a 17mm bar by simply undoing some circlips and slipping the blades off the end and onto another bar.

am quite aware of the weight of such an item made out of steel weighs though both in unsprung weight and in overall adding to the weight of the car.

your a engineering student right? i remember you saying you were in one of your posts, if thats the case then the following should not be too difficult for you.

If you got the bar already along with the basic dimensions then, work out/assume your roll angle then work out how much your bar twists.

from the twist angle you can then work out the torque in the rod and hence force on the joint between the arm and the rod.

if your going to make the arms out of alloy and the bar out of steel and join them with a machined slot in the ends of the bar, i dont think it would last too long especialy if its only 10mm thick plate for the arm.
I would go for a larger bar an thin down the centre section of the bar to 19mm with a splined connection or a change the arms to steel with a heavy press fit still with the larger ends.

could try having the larger ends with flats, but it still wouldn't be as good as the spline or the hevy press fit.

[Boost_4_Life]

we have just built an adjustable one at work for the boss' jag race car using servos. piece of piss mate

[touge_ae101]

yup i am an engineering student but more design engineering than mechanical. maybe i should define my question a bit better.

i was wanting to know primarily if my assumption about the spring force on the inside wheel is transferred via the swaybar (which also has some elasticity) through to the outside wheel when turning.
so the force going through the blades of the swaybar cannot be greater than the spring rate of the inside wheel? plus of course FOS for sudden bumps etc.

i was interested if anyone had any formulas for the twist of the chromoly bar also as i can't be bothered going through all my textbooks and finding when we got taught that stuff.

uni is gay in the repsect that you get taught a lot of stuff but never remember anything until you put it into practice.