NEED FORCES ON STRESS & SWAY BARS...

[rduarte01]

Hello everyone! This is my first time on here and I was wondering if you all could be of help. I am trying to design some sway bars & stress bars for my 2003 1.8T GTI. Do any of you have a diagram of all the forces that would be acting upon these points. I just dont know where to get started as far as if the weight of the car would be taken into consideration, flex... etc. My ultimate goal is to design a set of bars that do the job & are light as possible. Your help would be greatly appreciated.

 

[GregLocock]

sta bars are fairly easy to analyse, the maximum force is generated when one wheel is all the way up and the other is all the way down. Usually they have linkages that resemble ball joints, so you can treat them as pin joints.

The stresses at the mounting points may be more complex, but they don't seem to fail there, they usually fail where the 'lever' arm fairs into the main torsion element.

BTW you are on a hiding to nothing, they are already made out of the right material, as E/rho is important. Tubes are better than solid.


Cheers

Greg Locock

 

[NormPeterson]

Stabar-related failure points seem to be in either the endlinks or in the chassis bracketry rather than in the bar itself.

The endlink bushing arrangement that uses split bushings to approximate a ball joint is commonly to blame for endlink failure, particularly if the bushing material is fairly hard (like poly) and/or the endlink bolts are overtorqued. Usually they let go through the threads near the nut. Lest anyone think that this issue is confined to amateur suspension tinkering, Ford has experienced this very problem on its Explorer and Expedition lines with the hard bushings. I've seen that one up close and personal, having had to replace an endlink on my son's Expedition last year. Bottom line here seems to be if you're trying to eliminate endlink compliance to improve the linearity and increase the effectiveness of a given size bar you'll want a better approximation of a ball joint than a split bush.

Chassis side failures also exist, though these are mainly related to the installation of stabars that are substantially more rigid than OE. Hondas with aftermarket rear bars commonly tear out their attachments and I think it's the late-model F-bodies that have been known to bend/crack/tear the bracketry when aftermarket solid bars in the 35 mm range have been installed. So going stiffer suggests that some local chassis reinforcement might be in order. For details, I'll defer to somebody who has specific knowledge of what exists under your GTI in those regions.

Norm

 

[mloew]

Greg,

I am interested that you mentioned that tubes are better than solids. I had originally assumed the dame thing, but I went to a technical presentation by a major supplier of springs, and anti-roll bars and an opposite statement was made. For certain conditions where very high strengths are required a hollow section may not be desirable. This was due to the fact that the ID of the material can not be shot peened to produce the necessary residual stresses as they can be produced easily on the OD. This same supplier had a sophisticated way to perform a detailed First Order Analysis (FOA) of ARB design and performance.

One more thing, would you not want to analyze the connection that is in fact typically a ball-joint as it is rather than assuming a pin? Or were you simplifying to a 2-D case?



Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

Matthew, interesting. I wonder if our supplier knows that? Hope so. The almighty dollar means we use solid anyway.

I agree entirely with Norm about that design 'feature', the trouble is it is very very cheap compared with a ball joint. The improvement in on centre feel when you go to ball joints is noticeable even by me. When I was talking about failure at the mounting points I meant failure of the bar itself, not the mounting point on the chassis.





Cheers

Greg Locock

 

[mloew]

Greg,

The supplier was Muhr and Bender KG (also known as Mubea) . A large and respected supplier to many OEMs.

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NormPeterson]

I just dont know where to get started as far as if the weight of the car would be taken into consideration, flex... etc.
Sort of. I assume that you want your bars to be something other than lightweight but otherwise carbon-copies of the OE pieces right down to the rate. Are you also planning to swap your OE springs out?


My ultimate goal is to design a set of bars that do the job & are light as possible.
Build up your own 3-piece bars from tubular torsional sections and aluminum arms (with steel inserts so the splines won't strip)? The circle track crowd does it all the time. Look here for some ideas and one list of what's already available:

http://www.stockcarproducts.com/ftsusp.htm

Greg - As far as better ball joint approximations go, I'd started to see differently configured endlinks in a few vehicles, most memorably in the C5. But it isn't something that couldn't be done less expensively. Right now I've got a little experiment going on in an older car with the split bush pair contoured on the washer sides to better approximate a spherical surface when installed, re-shaped the washers to suit, and made the through-holes cone-shaped. I see no reason that these pieces couldn't be made that way in the first place (and probably should have been in the Ford SUV instances).

Norm

 

[rduarte01]

Norm,

I will eventually swap out the springs. So you say to check out that site for some ideas? I do want these things to be more than just lightweight, I do want them to function correctly but the main reason I am doing this is just to have a fun little project to work on. I go to a lot of car shows with my buddies and they purchase all the parts for their VW's (lightweith & underdriven pullies, sway & stress bars, intakes, wheels, flywheels, etc). I want to be able to say look at what I engineered and manufactured and be able to show them my calculations etc. My dad and uncle own a precision machine shop (I currently work here) where we make all kinda things so why not utilize what we got for equipment and use my engineering capabilities to make this stuff. Thanks guys.

 

[NormPeterson]

Sure. They say a picture is worth a thousand words, and all those components are illustrated. The bar info has links to bar rate information, though it's given in some rather unusual units (lbs @ 5 degree bar twist) rather than an effective rate at the ends in lb/in.

Re: the whole effort, unless you're going to autocross or open-track this car you'll want to maintain the overall front and rear roll resistances in at least a semblance of the stock balance. Expect to become very familiar with the assorted motion ratios. Otherwise it's entirely possible to end up with some, ummmm, exciting handling characteristics. So you'll need to consider the bar design and the spring selection in conjunction with each other; otherwise you'll end up tuning by throwing more new parts/part numbers at it than necessary.

Norm

 

[GregLocock]

We use M&M shaped rubbers in our version of that sta bar linkage, and cupped washers. I have seen suspsenions that just use flat washers and cylindrical rubbers. Which makes my skin crawl.



Cheers

Greg Locock

 

[FredEvans]

Could someone tell me what a split bush is ??

 

[NormPeterson]

In many designs, particularly those on older (US) domestic cars, the attachment of endlinks to the stabar (AKA antiroll bar) and the control arm is through compliant bushings, one at each end of the link. For cheap and for simplicity of installation (and probably for NVH reasons as well) these bushings are split in half, with one half going below whatever the endlink bolt is passing through and the other half going above.

While it's overall a poor attempt at simulating a proper ball joint, it's more than adequate for the vast majority of drivers.[/editorial]

Norm

 

[FredEvans]

Thanks for the info Norm-- I recognise the part from your description.
I am looking for information on a suspension system where there is incorporated into the arrangement a clevis or fork holding a rod which is then able to pivot on the clevis.
The information is required to show that this arrangement is existing technology . Any suspension in any road vehicle having this as a design feature would be of interest to me.
The feature that I am particulary interested in is the bushing arrangement as i am trynig to establish whether there is existing design like this having bushes in the clevis as well as in the rod.
Perhaps you know of this type of arrangement. The closest that I have been able to find is a similar arangement used in the Cord motor car suspension
I think that this arrangement is or was used extensively in motor cycle rear wheel suspension systems

best regards

 

[NormPeterson]

The closest thing that I can think of to a pin & clevis arrangement would be rod-ended or Heim-jointed designs. Such configurations are commonly associated with competition cars, though many late-model OE designs are similar. For instance, GM uses small ball studs and composite links in a number of its cars, including the C-5 Corvette.

Keep in mind that in the typical control arm suspension application the control arm end of the link moves in an arc that's essentially in the YZ plane while the bar end arc motion is contained in the XZ plane. So the endlink has to sort of "wobble" in 3-D as the suspension moves, which is motion that a tight-fitting pin in a clevis doesn't like (you end up with bending moments in the endlinks and local plate bending in the control arm attachment locations). Hence, these pivot points need to freely permit 3-D rotations.

I suppose that you could bush the pins in the clevises and rely on that compliance to accommodate the "wobble", but that sounds more complex, less reliable, and less rigid for sta-bar effectiveness purposes than using some form of balljoint.

A motorcycle rear suspension is a different animal; the motion of the swing arm is contained entirely in the XZ plane and the shock/spring assembly does not need to accommodate any Y-direction movement. So the simpler and less expensive single-plane rotational freedom of a pin & clevis is the proper solution there.

Norm

 

[etatv]

Is this what you were thinking about.

http://www.baldwinmotorsports.com/yfzProTrax.htm

Offroad guys have been using this for a few years.

 

[mloew]

The following SAE Technical Paper may be interesting:

2004-01-1550 Automotive Stabilizer Bar System Design and Reliability
Ian Czaja and Mohammad Hijawi
Ford Motor Company

Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bobqzzi04]

Well, no real need to design bars for such a well developed car. Try SRSVW.com for the rear bar- theirs has no links at all, but rather bolts directly into the rear beam.

My best suggestion for a front bar is to either leave in the stock one, or remove it. The last thing you want on a FWD car is a large front bar. The outside front tire is already overloaded.