Suspension design theory
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  1. #1
    Dub driving rodder wanabe dubwarrior2's Avatar
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    Suspension design theory

    I have been reading alot of threads really about building chassis' for Rods and they make the suspension look very easy indeed.
    I have got a lot of decent books on the theory but there is still something that I cant visualise/understand and its driving me mad.

    That is the following-

    How does a four bar rear end set up work when the pivots of the arms are not in alignment?
    I can see that with all four pivots at exactly 90 degrees to the chassis rails it would work no problem, but how does it work when some are at angles to the chassis rails?
    If the left and right side are not on the same axis, then surely the suspension wouldnt move at all or would destroy the bushes in no time at all.
    I can see that rose joints would help but only for a certain amount surely.

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    Off the Xmas card list kapri's Avatar
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    Not sure what you mean when left and right sides are not in alignment length ?Do you mean when top and bottom arms are not parallel or when the arms are parallel,with the fittings on axle above one another but the forward chassis mounts staggered?

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    Nowheresville JackGriffin's Avatar
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    Do you mean under body roll conditions, or turning into a steep driveway when the axle isn't parallel to the chassis? If so, and you are wondering how urethane bushes cope with the twist, the answer is "poorly". The torsional load on the bars and mounts likes to find the weakest part of the assembly. This can cause the adjustable (threaded) end of the bars coming loose. As you say, Rose joints will stop this happening, at the expense of a hard ride. A (possibly) better option is to use compliant rubber bushes instead of urethane ones - that's what the manufacturers use (e.g. Ford Cortina triangulated rear four links). These accomodate the twist and will also reduce the ride harshness.


    EDIT: Now I've read my answer, I think you might have been referring to triangulated four link setups anyway? The answer still applies

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    I'm Not Jed Clampett stueeee's Avatar
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    I think you are referring to the type of setup where the top two bars converge on the diff case in plan view, but are somewhere near parallel with the bottom two bars when viewed from the side of the car. This setup seems to be very popular for rod chassis for some reason, but I wouldn't use this arrangement myself unless I was forced to by obstructions etc. under the car.

    As Jack Griffin says, you really need some compliance in the bushes to make the setup work for a road car. The same compliance causes the bushes to wear out quite quickly (changing the void bushes was a fairly regular job on the later Cortinas that had this setup) And the same compliance in the bushes means that the lateral location of the axle isn't that great either.

    Uning a four bar setup where the bars are parallel to each other in plan view and where they converge slightly in side view using a Panhard rod, or better still, a Watts Linkage to take care of the lateral location would be my first choice every time.

    I think it'sa lot easier to know what's going on with the geometry, and the ability to change it easily is there with a non-converged four bar setup; e.g. changing the anti squat doesn't affect the roll centre. See the diagram below.

    Stuart.
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    Official RnS Addict WB54's Avatar
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    been reading up on this myself from a book called Chassi Engineering and the underlying theory is that all designs are a compromise and the vehicles intended use should be the underlying choice for suspension design. the limts of each design should be considered during design.

    With ref to converging 4 bars, the length of the top and botom bars and their distance apart determines where their theoretical projections would meet, ie their instant center. If you take into consideration the distance of the instant center from the axle mounts and move the suspension through it's full travel, the actual number of degrees the axle arc's in, is actually pretty small, small enought to either be taken up in the movement of a bushing or it's already catered for in the rod ends range of spherical movement. Obviously the shorter the instan center, the greater the arc of the axle, but simply having the longest possible IC or parallel bars won't solve everything as there are other aspects to consider.

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    hi dubwarrior

    are you refering to triangulated 4 bars.
    this is where the lower bars are parralell to chassis and the top bars run out at an angle

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    Official RnS Addict hellblue's Avatar
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    I think the layout we are talking about is that used on mk3 to 5 cortina's which as it happens shows how well this may not work !! even the heavey duty bushes didnt last long
    Even with parallel 4 bars the bushes need to flex a lot, (looking at the rear of the axle) if one wheel went up by 2" & the other down by 2" picture the flex you have now ! any live axle set up will have this problem & the more the travel the worse it becomes
    How often do people check their split wishbones for crack's !! (the rears will twist & flex the most cos the axle wont !)

  10. #8
    Nowheresville JackGriffin's Avatar
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    Quote Originally Posted by hellblue View Post
    I think the layout we are talking about is that used on mk3 to 5 cortina's which as it happens shows how well this may not work !! even the heavey duty bushes didnt last long
    Even with parallel 4 bars the bushes need to flex a lot, (looking at the rear of the axle) if one wheel went up by 2" & the other down by 2" picture the flex you have now ! any live axle set up will have this problem & the more the travel the worse it becomes
    How often do people check their split wishbones for crack's !! (the rears will twist & flex the most cos the axle wont !)
    I think the top arms are quite short on these (and also on a lot of GM cars like '60s and '70s Chevelles, etc.) to leave room for the rear seat, so if the arms are made longer on a rod/custom/whatever I think that should reduce the variation, but it will always be there to a certain extent. I had thought before about welding the ends parallel to the axle in this situation, but then the rubber bushes wouldn't give the required support under side loads, which is the whole idea behind them being at an angle in the first place.


    Someone recently pointed out that all suspension is a compromise, and the vehicle's use will dictate the priorites, e.g. handling, ride comfort, suspension travel, launch traction.

  11. #9
    Off the Xmas card list kapri's Avatar
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    Re welding the ends at angles my friend Steve Goodridge did this on his Y sedan back end and it has covered about 30,000 miles like that now , only requiring a refurb of the bushings about 3000 ago. Recent owners eemd to behappy with it's handling as well.? haven't driven it myself so can't say if it feels any different to convetial set up.

    As mentioned the issues with the Cortina /GM back ends is caused by shortening the upper arms due to packaging constraints. The void bushes were their to provide 'compliance', OEM speak for" it'll bind if we don't put a sod off big rubber in there" .

    Can't beat a nice pair of semi elliptic cart springs

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