Anti-roll bar loadings
- Thread starter Shed Racing
- Start date
Terry Oxandale
Skinny Man
I found that the front bar still remained a good size larger than the rear, but that was for my set-up. Think I'm running 1.062 vs .750. Of course, that is dependent on the bar's arms and specific suspension design.
Mike Pass
Supporter
The loading on the anti roll bars will depend on many things - spring rates, anti roll bar leverage arm length.The important thing is the anti roll bar rate at the wheel. A thick bar with long arms can have the same rate at the wheel as a thinner bar with short arms. The car should be set up with the correct spring rates and then the anti roll bar to fine tune the handling. When set up correctly changing the setting of the antiroll bars can give understeer, neutral and oversteer. The overall roll resistance will be the effect of the wheel spring rates plus the anti roll bar rates. The trick is to match the roll rate with the camber changes built into the suspension geometry so that the tyre is correctly presented to the road. A useful figure is the load on each tyre. A typical GT40 space frame replica with water, fuel and driver has about 260Kg on each front wheel and 330Kg on each rear wheel.
Cheers
Mike
Cheers
Mike
One of the best treatments regarding anti-roll bars is David Gould's approach outlined in a chapter of Allan Staniforth book; Competition Car Suspension. He approaches it in terms of weight transfer as a whole and takes into account springs, bars, mass distribution, suspension geometry, etc.
He also provides a worked example based on a rear-engined race car that I found wasn't too far away from the GT40 in dimensions or weight. I set up an excel spread sheet and worked through his analysis modified to the specifics of my car. It helped me realise that my bars weren't well matched to my springs specific to my geometry and was transferring a lot of weight to the rear. His analysis doesn't provide a lot of guidance as to which way to tune for specific handling characteristics, but makes the point that by going through the process of analyzing a baseline allows a better understanding for what is going on when you make adjustments.
He also provides a worked example based on a rear-engined race car that I found wasn't too far away from the GT40 in dimensions or weight. I set up an excel spread sheet and worked through his analysis modified to the specifics of my car. It helped me realise that my bars weren't well matched to my springs specific to my geometry and was transferring a lot of weight to the rear. His analysis doesn't provide a lot of guidance as to which way to tune for specific handling characteristics, but makes the point that by going through the process of analyzing a baseline allows a better understanding for what is going on when you make adjustments.
Wow!
Loads of sensible comment.
Damn glad I'm not trying to design any anti-roll bars.
I guess by trying to ask the question in a very basic fashion I managed to not give a clear indication of intent.
The piece of information I'm chasing is ultimately about the drop link end connections. My assumption is that these will experience the greatest loadings during heavy cornering / uneven surfaces etc.
I fully accept that the loadings onto the rods ends will be affected by many additional factors related to suspension geometry, spring / damping rates and bar stiffness but to find I have a pair of bars that have three different rod-end sizes being used on the drop links was a surprise.
My expectation would have been to find the same rod-end sizes on both ends of the drop links - not necessarily being the same on the front bar as the rear bar.
Perhaps what I should have said was ...
Assuming that a suspension designer has designated that the rear anti-roll bar drop link connections should be 3/8" ID spherical rod-ends to suit the maximum expected loads then would the expectation be that same size, larger or smaller diameter items would likely be required for the matched front anti-roll bar connection?
Loads of sensible comment.
Damn glad I'm not trying to design any anti-roll bars.
I guess by trying to ask the question in a very basic fashion I managed to not give a clear indication of intent.
The piece of information I'm chasing is ultimately about the drop link end connections. My assumption is that these will experience the greatest loadings during heavy cornering / uneven surfaces etc.
I fully accept that the loadings onto the rods ends will be affected by many additional factors related to suspension geometry, spring / damping rates and bar stiffness but to find I have a pair of bars that have three different rod-end sizes being used on the drop links was a surprise.
My expectation would have been to find the same rod-end sizes on both ends of the drop links - not necessarily being the same on the front bar as the rear bar.
Perhaps what I should have said was ...
Assuming that a suspension designer has designated that the rear anti-roll bar drop link connections should be 3/8" ID spherical rod-ends to suit the maximum expected loads then would the expectation be that same size, larger or smaller diameter items would likely be required for the matched front anti-roll bar connection?
Sorry, got the wrong end of the stick. Without knowing the details of how the links are mounted, do the bearings at both ends of your link remain more or less perpendicular to their housings as they move through full movement of the suspension travel or does one end move out of perpendicular (if you see what I mean). The more it moves out of perpendicular, the more axial load it puts on the bearing and therefore might require a larger bearing to match the load. If both ends remain perpendicular then I'm not sure how to explain any difference.
I am just an old amateur, but I use a different way altogether. It depends on whether you are using the bar as a beam to just transfer load across the car, or using the it as a torsion bar that twists with load to act as form of a cross car spring, these then controlled by rotational beams to allow stiffness settings individually each side of the suspension. (Based on led as used by Fords competition cars , rallying etc.) I find that this works very very well on the 40s. See picture to hopefully explain my poor descriptive effort!
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Terry Oxandale
Skinny Man
Haven't seen, nor could I calculate the dynamic tension on even a perfect "perpendicular" geometry that Trevor notes. If one knows that value, I assume one could determine the need. I've got 3/8" chromoly rod ends (9K pound tensile strength) on all of mine, with some slight angularity in the links. Front bar's wheel rate is comparable to the rate of the springs, and much higher than the rear bar's rate. Probably not much help there.
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"Just an old amateur"
Yikes I dread to think what that makes me then!
Can we at least add the word "gifted"?
Your descriptive effort Frank makes perfect sense.
If the car was going to be a track only (and smooth track at that) car then the bar would doubtless be designed for load transfer across the axle - and no doubt be of a battle ship design! This I believe is the likely "worst case" scenario for loads through the drop links.
The bar in question is a 3/4" solid item and more likely to act as a torsion bar I think to retain a greater amount of independent control at each wheel.
I guess where I'm going with all this is that instinctively I feel that 5/16" rod-ends may not have great longevity on a GT40.
With Terry having 3/8" items fitted and many other cars seeming to have rod-ends of a similar or larger size then I'm going to find a way to revise drop links being used so that all use 3/8" items. It only causes a slight issue in one area but nothing I perceive to be insurmountable.
Just that 1/16" increase gives a 50% greater load capacity and increased margin against failure.
It'll also get rid of the 5/16" one end to 1/2" other end drop link that, in my opinion, is an unusual disparity and triggers my OCD.
I'd better go and purchase as many suspension design books as I can now and see if I can ever get to understand the "voodoo" of suspension.
So much more fun testing and adjusting by the seat of the pants and hoping to keep things out of the hedge.
Mark, what make of car are you building ?
maybe this is obvious, but...until you track the car, you don't really know what it needs. Roll bars are meant to balance under/over steer in the car, and keep the chassis at appropriate roll to work with the suspension geometry. You'd expect, all things equal, that the necessarily higher spring rates in the rear require a heavier bar/anti-roll rate setup, consequently heavier end links make sense. Equally, you'd expect to find the same size end links across the bar.
I would agree that any final setup would need to be the result of on track testing.
At this stage my target was to understand whether, in a general sense, the potential expected loads put through the drop links would be in excess of what the supplied spherical rod ends would cope with.
With the sizes I currently have I am now fairly confident that bearing damage would occur after fairly limited track mileage - certainly with my track history!
Track time costs money so by eliminating this expected failure in advance is a sensible approach. And, as the changes to be made are a lot simpler if applied before installation of the anti-roll bars, it makes sense to revise things at this stage.
At this stage my target was to understand whether, in a general sense, the potential expected loads put through the drop links would be in excess of what the supplied spherical rod ends would cope with.
With the sizes I currently have I am now fairly confident that bearing damage would occur after fairly limited track mileage - certainly with my track history!
Track time costs money so by eliminating this expected failure in advance is a sensible approach. And, as the changes to be made are a lot simpler if applied before installation of the anti-roll bars, it makes sense to revise things at this stage.
