cheapracer
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| posted on 8/6/12 at 06:56 PM |
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Locost 4 link solution .....
Another "Bex Solution" by cheapracer, because Bex is Better!
4 links trailing arms should not be parallel!! .... even though 99% of the mod car population thinks that it goes without saying they should
be.
Trailing arms do not roll around a center point between them, nor do they rise in a 2 dimensional plane in roll, they roll 3 dimensionally controlled
by points (RC and chassis pivot) some distance away, there's 2 sides of them and they are at different points of that arc hence their
longitudinal chord lengths change at different rates. This is pyhsics that can't be changed regardless that your brain is now screaming
"Trailing links should be parallel!!".
Why do links bind? Because they immediately become non-parallel (longitudinally) at the first instance of roll and the change in the link's
chord lengths opposes the opposite side's trailing links that are also trying to do their thing causing bind.. specifically the top link in bump
shortens faster than the opposite side's top link in droop and the axle won't twist to compensate.
By having non-parallel links we have one side's top link chord length shorten in roll (bump) while the opposite side's top link
shortens at closer to the same rate while in droop.
The test setup;
For a practical sample I chose the very popular Locost 4 link rear trailing link setup as per the book specifications. Although using some scrap
steel, great care was taken in dimensional accuracy. Note horizontal heims were used on the dummy "axle" upper pivots to enable quick
height adjustment for the test ..
Links and jig;
Links were 292mm in length as per book, initially 140mm vertical spread and all 4 jigged for accurate length ..
The result;
With the links parallel as per the book specs and common mythology that 4 links should be parallel(not to mention 99% practical application), bind was
quickly established at around 6 degrees. Axle had to be mildly forced to this limit, would not fall under it's own weight after about 4 degrees
and binding was felt well before limit. Note at this point all 4 links were very tight to swivel due to the binding and had not reached their travel
limit ....
With the links set non-parallel (rear pivots set 8mm closer together), bind was non-existent, the axle fell under it's own weight until travel
limit of horizontal heims was reached. note that the only reason the axle stopped at this point was because the horizontal heims ran out of travel -
THE AXLE WOULD HAVE TRAVELED MUCH FURTHER FREELY with vertical heims. The lower links swivelled freely and easily indicating little bind. Stupid
picture should read "255mm" on the left..
The resulting "winning" dimensions that you might use on your Locost, note the horizontal hiems have run out of travel ....
I also tested upside down with the RC centered to a live axle (above test with RC 50mm below wheel centerline) with similar results... note in the
lower picture the trailing arms have reached horizontal having traveled further and again i stress that it only stopped there because the horizontal
hiems ran out of travel, not because of bind!
I take these tests very seriously and was very careful to check and recheck before taking results.
Should you do it even though theres thousands of Locost's running around today successfully? Well that's entirely up to you now you have
the information, note that the serious binding starts near the end of suspension travel limits on a typical Locost but it is there. At maximum bump
this will tend to creat oversteer due to the binding creating anti-roll besides the higher wear/stress on your heims and chassis mount stress. Of
course rubber bushings masks this (but doesn't remove the binding anti-roll) but that's not good enough for me and it shouldn't be
for you.
On a Locost this should be very easy to check/modify for yourself by merely adding 2 extra holes on the rear axle brackets as shown (can weld a washer
over later) or raise the front mounts the same amount. Note that at 120mm test seperation binding had seriously returned, 132mm was the sweet spot (no
need to take that too literally, 130mm is also fine);
As another proof of point, I currently have a mock up of another 4 link (DeDion) with 710mm long non-parallel trailing arms with vertical seperation
of the pivots at 310mm front and 290mm rear - a full 20mm difference. With the trailing arms parallel I get serious bind within 100mm of travel but
with the rear pivots 20mm closer I can lift either side a full 200mm freely using 1 finger and only limited by chassis interference in this case.
Pictures another day, phone battery dead.
Don't believe me? Make up your own test rig, use some wood, nails and hook screws or try this on your current 4 link, it's not that
difficult to prove or understand once "you see it"!
Some people will not be convinced because of the mindset about what 4 links should be ie; "Parallel!!" but a few of you more
flexible thinking might benefit from this for your builds ;-)
It's coming....
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v8kid
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| posted on 8/6/12 at 07:32 PM |
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Nice experimental procedure. VG info
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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PSpirine
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| posted on 8/6/12 at 07:32 PM |
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Very interesting! Awesome post.
I've got a Westfield with a 4 link rear, I wonder whether that uses standard book parallel arms... May have to do some measuring and
re-arranging the brackets..
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MikeRJ
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| posted on 8/6/12 at 08:10 PM |
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Good post! I suspect that if bushes with some compliance were used (e.g. as per book design) rather than rod ends then binding is unlikely to be a
problem with typical Locost axle articulations.
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cheapracer
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| posted on 9/6/12 at 04:55 AM |
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quote:
Originally posted by PSpirine
Very interesting! Awesome post.
I've got a Westfield with a 4 link rear, I wonder whether that uses standard book parallel arms... May have to do some measuring and
re-arranging the brackets..
I would encourage you to first remove you shocks and springs, check for binding first, then remove one upper link bolt and pull the link end away from
the mounting and check again.
Some people don't know what binding feels like and this will help you to feel the difference, if there's no difference and it all
articulates freely please leave sleeping dogs lay, ie; don't do this just cause it looks good.
A reason is in production, the variations of holes and mounting welds may actually have already caused the links to be able to articulate freely, but
on the other hand it also can go the other way so please check first.
If the Westfield is different please tell me the 3 dimensions, link length, link vertical separation and distance apart (side to side) - my jig might
be close enough for me to test it for you easily.
The book dimensions are; 292, 140 and 1050 (rounded off).
quote:
Originally posted by MikeRJ
I suspect that if bushes with some compliance were used rather than rod ends then binding is unlikely to be a problem with typical Locost axle
articulations.
Yes, this takes some stress away from the mounts and links via the rubber absorbing that energy in compression but still creates anti-roll
characteristics as that energy attempts to dissipate. Newton's 3rd ...
At the end of the day and as i mentioned, there's thousands of Locost's running around with happy owners and this is just a
"meh" issue to them and good luck to them ;-)
[Edited on 9/6/12 by cheapracer]
It's coming....
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britishtrident
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| posted on 9/6/12 at 07:22 AM |
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i remember a few years back pointing out the standard book 5 link suspension was over located and getting shouted down.
On the S1 to S3 Lotus Seven Chapman used a 4 link system (2 upper links + A frame, the lower A frame counts as 2 links) but that has problems
because after a few thousand miles English Banjo style axles which were not designed for to loads crack adjacent to central mounting point.
[Edited on 9/6/12 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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cheapracer
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| posted on 9/6/12 at 03:39 PM |
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Above I mentioned;
As another proof of point, I currently have a mock up of another 4 link (DeDion) with 710mm long non-parallel trailing arms with vertical
seperation of the pivots at 310mm front and 290mm rear - a full 20mm difference. With the trailing arms parallel I get serious bind within 100mm of
travel but with the rear pivots 20mm closer ...
..here is a video of me running it through 10" of movement with 2 fingers and totally free movement. The Dedion is lateraly located - that
should be clear actually as it obviously goes up and down on the same path. - to make matters worse the opposite side is even at full droop and yet
..
HTH someone, interesting even if it doesn't :-)
It's coming....
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