Jesus-Ninja
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| posted on 15/8/08 at 02:29 PM |
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Degrees of precision
How precise does one have to be with things? Of coure - "very", but clearly it's unlikely the average home builder can align the
suspension pick up points of each end of the vehicle in relation to each other, to the nearest 1/100th mm, and clearly the nearest 10cms is no
good!
Where's the happy medium? Is the nearest mm enough, or even the nearest 2 or 3, and any correction can be dialed out through adjustment of
suspension components that allow it?
I'm assuming it's the nearest mm...
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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blakep82
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| posted on 15/8/08 at 02:37 PM |
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i'm probably wrong, but i'd be happy with the nearest mm. maybe even 2. but i am usining rose joints in all my suspension, so
there's loads of adjustment to be had there
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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Doug68
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| posted on 15/8/08 at 02:49 PM |
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What you can't dial out though is if the hard points are not where you meant them to be.
I've been thinking long and hard about this myself lately.
I intend to write up how hopefully I've got my suspension pivot points attached to the chassis at the points I wanted them to be. It's
past my bedtime right now though, so that'll wait for another day.
Realistically the best I think the home builder can hope for is + - 1 or 2 mm and if you think you're doing better than that you're
probably fooling yourself.
Doug. 1TG
Sports Car Builders WA
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Jesus-Ninja
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| posted on 15/8/08 at 02:50 PM |
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Me too.
I'm getting the front and rear jigged up, and they are both bang across the centre line, but of assorted diagonals a couple aren't quite
adding up. A few mm out.
[Edited on 15/8/08 by Jesus-Ninja]
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Jesus-Ninja
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| posted on 15/8/08 at 02:53 PM |
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Oh, the original pick up points are oval (12mm x 18mm) to allow for adjustment and correction, so I'm not too panicky.
Although I am at the point of joining the front and the rear and getting some points welded in....
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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kb58
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| posted on 15/8/08 at 03:33 PM |
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Welding distortion will cause the chassis to bend all over the place. Feel fortunate if the front and back are level within a cm. Yes, really.
The secret is to attach the in-board suspension brackets last, after all other welding is done. That way, it doesn't matter how far off
it is.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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Liam
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| posted on 15/8/08 at 04:42 PM |
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As above make sure you fully weld the whole chassis before adding the wishbone brackets! Then jig them up carefully and you'll be able to get
everything within a mm or so, which is as accurately as you can measure it afterwards anyway.
I went a step further checking mine by carefully measuring up my lower balljoint postions all round with the wishbones on. Managed to end up with my
wheels within a mm or so of where i wanted them with a square footprint so was well pleased.
I've often read, on this topic, that production car suspension location tolerances are +/- a good several mm, though personally I doubt they
could be that innaccurate the way they're made. Still with the amount of slop in production car bushes, and other sources of flex, the wheels
must be literally all over the place dynamically anyway.
Just make yourself a decent base to build on, carefully jig your brackets and bones, get the suspension setup properly when it's finished, and
you'll be fine.
Liam
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mr henderson
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| posted on 15/8/08 at 04:48 PM |
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Probably better to express tolerances in terms of percentages rather than absolute numbers.
So, say 10mm out over a distance of 2000mm = .5%. Helps to put things in perspective
John
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Bigheppy
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| posted on 15/8/08 at 05:24 PM |
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Was watching wrecks to riches when they were rebuilding a toyota celica after a bad smash. When they were jigging it the guy said that he was allowed
3mm tolerance in the chassis and 5mm in the upper body. Dont know if this is relevant to your queery tho 
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Jesus-Ninja
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| posted on 15/8/08 at 05:40 PM |
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Great stuff guys, thanks - very help ful! 
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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smart51
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| posted on 15/8/08 at 06:14 PM |
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I was part of a team that made a machine to assemble the multi link rear beam for a popular car. The beam itself was made from the most rougharsed
old pressings and welded together with a haste that would make most blush. They were, shall we say, not meant to be seen. Using laser measuring and
automated adjustment, I made the machine adjust the toe and camber to within 20um, if memory serves.
There is a time for precision and a time to just make it quickly or cheaply. Knowing when to use precision is the clever part.
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Jesus-Ninja
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| posted on 15/8/08 at 06:18 PM |
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quote:
Originally posted by smart51
I was part of a team that made a machine to assemble the multi link rear beam for a popular car. The beam itself was made from the most rougharsed
old pressings and welded together with a haste that would make most blush. They were, shall we say, not meant to be seen. Using laser measuring and
automated adjustment, I made the machine adjust the toe and camber to within 20um, if memory serves.
There is a time for precision and a time to just make it quickly or cheaply. Knowing when to use precision is the clever part.
So are you saying that the points were not precisely jigged, but that one built, the suspension was adjusted to dial out error?
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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johnston
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| posted on 15/8/08 at 07:54 PM |
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Has anyone ever put a newish car on a jig or a full laser set up ? How close do manufacturers get it?
Only thing i've ever seen was a 3 or 4 year old audi on a pro laser machine when doing a course few years back. Fella that owned it was bummin
and blowing about how good it was and how good audi build cars.
That was until instructor showed us the print out showing one side sitting back and that it was sitting diagonal against the centre line.
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Jesus-Ninja
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| posted on 15/8/08 at 08:26 PM |
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quote:
Originally posted by johnston
Has anyone ever put a newish car on a jig or a full laser set up ? How close do manufacturers get it?
Only thing i've ever seen was a 3 or 4 year old audi on a pro laser machine when doing a course few years back. Fella that owned it was bummin
and blowing about how good it was and how good audi build cars.
That was until instructor showed us the print out showing one side sitting back and that it was sitting diagonal against the centre line.
I'm jigging of my existing monocoque, and someone asked the question "You're making the assumption that your existing shell is
straight"
But to me it shouldn't matter, the principle is that the current chassis handles fine, so provided I replicate it....
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Ian-B
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| posted on 15/8/08 at 08:30 PM |
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The most important aspect to determining how precise the positioning of the pick ups needs to be is the design of the geometry. If you have designed
the geometry yourself it is worth re doing the calculations with the expected build variations to see if the changes are acceptable (even if it is not
your own design it should be possible to reverse engineer to find this), this will also highlight which dimensions are most critical.
As others have said you are unlikely to be able to control the general tolerances on a welded assembly to less than +/- 2mm but to is possible to
control specfic key features to less than 1mm with carefull planning
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Jesus-Ninja
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| posted on 15/8/08 at 08:34 PM |
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Cheers!
Yeah, not my own design, as such. I'm jigging the points from an existing vehicle. Came to do some verification (matching diagonals), and most
agree, but there's on particular point which throws slightly anomolous measurements compared with it's counterpart - about 5mm difference
in comparing diagonals with key points in the front suspension.
Could be a crooked subframe.. It's actually thre point where the rear camber arms go which are in question, and both arms are rose jointed and
adjustable.
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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02GF74
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| posted on 16/8/08 at 08:47 AM |
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I would say think in percentages than in absolute distance.
For example you a building a chassis - 2 mm out ened to end is not too bad but if you are 2 mm out drilling the hole for the wishbone bracket that
would make a bigger difference.
Anything less than 5 % would probably be good enoug (just a finger in air type of guess). 1 % would be fantastic!
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Jesus-Ninja
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| posted on 16/8/08 at 11:45 AM |
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ok, that sounds reasonable. I'm worrying about 5mm over a distance of more than 2000mm which is 0.25%
So that's OK then!
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Doug68
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| posted on 17/8/08 at 01:03 PM |
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My write up on how I'm attempting to get my suspension location points in the designed
positions.
Doug. 1TG
Sports Car Builders WA
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Jesus-Ninja
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| posted on 17/8/08 at 05:17 PM |
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quote:
Originally posted by Doug68
My write up on how I'm attempting to get my suspension location points in the designed
positions.
WOW!
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Jesus-Ninja
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| posted on 18/8/08 at 10:54 AM |
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As I have all the pickup points for the suspension jigged and square on the platform. I have decided that I shall leave the jogs in place, with the
suspension brakets bolted to them, but I will not weld the bracket.
I'll build the rest of the chassis, and the very last thing I do will be to weld in the chassis pieces which the bracketsweld to, and then weld
the brackets.
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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