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PostPosted: March 20, 2018, 5:21 am 
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Joined: March 15, 2018, 6:03 am
Posts: 118
Gentlemen and Ladies (hopefully this isn’t an all-male preserve) I would like to solicit a little basic help, please.

I am designing a ‘reverse-trike’ (which I hope won’t be frowned upon by you 4w guys) and hope to start building this summer - I am currently trying to source some basic items which seem to be in very poor supply in my neck of the jungle... Uprights, especially...!

I enclose two graphics of my vehicle which I hope will clarify my immediate concern.
Attachment:
mangpong-a.jpg
mangpong-a.jpg [ 82.16 KiB | Viewed 1688 times ]

‘mangpong-a’ shows the basic layout, and ‘mangpong-b’ shows the front suspension in more detail. Like most designs the chassis sides converge towards the front, and I have always intended to connect the ‘wishbones’/control arms in line with the chassis sides i.e. at an angle of 8º to the longitudinal axis of the car... This will give a sort of ‘leading’ feature to the wishbones...
Now... although there seems to be nothing fundamentally ‘wrong’ with this layout (perhaps a little harsher reaction to potholes...) I have recently come across a couple of comments online asserting that wishbone mounting brackets should be mounted parallel to the central axis - but I have never seen anybody explain, Why...?

As it is possible to have semi-trailing arms, which immediately contradicts this ‘theory’, maybe it is not completely impossible to have leading arms - especially as they are only at 8º.

I should add that I am hoping to build a car that is aesthetically smooth and smart. Thus I do not want to have any suspension brackets visible outside the bodywork - just the control arms (with the ball-joints hidden), the pushrod (with the suspension inboard and also hidden), and the steering arms.

If I have to have the wishbone connections parallel I will endeavour to position the rearward mounts further inside the bodywork - but this would (I think) create a little extra complication, structurally. It would also then require two little internal ‘bumps’ (covers), the upper, rearward, one of which would be level with my knee, and the other level with my ankle - and there isn’t too much space in this area as it is...
Attachment:
MangPong-b.jpg
MangPong-b.jpg [ 113.49 KiB | Viewed 1688 times ]

So, simple question (but please make your answers as complex as you like): Is it really essential to have the mounting points parallel to the car’s central axis...? and if I don’t do this, what can I expect to go wrong...?
_____

Part 2: I would also appreciate some assistance in deciding the diameter of the wishbone struts. Again, for aesthetics, I would like to have these as small as possible. Each individual arm will be straight. Also, am I best to use solid bar and drill & tap to take the ball-joints, or use thick-wall tubing and tap that, or insert a threaded section. I imagine 1/2” OD would be too small but 1” would be too ugly, to my eye... The perpendicular arm length is about 11”, and the lower trailing arm is about 17”... Any comments would be appreciated.
_____

I will probably have other questions but, for the moment - That’s All Folks...

Mangpong.


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PostPosted: March 20, 2018, 11:16 am 
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Joined: February 20, 2015, 12:04 pm
Posts: 286
Location: Cotswolds - UK
Part 1: No, it's not essential to have the wishbone pickups parallel to the centreline of the chassis. Plenty of Lotus's don't (for example the Lotus 23). It does mean that your caster, and therefore steering geometry, changes slightly through the range of suspension movement, but not by enough to cause major difficulties.

Part 2: Wishbone tube size will depend on loadings (hence tyre size and vehicle weight, along with where on the wishbones the springs/pushrods/pullrods are mounted), but if your trike is light enough, with skinny tyres and no aero, you should get away with 5/8" x 16g CDS without any problems, if designed correctly. Single-seat hillclimb cars (with very sticky tyres and lots of aero) sometimes go down to 5/8" x 18g., but that won't reduce the external diameter and I'd be inclined to build a bit more robustly for road use.


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PostPosted: March 20, 2018, 2:33 pm 
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Joined: March 15, 2018, 6:03 am
Posts: 118
Hello Sam-68
Thanks for responding, and for so positively reassuring me. I also think I noticed this on an early Lola drawing I saw online... All else being equal, would you think reducing the castor angle by perhaps 2º would help or hinder...? Or stay as I am.

I am expecting the car to be light and, being rear-engined, perhaps too light on the front. I'm also hoping for narrow wheels/tyres but I need to source uprights/spindles before I can see what’s available. And I can promise the world there will be NO visible aero...! lol. The lower end of the pushrods will be as close to the outer end of the wishbone as possible, although this will make for a very shallow pushrod angle so I might have to compromise here.

Forgive my ignorance - I'm not quite up to speed with the terminology - what is CDS...? Wiki was no help. Does this refer to the size of the heim connecters (5/8” D, 16 TPI, threaded ends...?). If I am close, does this mean: 7/8” bar drilled out to about 1/2” and tapped at each end to 5/8”...? Would it seem crazy to you to use 3/4” rod (or tube with 1/8” walls) tapped to 5/8”...?

Many thanks, Mangpong.


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PostPosted: March 20, 2018, 2:35 pm 
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Mid-Engined Maniac

Joined: April 23, 2006, 8:26 pm
Posts: 5963
Location: SoCal
Chassis design aside, I highly recommend doing everything you can to move the CG just as far forward as you can. There are many benefits and no problems, other than packaging, which is a real bear.

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PostPosted: March 20, 2018, 9:31 pm 
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Joined: March 15, 2018, 6:03 am
Posts: 118
Many thanks for your confirmation... I have been thinking this for a while. If my engine turns out to be water-cooled (which I expect) I can put the rad. in the front, along with the battery, fuel tank & pump. Other than that I can only think of ballast...!! As it is, the occupants are already fairly well forward, and the backs of the front wheels are level with the dashboard... I don’t know if I fancy sitting right up ‘between’ the front wheels... Very vulnerable.

Last night I rewatched The World’s Fastest Indian... a wonderful film about the most brilliant ‘locost’ motorcyclist ever. At high speed his bike developed a wobble which he tried to cure making a ‘lead brick’ by melting down two dozen car batteries...
There is precedent... lol.

I shall keep this problem in mind.

Many thanks, Mangpong


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PostPosted: March 20, 2018, 10:42 pm 
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Joined: April 17, 2009, 1:28 am
Posts: 172
Location: San Tan Valley, Arizona
Definition of CDS (tube) -- Cold Drawn Seamless Tube (CDS) Produced into A519 from 1020/1026 carbon steel and various alloy grades, cold drawn seamless (CDS) tubes have
closer dimensional tolerances, better surface finish and higher mechanical properties than the corresponding grades of hot finished seamless tubes.


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PostPosted: March 21, 2018, 9:30 pm 
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Joined: March 15, 2018, 6:03 am
Posts: 118
waitj - many thanks... I was thinking of BDMS (bright drawn mild steel) from my childhood - I didn't think of 'seamless'.
How the world has changed while I've been asleep...
mangpong (scorpion)


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