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PostPosted: November 13, 2018, 6:39 pm 
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I'm almost to the next phase in my build (plotting the rear suspension), and I need to fab in my rear upper and lower control arms (uprights are sourced from an NA Miata)

I was able to find enough info to get my Miata front suspension plotted, but I'm wondering about how to do the rear?

I believe it's best to set the lower control arms to parallel to the ground, yes? I don't know though what to do with the uppers.. do they angle 'downhill' toward the differential, or to they run parallel to the LCAs?

Also, what's the ratio in arm size upper to lower?

--Finally, in my particular build, my lower arms are only 6.5" long (this car is tiny), so I'm thinking my upper control arms will end up being quite short (as the uppers on the front suspension are)... Because all of my control arms are so short, there will only be a few inches of travel (maybe 3 or 4) once it's all done...

As always- thanks for the help :cheers:

--ccrunner

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1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
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1959 Berkeley SE492 Restomod...
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"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com


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PostPosted: November 13, 2018, 8:03 pm 
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ccrunner wrote:
I believe it's best to set the lower control arms to parallel to the ground, yes? I don't know though what to do with the uppers.. do they angle 'downhill' toward the differential, or to they run parallel to the LCAs?


Setting the lower arm parallel to the ground is the easiest thing to do as it helps to minimize the track width change through the range of travel but it isn't necessary. A line through the axis of each arm will meet at a point and a line between that point and the tire contact patch determines the length of the suspension's instant center (and the kinematic roll center is where this line for the left suspension crosses the same line for the right suspension.) The shorter this distance, the greater camber gain you will have. Typically you want less camber gain on the front (longer instant center) since the outer tire will gain camber via caster as it steers.

How much camber gain you need is related to chassis roll stiffness. If you have CAD, you can draw a suspension and roll the chassis 2 or 3 degrees and see what the camber does. If you get something that doesn't go positive (or very positive) and a roll center that doesn't do anything crazy, you are probably in the ballpark.

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PostPosted: November 13, 2018, 8:15 pm 
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Given that the arms are going to be short, one consideration you'll need to add to the rear suspension evaluation is the amount of horizontal travel that's available in the half-shafts. With such a small radius arc, two inches of vertical travel will result in 3/8" shortening of the cv joint, if the upper was also 6" long, but since the upper will be shorter, the travel may be about 1/2". Take the location of the CV joint within it's travel into consideration when deciding where the components are going to be located.


Last edited by seven13bt on November 13, 2018, 8:17 pm, edited 1 time in total.

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PostPosted: November 13, 2018, 8:17 pm 
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To add to what Andrew has stated, also look into toe changes through bump and droop. Toe out = bad; toe in = good/ok; no toe change = best.


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PostPosted: November 13, 2018, 9:44 pm 
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The upper arms should angle downward towards the diff, just like the front. I would say most of the same rules apply to the rear as the front. The reason for this is so you get negative camber gain on the outside in a corner.


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PostPosted: November 14, 2018, 9:45 am 
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In addition to the above comments, you should be looking at how much camber gain per degree of roll. A low profile/wide tire will tolerate less, then a narrow high profile tire. The best compromise is about .8 of a degree per degree of roll within the typical 2 to 3* of roll. With shorter control arms you usually end up with a much greater camber gain range, which does not help because you do not want a lot of static camber at ride height on the rear wheels. DaveW


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PostPosted: November 14, 2018, 12:20 pm 
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Thanks for the feedback guys... here's where I am so far.. (I don't have CAD, so I'm designing by generalities and space limitations at this point)

Rear LCAs (and axles) are parallel to the ground at ride height; LCAs are 6.5" long.. (I'm married to this short length as demanded by packaging)...

Tires are not very wide (6" at contact patch; there's really no room to go bigger, and I won't flare the car, so 6" is it; 155-70-13).. car will weigh about 850/900lbs.. suspension will be on the stiff side (this is more go-kart than Cadillac :lol: ).. total suspension travel will be dictated by CV range and/or body clearances (likely 3-4" max; 5" would surprise me)..

---------------------------------------

*** What I'm gleaning from the comments here is the rear UCAs need a 'slight' downhill-toward-the-pumpkin rake; 5-ish degrees?... -- (for what it's worth, my front UCAs are at 10 degrees, but I can of course alter that if needed)... And also, because my travel arc is so tiny, maybe I build the rear UCAs to be only slightly shorter than the rear lowers? Thought being not to introduce too much camber at bump.. (again, for what it's worth, my front UCAs are 2/3 the length of my front LCAs)

**This car's strength will be it's power-to-weight (maybe 7.5-ish:1) and it's overall lightness (sub 1K).. I'm not designing it to be a competitive race car, but instead a fun street legal play car (that won't embarrass me with poor handling on it's occasional track/autocross day).. I wish I was better versed in suspension theory, but I get lost quickly in all of the variables.. I appreciate your help as I try to design around all of the compromises..

--ccrunner

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1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com


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PostPosted: November 14, 2018, 1:53 pm 
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I think that is a substantial compromise to make. Consider raising the car a couple inches. This will allow a much longer lca to clear the diff that will still minimize the size of the tunnel eating pax space. It will also help a lot with the front crossmembers while still allowing the hood to clear the engine.

Update: Idea from your last post for reverse clearance.


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PostPosted: November 15, 2018, 10:01 am 
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I would go one more step then MV8. Mount the control arm pivot points under and above the diff. Shot for as close to the center line as you can get. This would allow you to pick up another 2 or 3" in control arm length. Cut away the floor pan, then add structure to carry the suspension loads. A 6" control arm is going to limit travel and it's going to ride like a go-cart.
DaveW


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PostPosted: November 15, 2018, 11:28 am 
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I hear ya guys.. I'm going to first get this starter/reverse set up in place and make sure it works (If I need to end up scrapping it, it would buy back some much needed LCA space).. even if the reverse stays, I'll do a rework of the LCA inner pivots and get them closer to center to bump up their length a bit..

Thanks for the help- this isn't something I want to do twice :lol:

--ccrunner

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1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com


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PostPosted: November 16, 2018, 12:17 pm 
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Rear suspension option No. 741 :mrgreen:
Look at a Semi-trailing arm layout. You already have the sides structure built up with 1"sq tubing. Continue that type design and make two smaller "U" box sections on each side of the drive shaft. You could package the pivot points on an angled vertical wall of the structure and just set your seat on the raised platform. That would give you more travel and better scrub and camber then a 6" control arm would. It would only require a stronger structure under the seats to carry the trailing arm attachment loads.
You would still need CV joints because of the narrow track.
DaveW


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