Ncrca'S Vs. Rca'S

[Grand Master of Awesome]

After asking around a few forums and not really getting a good answer. I'm putting my car back together for track purposes and was looking at RCA's and NCRCA's.

 

Now i know majority of people will run a set up consisting of, Longer lower control arms (xt130, sigma etc) coilovers (~8kg springs), camber tops, lock spacer, ae86 PS arms and RCA's. Which gives you, more track, negative camber and reduced bump steer

So i currently have t3 camber tops, stock LCA's, t3 coilover kit on ae86 struts (8kg springs, trd yellows), ae86 PS arms, lock spacer and have been considering NCRCA's but the general feedback is not to run them because they are "crap". But they do the same thing the set up ive described above, track, negative camber and reduced bumpsteer.

 

Fair few car builds ive read from the USA all run NCRCAs and there doesnt seem to be a mention of lower control arms being changed.

 

So i was really just wondering whats the benefit of each set up? or a detailed answer as to why NCRCA's are considered to be bad for a suspension set up.

 

thanks in advance.

[ke70dave]

my understanding is that NRCA's are like a cheat way to get a bit of camber while getting the effects of the RCA.

 

have a google around on what an RCA does, there are some good explainations out there.

 

as for the "bad" points of NRCA's. in reality there probably isnt anything "bad" about them.

 

but for me personally, they just don't sit right. the idea of stepping out the connection between the strut and the LCA for the purpose of getting a bit of camber, is just silly.

 

either lengthening the LCA or using camber tops is the proper way of going about gaining camber. with the NRCA's you are introducing that step in the strut (down to the LCA) which creates further failure points.

 

I am a mechanical engineer, and when ever we design stuff at work, the first thing we ask outselves is "does it look right". and even if all the analysis proves that it will be fine, if it doesnt look right, or sit right, then a new design is found.

 

NRC's just don't look right!

 

actually for an academic excercise, if someone with some NRCA's could get me some dimensions (ie the offset of the holes) ill model it up in FEA anlaysis program, and show you why i don't like them.

 

edit: have just done some FEA analysis on an RCA and NRCA, i guessed the dimensions, but the results are fairly conclusive! ill put the images up when i get home.

Edited January 6, 2011 by ke70dave

[wagn]

do you even need more camber than you can get with your camber tops?

[greenmac80]

NCRCA's ruin your scrub radius

 

 

Edited January 6, 2011 by greenmac80

[KENut]

NCRCA's ruin your scrub radius

They do. Because the NRCA's turn with the wheel, they not only affect the camber while the wheels a pointing straight ahead, they completely change the dynamics as the steering angle is changed. Longer LCAs/camber tops do not have the same effect.

 

So yeah, generally not good.

[ke70dave]

photos of my FEA as shown:

 

for those not familiar with FEA it is a computational way of seeing how a body acts when forces are applied.

 

in my model i fixed one side (the steering arm side) and applied the load directly down on the other side (like where the strut would sit on the RCA/NRCA)

 

now the scale you see in the drawings is in MPa or megapascal, this is basically a measure of stress in the material. i chose a standard alluminium for the material which has a yeild strength of about 620MPa, which means if the stress gets above this value then the material will yeild, or deflect or "bend".

 

now i chose a force of 100,000N, which is equivilent to about 10,000kg or 10ton. i chose this fairly arbiarilly, but its sort of a worse case scenario, say your driving at 160km/h, you hit a giant pot hole or ripple strip. the impact force on this thing is going to be huge. but its difficult to get a real number, so 10ton was chosen.

 

so here is a quick sketch of the cross section of my NRCA, i guess the dimensions. this is more of a comparison between the NRCA and the RCA so i figured the dimensions werent that important:

 

 

now for the RCA i just changed that sketch a bit to make it smaller, and here are the stress results that i got:

 

 

and one from underneith (this is where it is fixed.

 

 

so the stress is around the 30MPa mark, with peak stresses hitting 50ish. in FEA you get peak stresses where the mesh stuffs up, so you sort of ignore them a bit (hard to explain)

 

this is a cut away of the RCA, you can see the stress is distributed nicely throughout the item, so the whole item is taking the load, which is a good thing.

 

 

now onto the NRCA...this is where things get interesting......

 

ill just jump straight into it:

 

the main problem with NRCA's is because of the bolt offsets they cause there to be what is called a "moment" applied to the item, which would put enormouse forces on the bolts and the NRCA itself. this is not a problem with the RCA because the forces are applied purely in the same plane as the strut (ie straight "down" through the RCA, not stepped like in the NRCA). this twisting can be seen in the following plot, the numbers on the right are mm of movement, the plot has been greatly exadurated.

 

 

 

now onto the stress plots:

 

 

 

in that second photo you can see that there is high stress concentrated around what would be the "outside" of the steering arm, because of the fact that there is now twisting being applied to the NRCA, the stersses are no longer uniform throughout the item.

 

not to mention how much higher the stresses are.

 

this is a cut away of the NRCA:

 

 

you can see how the stresses are concentrated along that edge, which again is from that rotation.

 

so thats why i don't like NRCAs!

 

keep in midn this is a fairly crude analysis, but it was more just to show why i don't think they are a good idea by giving some sort of evidence.

 

not to mention they do ruin your scrub radius as mentioned previously, which i think is a good enough reason not to use one at all......

Edited January 6, 2011 by ke70dave

[Grand Master of Awesome]

thanks everyone for the info, especially ke70dave. Decided on doing ke25 LCA's and rca's. undecided on 40mm or 50mm though? 50mm would be for super low cars, so probably just live with 40mm.

 

As for the more camber, I'm not running camber tops for more I'm running them so i can adjust it if needed and not be stuck with constant 2 degree.

[philbey]

Ahhh cosmosworks. Good times.

 

I woudnt worry about those stresses in the ncrca model it's a bit of a false positive because you've used a split face you'll get spikes at the phantom edge. Mind you I was impressed by the deflection value,seems pretty high.

 

One of the speedway boys posted a picture of his half inch steel plate version, run an analysis on that next.

 

Regardless of the fea, I reckon ncrcas are dodgy, especially given how cheap a set of camber tops are.

[ke70dave]

Ahhh cosmosworks. Good times.

 

I woudnt worry about those stresses in the ncrca model it's a bit of a false positive because you've used a split face you'll get spikes at the phantom edge. Mind you I was impressed by the deflection value,seems pretty high.

 

One of the speedway boys posted a picture of his half inch steel plate version, run an analysis on that next.

 

Regardless of the fea, I reckon ncrcas are dodgy, especially given how cheap a set of camber tops are.

 

bit of a cosmosworks/simulation fan? bit better than strand7 we had at uni.

 

yeah its pretty dodgy analysis thats for sure, but was just a quick copy and paste. like you say the actual numbers don't mean a whole lot, was more just a comparison, and yeah the rotation is interesting, thats probably the root of the "problem" with NRCA's.

 

yeah not sure i want to touch that speedway plate, i don't think 10T will be very kind to it.

[philbey]

Yeh I used ansys 11 a fair bit at my old job, using solid works for design, I ran a bunch of analyses in COSMOS to compare and they were all spot on.

 

Don't fiddle with it much these days. Done a couple of moonlighting jobs and that's it.

[Andy43]

We use a camber plate at the bottom for speedway.

The design is simlar to the sketch but we also move the strut forward to increase castor.

As we only turn left and spend more time turning than going straight, The plate works fine.

 

Stresses on the plate don't seem to be a issue, However the lower balljoint gets forced into a strange angle, If you make the plate any thicker than 14mm or push the strut out more than 40mm, you will reach the limit of the balljoints movment. (Can Supply pix if interested )

 

We hit the wall a few weeks back, this hit moved the wheel back some 60mm and also moved the subframe across some 30mm measured at the rear k frame bolts.

The items destroyed were Mag wheel, Lower control arm, strut radius rod, K frame, steering box, and subframe, The castor plate remained unbent.

We have used strut bends in the past and had them fail, We found the plate a much more reliable option.

We only run the plate on the right.

 

The strut plate does look dodgey, and stresses on the balljoint are a concern, I wont use them on my street car.

 

I would be interested if you could model the stresses on the plate and am happy to supply dimentions.

 

We are governed rules and can only use one of three options.

Adjustable tops.

Strut bends.

Camber plate.

 

For us the camber plate is the best option, I would like to run longer LCA's with coilovers but rules wont allow.

 

At the end of the day, I don't like the camber plate but its our best option for the rules.

 

 

 

 

[pozman]

thanks everyone for the info, especially ke70dave. Decided on doing ke25 LCA's and rca's. undecided on 40mm or 50mm though? 50mm would be for super low cars, so probably just live with 40mm.

 

As for the more camber, I'm not running camber tops for more I'm running them so i can adjust it if needed and not be stuck with constant 2 degree.

 

 

what car are the ke25 lca's going in? cos I'm pretty sure ive read they don't fit later corolla's, sigma lca's do fit tho with minor mods, and there around 325-330mm

Edited January 9, 2011 by pozman

[Grand Master of Awesome]

ke70, i know they fit because they've already been in it and then into my panelvan aswell. No mods needed.

[pozman]

that's alright then, the wonders of info on the net

[Grand Master of Awesome]

wait I'm an idiot haha had to drill new holes for sway bar. Longer than corona arms though so its win.