Moving LCA up, UCA stays put..

[DRJDVM's '69]

Okay I'm a real newbie on front suspension design. I've read alot about it but it still confuses the hell out of me.

So..... I have RMS front suspension

When we did the mockup to the front of may car, it sat WAY too high. The inner fenders were already custom done, and the design includes the UCA mounts. So in an attempt to not re-do a bunch of stuff, we notched the original frame and moved the k-frame up into the original frame (rather than bolting to the underside.....to get you oriented the two "ear" bolt to the underside of the original frame and the upper shock mount goes up the outside edge of the frame)

So essentially the entire k-frame all moved up as one unit, so the geomerty relative to each piece stayed the same. However the UCA mounts stayed in the original position ( that bolts to the mounts on the original frame).

The entire k-frame unit was moved up 1.5 inches so essentially we moved the distance from UCA mount to LCA mount quite a bit closer.

So....... in discussing it with the shop that is building part of my car, they feel very confident that this wont negatively impact the handling of my car. I'll lose some camber gain, but I shouldnt have bumpsteer issues etc etc... The car will be mostly street driven, but I dont want it to drive like crap no matter what

So what do you guys think ? I'm just looking for some expert second opinons....its making me nervous that I'll discover issues once the car is actually running.

[aosborn]

With that modification you will greatly increase camber gain and will likely have increased bump and roll steer. You have also drastically changed the position of the roll center.

Did you contact the manufacturer about the ride height issue?

Andy

[Hidro]

Did you contact the manufacturer about the ride height issue?

Andy

x2 sounds like something was done wrong somewhere.Why guess call the manufacturer.

[Bryce]

what is the angle of the UCA and LCA in relation to the ground at ride height?

another question, did you change wheels and tires at the same time as the suspension swap.

Lastly suspension can settle about one to two inches.

[79T/Aman]

you have also raised the engine by 1.5 " this raises the CG and may cause fit issues with hood, exhausttrans tunnel

[rjsjea]

Ned, you have seen my car's front suspension? I wanted to keep the UCA's and LCA's in the same orientation to each other as Bill designed them.

Is the problem that you already had the inners fenders done and now you wanna move up the UCA's mounts and it will interfere with them? Having your wheels and tires seems kinda mandatory at this point I would bet.

[DRJDVM's '69]

I think I posted this in the wrong forum... I meant to post it in the Advanced Tech and Perf section...or did it get moved?

Theres no manufacturer issues...... no defecrs or isntall issues....the stance to the car just wasnt right for what I wanted. Nothing was done wrong and its not a "settle issue" (the car isnt on springs etc...its on air ride). Hood and engine clearence issues are not a problem

It was a matter of getting the car to have the stance I wanted without re-doing a bunch of stuff where alot of time and $$ was already spent.

I think I'm going to copy this post over to the advanced tech forum and see if someone with advanced suspension knowledge will chime in

[DRJDVM's '69]

I'm not sure if this got moved or I screwed up or I didnt put the post where I had intended before...so I'm posting it again.... this was where I intended it to be. I really am looking for advanced discussion on the design of my suspension and how changing that design will alter the suspension performance. If it got moved I was curious on why.....

Okay I'm a real newbie on front suspension design. I've read alot about it but it still confuses the hell out of me.

So..... I have RMS front suspension

When we did the mockup to the front of may car, it sat WAY too high for my taste. The inner fenders were already custom done, and the design includes the UCA mounts. So in an attempt to not re-do a bunch of stuff, we notched the original frame and moved the k-frame up into the original frame (rather than bolting to the underside.....to get you oriented the two "ear" bolt to the underside of the original frame and the upper shock mount goes up the outside edge of the frame)

So essentially the entire k-frame all moved up as one unit, so the geomerty relative to each piece stayed the same. However the UCA mounts stayed in the original position ( that bolts to the mounts on the original frame).

The entire k-frame unit was moved up 1.5 inches so essentially we moved the distance from UCA mount to LCA mount quite a bit closer.

So....... in discussing it with the shop that is building part of my car, they feel very confident that this wont negatively impact the handling of my car. I'll change some camber gain, but I shouldnt have bumpsteer issues etc etc... The car will be mostly street driven, but I dont want it to drive like crap no matter what

So what do you guys think ? I'm just looking for some expert second opinons....its making me nervous that I'll discover issues once the car is actually running.

I'm looking for some real advanced tech knowledge to either reassure me or tell me I'm going to have big issues so I can do something now rather than later. The more detail the better..

[Bryce]

okay i think we are all trying to understand the overall problem and help you with the situation.

so if you raise all of the Kmember and suspension except for the UCA. then in theory you lowered the UCA. this is like the guldstrand (sp?) mod for camaros or the shelby drop for mustangs.

These mods increase the camber gain. or more negative camber.

DId your kit already address this issue/mod?

[derekf]

1.5" is a lot.

It sounds like you got the answer pretty well immediately with Andy's

With that modification you will greatly increase camber gain and will likely have increased bump and roll steer. You have also drastically changed the position of the roll center.

I'm not too sure what you're looking for beyond that. Were I in your shoes, I'd be looking for a method to recover from moving everything else by possibly raising the UCA mounts an equivalent amount, and verifying the other angles - your rack moved up as well, but what about the column angle? Will it be driveable? It surely won't be optimal... but whether or not you'll find it acceptable is really up to you. Does seem like you spent a lot of money on the k-member to try and re-engineer the angles on the fly, you might contact the manufacturer as well -- it's too late to un-notch the frame to fit up in there, but they may have useful input on the lowered UCA mounts. (and, of course, please keep in mind that I know very little about pretty much anything so don't take anything I say as gospel)

As for why this post was moved (if it was indeed moved) the first time - the Advanced forum is intended for theory, not so much practical application. The post has been left here this time, for now, in case someone has something to add but expect that in the next couple of days it'll be moved to the proper location in the Suspension forum. The expert suspension folks seem to read both anyway, so I'd be of a mind to assume that the lack of additional posting is mainly because Andy hit the nail on the head immediately.

[Damn True]

OP had the same question posted in both forums. I merged the two with the intention of moving the whole thing to the normal suspension forum once the issue was resolved.

[rjsjea]

Bill Reilly would have the answers....I would call him first, since he designed it.

[DRJDVM's '69]

Thank you for merging the threads..... I know how complex the geomtry of the front suspension can get. Thats why I was hoping for some of the hard core suspension guys that know all the theories and math etc to take a look... I figured this was a better place to get detailed analysis of the situation. I'm looking for real data to support how and why it will change things..

I did email Bill Reilly....as expected he didnt really have an answer for me...just he was concerned it could cause some issue but had no explanation

To symplify the issue........as was alluded to above...... think of it in simple terms.....keep everything the same, but lower the mount to the UCA....how will that impact roll center, camber gain, bumpsteer and all the other geomtry factors that effect handling

[Bryce]

get herb adams chassis engineering book. It will explain a lot. everything will be impacted by the UCA drop.

I just built my own front suspension for my falcon i have done all the calculations. to give you an idea i dropped my UCA 1.75" from stock location. this was the best for my ride and setup.

[DRJDVM's '69]

I have full plans to get a suspension book to learn alot more....I have alot to learn.

Right now at least, I'm dying to get some help so we can make some changes ASAP if needed.

Thanks for all you help

I did look at the Herb Adams book on Amazon....some loved it...some hated it...it was published in 1992..anything more recent you would recommend?

[Rhino]

Without seeing the geometry the way it was planned, or very accurate measurements of your current set up, there's really no way to give a good suggestion for what should be done.

As was stated earlier, by raising the K and all lower mount points without raising the UCA will most definately increase your camber gain. Is it too much? That can only be determined by measuring or knowing how much camber gain was built in to begin with. They typically say you want, at max, 3/4* of gain for every 1* of roll. Outside of this boundary can negatively impact handling.

I'll lose some camber gain, but I shouldnt have bumpsteer issues etc etc... The car will be mostly street driven, but I dont want it to drive like crap no matter what

In relation to this particular portion of your post, you state it will be mostly street driven. When it's not on the street, what else is it's intended purpose? If it's mostly street, and a little bit at the drag, you're likely to notice little difference. It's only near the edge that you really see the true personality of a suspension come out.

Bump steer characteristics are determined by your steering rack pivot points and their relation to your LCA/Steering arm. If none of these items have changed you will probably not experience significant bump steer issues, although the bump steer the stock height suspension has could be exaggerated. It will probably affect it in some manor.

You mention that the suspension is too high for your liking, but do you know what height it was actually designed for? How does that differ from the way your car sat before raising the K?
How much weight was on the front of the car? I'm assuming the engine was already installed? Do you happen to know the rate of the springs installed at the time of mock up?

[derekf]

Thats why I was hoping for some of the hard core suspension guys that know all the theories and math etc to take a look

...keep everything the same, but lower the mount to the UCA....how will that impact roll center, camber gain, bumpsteer and all the other geomtry factors that effect handling

If you're wanting math and true analysis done, you'll likely need to provide a lot of measurements; the only measurement we've got so far is 1.5 inches, and I'm not even sure that that's an exact measurement or a guesstimate.

Without measurements you're not going to get too much more than what you've got - namely, that yes, you've changed the camber curve a lot (if memory serves, the Guldstrand mod is only like .75 inch down but I am not sure since I've never done it).

[Norm Peterson]

Numerically, I'm a little more familiar with Shelby's (Mustang) version of this mod, and that has been done with anything from 1" UCA relocation up to I think 1.75".

But I'd be very wary about comparing the amounts of UCA pivot relocation from either Shelby or Guldstrand directly against a third suspension that we still don't know all that much about. Order-of-magnitude numbers for discussion is one thing, hard numbers for design quite another.

I would not be at all surprised if it was found that the RC moved up by 3" here.


Norm

[DRJDVM's '69]

Thanks guys.... I know without exact measurements etc no one can be sure on the magnitude of impact the change will make.

When I say the car will be 99% street driven, what I mean is that I will likely take it to an autocross or road coarse a couple of times over its lifespan....but not a ton of times. I'm not looking for a dead on handling track car.

My goal....have it handle alot better than stock (which shouldnt be hard...its a Mopar ...).......not be twitchy on the road or have the sensation that after every bump I hit that it wants to jerk me into the guardrail or oncoming traffic.. I want to be able to take it on
some curvely roads and not be afraid to push it. I dont want to struggle with the car while just driving it on the street

The ride height we set was with the motor etc etc....but once again I'm on air ride so I can tune that alot. We set the stance based on the look of the car and maintaining decent ground clearence...both at ride height and dropped stance. It was aimed at the stance and look. I'm willing to concede some performance to get the stance I want but dont want to make the handling a disaster after I do.

[David Pozzi]

That suspension is likely narrowed 4 to 5 inches over a stock vette, then you raised it, so it's now quite different than GM intended. I think the GM design is very conservative as far as camber gain, the upper A arms could stand to be lowered a bit without much trouble, but 1.5" is probably too much, especially for a street car.

I'd raise the upper A arm mounts 1.5" to match what you did to the lower.
David