suspension pickup points

[jdsoza]

Been doing a lot of reading and browsing the forums. There are numerous discussion on roll center, camber, caster, toe, scrub radius. I want to talk about pickup points and how and why they would affect being moved. Obviously Robispec built a subframe mod that raises the inboard pickup to improve roll center as well as build roll resistance into the geometry. That's a great start and a huge help, however I have other questions. 240SX guys cut and modify knuckles (believe me I know there's not enough meat on the knuckle to modify). Lets just say there is, what would be the benefit of having the lower control arm mounts lowered 25mm or more? Would this raise and correct roll center? Would this be more effective than raising at the subframe? I have thought about having my fabricator build a tubular front subframe but i have questions there. The subframe mounts only the lower arm and anti-dive arm, but the anti-dive arm is mounted to the chassis at one end. It would give an adjustable roll center, but it would leave our anti-dive stuck in an awkward position right? Now to talk about the upper portion of the knuckle. I know it's next to impossible to change the arm mounts on the upper arm. heres whatgixxer drew told me in a different thread "

I custom spindle is a good solution, but for FWD especially and probably AWD I would like to retain the tall spindle mode for reasons of steering axis geometry. Going to a racing style upright was considered and ruled out for this reason. Something that extended the ball joint mounts down would be a nice addition. I dumped all my knowledge on this stuff to Andrew at Frontline Fab but I dont know what ever came of it all." can somebody explain a little more on steering axis geometry? I hope this ins't too much, but it makes me want to learn more. Thats why im taking a geomtery math class right now in college......

 

[bjones18]

I had your understanding of suspension geometry 24 years ago in engineering school. Thinking about modifying cars to gain a better understanding. If you would like to learn from someone who has been paid as an expert in this field, and designed three suspensions, millions of them on the road, read on.

Modifying vehicles with your level of understanding will only confuse your learning.

Read, read, read (Carroll Smith & others). Then get your hands on software and run models then make changes. Look at the results. Join a Formula SAE team. Then read more. I have posted a few threads in this forum you may find interesting to read.

I'm not saying that I am the 'end-all' in knowledge. What I am saying... is that if you want to start understanding suspension geometry (much less compliances) by modifying hardware... good luck. You can change geometry & compliances in a computer model 100x in the same time it takes to make one simple mod on hardware. An automated DOE approach can run hundreds of models in a few hours.

What toolset do you chose?
a) hammer, torch, grinder, welder, socket set.
b) computer & software.

Learn the hard way... or learn from the mistakes and successes of others. Read.

 

[jtmcinder]

Looks like the OP has already done some reading.

Yes, lowering the mounting points on the knuckle (which is what Whiteline roll-center kits do, for example) will help. It's exactly the same, mathematically, as raising the inboard pick-up points. And, yes, to get the anti-dive back after lowering the car, you need to lower both sets of outboard mounting points (i.e., those on the knuckle). As long as you don't end up hitting the inside of your wheels, have at it.

 

[bjones18]

Agreed.

The issue is more along the lines of starting to modify hardpoints to change roll center height. With the complication of roll/bump steer changes (unless you know what you are doing) cutting metal is a hard and expensive way to learn.

There are soooo many bigger-bang for your effort - things to do - in my opinion it is not worth doing. I have driven cars with roll center changes (equivalent roll steer). It takes consistant driving technique to dicern the change. It is not a WOW difference (unless you do not correct roll steer).

If you want better handling, 1. tires 2. camber F/R (static & gains) 3. reduce & balance weight. 4. take a week long high performance driving class. 5. dampers & ride height.

Many beginners want to "redesign" their vehicle for better handling. Simple tuning changes are much more productive... until you have a clean-sheet vehicle design.

 

[jtmcinder]

Yeah, OK, because we're here talking about a 2G, it's not as critical, but if you ever try two Evo Xs, both lowered the same amount, one with OE ball-joints and one with Whitelines, you will start saying that front roll-centers can be a huge issue and can be felt by just about anyone. (For all I know, it's a huge issue on any car with struts, since that's what the math says, but I can only say it for sure for Evo Xs.)

 

[bjones18]

Yes,

1. a trained driver can tell minor differences, paid drivers are really good.
2. an untrained driver can tell a good set-up from a poor set up.
3. an untrained driver making geometry changes he/she does not fully understand is most likely experiencing a combination of multiple changes when driving their car with modified geometry.

People should feel free to change and modify their car. I'm just saying their are alot of great cars out there with less than optimal "designs" which have been set up (tuned) very well, with relatively little effort. (cut/weld knuckles & subframes is alot of work)

Strut vs SLA, in example... SLA is better than strut for "geometry." This does not mean you will find all winning cars with SLA suspensions. More-over the good driver who has invested in setting his car up well will be at the front of the pack.

 

[bjones18]

Back to the original post.

....roll center, camber, caster, toe, scrub radius.
...pickup points and how and why they would affect being moved. ....
cut and modify knuckles ....
benefit of having the lower control arm (outer)mounts lowered 25mm or more? ...raise...roll center? ...(vs) raising at the subframe?
... fabricator build a tubular front subframe ... anti-dive arm, ... an adjustable roll center,
...impossible to change the arm mounts on the upper arm.... custom spindle is a good solution... retain the tall spindle ... steering axis geometry. ... racing style upright
... extended the ball joint mounts down .... explain a little more on steering axis geometry?....
taking a geomtery math class right now in college......

I read two primary subjects in this note above:
1. learning more about geometry.
2. modifying a vehicle for undefined objectives (raise RC is geometry mod, not a performace objective)

As you might read (from my previous answers), time learning and modifying a vehicle would be better spent as: time learning and computer simulation. Once you understand your objective, then modify a vehicle to match the hardpoints from your model. Keep in mind, that millimeters are important here.

Further complication for a 2g is the fact the outer LCA pivots form a "double ball joint" or one end of a virtual kingpin axis, a kpa which changes as wheels are steered.

As for UCA pivots, This was a relatively easy cut/weld project to correct my static camber after lowering my car:
1. remove UCA bushings
2. push bushings out of loops
3. cut loops from studs
4. weld loops to move hardpoint outboard (and move up to reduce camber gain effect from lowering)
5. re assemble.

clearly, I needed to know how far outboard my hardpoints needed to move before cut, jig, and weld.