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Perhaps it's the Evo tie rods that alter the feel and mine is a a '95 car which may well have the 964 rack as it looks quite different to the later cars. So the RS does have a different rack? I think I confused the conversion of 964 uprights to 993. I bow to your superior knowledge.
EVo is sort of vague
From Porsche there are 3 different inner tie rod versions and 2 different out versions
They can be mixed and matched
for outers there is RS and standard, RS are only used w/ RS wheel carriers and standard w/ standard wheel carriers
for inners there is standard w/ rubber connector, RS w/ stiffer connector and RSR/cup w/ a mono-ball connector.
the inners are listed in order of reduced play, yes there is a difference in feel w/ the RSR/VCup versions having no slop and very precise action.
lastly there are several different aftermarket versions of the outers w/ o/s ends
When you lower a 993, the angle of the tie rods, which connect the steering rack to the wheels, changes. At normal ride the tie rods are roughly horizontal or parallel to flat ground. Imagine now that the car (and thus steering rack) is lowered. As the wheels+hubs+uprights (the big knuckle shaped piece of metal that the wheel and bearing and strut all hang onto) stay in the same position (their height is determined by the rim size and tyre profile) the tie rods are now lower on their inboard ends and are no longer parallel to the floor. You have changed the fundamental relationships of the front suspension. The tie rods no longer travel on the same arc as the suspension. You will have increaed the amount of bumpsteer the car will suffer when the front suspension travels up and down.
At sport height you can get away with an awful lot but at the lower RS height and with stiffer less compliant/forgiving spring and damper rates, the effects can be quite intrusive and unwanted and make for a car that is very unsettled on bumpy roads, a car that darts and weaves about quite violently on rough roads or across crowns or compressions.
When Porsche built the 993 RS with RS geometry and RS suspension they deemed it necessary to correct all of this by fitting the car with different uprights. Quite simply, the mounting point for the outer tie rod ball joint is higher as denoted by the blue arrows in the picture. This means that for RS ride height, the tie rod's position is corrected giving the correct bumpsteer profile.
I want to make it absolutely clear though that plenty of people have lowered their 993's and not personally discovered anything detrimental. This could be for a number of reasons. Maybe they have a geometry setup, or softer suspension which masks the bumpsteer. Maybe they don't listen to what the car does very much or have low expectations from it or maybe their idea of rough roads is very different to yours. If it aint broke then don't fix it. I can say that my car needs the upgrade and I am a fussy beggar at the best of times.
Take a look at the picture below comparing RS to standard uprights. Not only is the mount point for the tie rod different (and the fact that the tie rod mounts UP into the blue arrow end on the RS rather than down onto the std. mount) but the steering arm, or the distance from the tie rod mount to the wishbone ball joint mounting point (or the axis where the upright turns) is shorter. This means for any given movement of the steering wheel, the RS upright will move MORE. That's right, with RS uprights and keeping your normal 993 steering rack, your steering will be quicker and it will feel quicker. Remember that the rack fitted to the actual 993rs was a 964 rack which is slower lock to lock so that brings the total tuns lock to lock (the speed of the rack) back to around what a normal 993 is. I think that the cup cars ran with the RS uprights but the 993 rack which makes for the quickest overall steering. A number of 993 users across the pond report favouring this setup as well as it's very sensitive & responsive.
One last thing about the uprights. There seems to be a lot of confusion on the internet. There are no GT2 or EVO or GT2EVO uprights. Maybe someone somewhere once referred to the uprights + tie rods as the GT2EVO kit and it's stuck. Whatever the case, you either get normal uprights or RS uprights. RS uprights were fitted to the RS M002, RS M003 (clubsport), the 993 GT2 and the race cars. There is an 'EVO' tie rod which is a motorsport part ... let's look at that a little closer.
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Tie rods
This can get slightly confusing and is another source of debate on the internet. Let's start by saying that the 993 tie rod comes in 2 parts, the inner tie rod that attaches onto the steering rack, and the outer tie rod that attaches onto the upright (steering knuckle or hub carrier as it is sometimes referred to). These 2 join together with a lock nut and its with this connection that one adjusts the car's static toe angle or 'tracking'.
There are 3 types of tie rods for the 993, the standard part, the RS tie rod and then there is something called the GT2 EVO tie rod. As one goes from standard to GT2 EVO the connection becomes more solid with less compliance. Also, both the RS and EVO tie rods have a different shaped outer portion so that they fit onto the different RS uprights.
Nice of you to quote info that was mostly garnered here on Rennlist
Porsche made 3 different tire rods sets, each has an outer and inner piece which can be mixed
993.347.031.80 GT2/RSR tie rod(these are nla from Porsche but versions are available from Tarrett)
993.347.031.03 993 tie rod
993.347.031.81 993RS tie rod
Top stock 993 tie rod, bottom 993RSR tierod, the arrow points to the geometry difference between them, at the left 2 different inner pieces are seen, the standard has a rubber connection, the RSR has a mon-ball connection
2 more tie rods, top standard 993, bottom 993RS, the differences are in the outer bend as noted above and the Rs has a stiffer rubber connection but still looks the same extrernally
993RS tierod
RS tierod and RS wheel carrier
the connection to the wheel carrier is different between standard and RS wheel carriers, RS & RSR mounts from below, standard mounts from above
Kudos to Elephant Racing for offering a kit that helps bump steer. This seems appropriate for those who are not too low. What that height is seems to be somewhat debatable.
Bill said it best when he explained there is no "cliff" you fall off and suddenly need to address bump steer. I've owned 2 different 993's at similar ride heights for 15+ years I never considered it a "problem". During spirited drives on Hwy 1 when the right wheel had to deal with a lot of changing terrain I did notice the need to correct steering to keep me in the lane. On the track I can't say I noticed anything negative either.
About 6 months ago I upgraded the wheel carriers to RS along with outer RS tie rods. Everything else in the front remained the same. I re-aligned to the same specs as well. So this was a good comparison of just the carriers. I was shocked to how much more responsive the front felt. Digging through corners the front just seemed to understeer less.
What drove me to upgrade the carriers without any bumpsteer symptoms? It all started with one of our BMW race cars. We were fighting understeer and started playing with spring rates, bumpsteer tie rods and all the other usual stuff. It was not until the Tim at TFB Racing pointed out the lower control arm was beyond horizontal. It was angled up quite a bit. We then used an extended ball joint to get the control arm angle back to a good angle. The results were surprising to say the least. He went on to explain strut based suspension get all weird and just don't work well when the control arm angle gets past 90 degrees at rest. At that angle you get camber gain during initial movement as opposed to camber loss when angles get severe.
I just so happened to have my 993 at his shop one day and asked for his opinion on my control arm angle. He said it was not good. That's what drove me to the RS bumpsteeer kit.
Cliff notes.....let proper front suspension geometry dictate when you need to address bump steer kit. Not when you notice it. If the angles are all wrong you will notice the after difference.
While the ELephant Racing tie rod is a good product I feel it misses the mark by not addressing the control arm angles.
It's for the front trailer tie downs, likely provided by Autometrics when they used to include them for folks who purchased their tie down kit. No longer available from them when you buy the rear tie downs but easily procured at Home Depot.
Bill, MarinS4, thanks for weighing in, that's the information I was after when I started the thread. The rod angles were definitely off when I started the project, I will report after installation, their effectivity.
Problem is, it's not an isolated modification. I am also doing bushings and ball joints. Thanks again.
Now if I can just find a good alignment shop in metro Detroit area?!
I guess I am one of the dissenting drivers who never saw a huge benefit in my RS uprights. When I first started DEing my former 993, I bought the RS uprights and tie rods because they were part of the de riguer mods that one needed to have for a car that was equipped with Moton Club Sports and as low as mine was. As I started to race the car, I eventually learned that these uprights were illegal in a PCA stock class. Oops, not cool. After the first track event upon removal, I expected an ill-handling vehicle, especially at Summit Point's notoriously bumpy (at the time) and fast track.
Strangely, the car felt the same to me with no perceived loss of confidence, and my lap times remained the same. At the time, I was usually at the pointy end of the grid, so I wasn't just duffing around doing fast parade laps. In terms of physics, I completely understand the need for the RS uprights, but, in reality, they just did not make much of a difference for me.
I equate this experience to the time when I had an issue with my Fikse wheels and had to borrow a set of wheels from Chaos aka Gregg L. He lent me a set of 18", mesh sport classics for a race at the Glen, and, boy, were they boat anchors. I expected a drop in my lap times, but, nope, my lap times remained the same. This made no sense from a physics perspective but that was my experience. YMMV.
You keep posting this, it is wrong
cabrio993 went to the trouble of actually measuring the differences between the standard and Rs wheel carriers.
The measuremnts are wrong as they are made in the wrong place. As your illustrations show the track bar ends are mounted in a different position which results if a different effective steering arm length. I stand by my comment.
I guess I am one of the dissenting drivers who never saw a huge benefit in my RS uprights. When I first started DEing my former 993, I bought the RS uprights and tie rods because they were part of the de riguer mods that one needed to have for a car that was equipped with Moton Club Sports and as low as mine was. As I started to race the car, I eventually learned that these uprights were illegal in a PCA stock class. Oops, not cool. After the first track event upon removal, I expected an ill-handling vehicle, especially at Summit Point's notoriously bumpy (at the time) and fast track.
Strangely, the car felt the same to me with no perceived loss of confidence, and my lap times remained the same. At the time, I was usually at the pointy end of the grid, so I wasn't just duffing around doing fast parade laps. In terms of physics, I completely understand the need for the RS uprights, but, in reality, they just did not make much of a difference for me.
I equate this experience to the time when I had an issue with my Fikse wheels and had to borrow a set of wheels from Chaos aka Gregg L. He lent me a set of 18", mesh sport classics for a race at the Glen, and, boy, were they boat anchors. I expected a drop in my lap times, but, nope, my lap times remained the same. This made no sense from a physics perspective but that was my experience. YMMV.
That's because a 993 has a fairly benign bump curve
some basics, the best toe curve wrt to bump steer is a vertical line, the worst is horizontal. The compressed side is the most important to handling the droop side much less so. The stiffer the suspension the less travel the less compression and the less droop(ie the less bump steer just from stiffness) Most track cars will have a very stiff suspension usually in the 600 to 1000lb/in range,600 is over 5x what even RoW M030 is.
This shows what is happening, red line is the toe change correlated to suspension travel, when the car is lowered w/ no other changes the x axis is moved up by the same amount as the car is lowered from ~154mm to RS ~124mm the slope of the toe curve hardly changes at all, but what the RS wheel carriers do is move the x axis to the blue dashed line for the same height. The is a better place to be as the curve is more nearly vertical, with further compression the toe change is similar to before, but the droop is much improved.
You keep posting this, it is wrong
cabrio993 went to the trouble of actually measuring the differences between the standard and Rs wheel carriers.
The measuremnts are wrong as they are made in the wrong place. As your illustrations show the track bar ends are mounted in a different position which results if a different effective steering arm length. I stand by my comment.
Although the steering arm is longer (which would give the appearance of being quicker), the (standard) tie rod, needs that extra room to mount from above. Where as the RS upright (with the perceived shorter) steering arm, actually isn't much different, since the RS tie rod mounts from underneath. Changes in steering ratio are negligible at best. If someone has factual measurements, that can prove otherwise, I am not sure if there's much (if any) difference. Aloha
Although the steering arm is longer (which would give the appearance of being quicker), the (standard) tie rod, needs that extra room to mount from above. Where as the RS upright (with the perceived shorter) steering arm, actually isn't much different, since the RS tie rod mounts from underneath. Changes in steering ratio are negligible at best. If someone has factual measurements, that can prove otherwise, I am not sure if there's much (if any) difference. Aloha
01-27-2017, 04:18 PM
