STX Ride Height

[Icecreamtruk]

Originally Posted by ApexEight

Is higher spring rate in the back the way to go? I always though stiff front soft rear is the way to go oan RWD cars m

It depends greatly on the car. But basically, the softer should have more grip than the harder one. So if you go harder on the front you would be promoting understeer, while harder on the back would be promoting oversteer. You also have to keep in mind that the rear has a motion ratio of around 0.7 while the front is close to 1. Which means for example, that 7/7 even rates end up being around 7/5 when converted to wheel rates. Understeer is easier to control and easier to be fast on than oversteer, usually safer as well, which is why people prefer it most of the time.

Edit: before people start jumping on details, this is a gross over simplification of how things work for the sake of explaining, I know there is more involved (sway bars, geometrie, camber, toe, tire, track width, and more).

[smg1138]

Originally Posted by ApexEight

Is higher spring rate in the back the way to go? I always though stiff front soft rear is the way to go oan RWD cars m

I think it's mostly a matter of personal preference and driving style. Generally speaking though, even spring rates or stiffer fronts seem to be more preferred in Autocross applications due to the fast transitions.

[strat61caster]

Originally Posted by ApexEight

Is higher spring rate in the back the way to go? I always though stiff front soft rear is the way to go oan RWD cars m

Most view suspension tuning as a black art and the vast majority confine themselves to buying off the shelf proven setups.

I think over time a stiffer rear rate (not spring rate, wheel rate) will emerge as successful on this car once people work it out and make their setups public, but like oinojo mentions it's not just springs+dampers=win, you need sway bars, alignment, ride heights, tires, dampers, driving style will all come into play to make a setup work, and very few people are able to hand you a good starting point right now.

I'm starting to think even spring rates have become so common because once you go stiffer than street rates you need to fully understand the sway bars, tires, geometry, dynamics etc. and potentially tweak all of them while the 'soft rear, stock-ish sways, any tire works ok' is much more appealing to move a few thousand coilover kits off the shelves.

But I'm just armchair quarterbacking until I do the math, I've got a few texts on the way to help me out with that (waiting until I get through the lighter stuff before I order the real textbooks). Just want to counterpoint since you got 2 people saying that the softer rear is good.

[renfield90]

I will say I run a stiffer rear spring than in the front, and my setup started at 400 lbs/in front and 500 lbs/in rear. Motion ratio is one part of the reason I selected those, but even at those rates (if my math is right) I still ended up with a faster natural frequency in the rear. The short answer is you get excessive chassis pitch or roll with equal or higher front rates; by making the rear slightly faster, when you go over a bump the front wheel will compress and rebound first followed by the rear, and the slightly faster rate allows the rear to catch up and finish its rebound motion the same time the front is. You can run a higher NF in the front and still overcome this with good shock tuning, but the higher rear NF will naturally make the car better over bumps.

I will admit I followed the rule of thumb as fiat without totally understanding it, but I've seen it in more than one place and I personally like the car better this way (I have tried a square setup).

Here is a guy from an FSAE forum. http://www.fsae.com/forums/archive/i...p/t-5177.html?

The out of phase motion between front and rear vertical motion, caused by the time delay
between when the front wheel and rear wheel hit the bump, is accentuated by the frequency
difference. A result of the phase difference is pitching of the body. To reduce the pitch
induced by hitting a bump, the rear needs to have a higher natural frequency to “catch up”
with the front. This notion is called producing a “flat ride”, meaning that the induced body pitch from road bumps is minimized.....

For a given wheelbase and speed, a frequency split front to rear can be calculated to minimize
pitching of the body due to road bumps. A common split is 10 – 20% front to rear.
The above theory was originally developed for passenger cars, where comfort takes priority
over performance, which leads to low damping ratios, and minimum pitching over bumps.
Race cars in general run higher damping ratios, and have a much smaller concern for comfort,
leading to some race cars using higher front ride frequencies. The higher damping ratios will
reduce the amount of oscillation resultant from road bumps, in return reducing the need for a
flat ride. Damping ratios will be explained in the next tech tip in detail. A higher front ride
frequency in a race car allows faster transient response at corner entry, less ride height
variation on the front (the aerodynamics are usually more pitch sensitive on the front of the
car) and allows for better rear wheel traction (for rear wheel drive cars) on corner exit. The
ride frequency split should be chosen based on which is more important on the car you are
racing, the track surface, the speed, pitch sensitivity, etc.......

Additionally, Dennis Grant at Autocross to Win suggested the same thing. You're not necessarily wrong if you don't follow this, and certainly plenty of people are doing extremely well on 400 lbs/in square, but it can be help your shock tuning to understand the dynamic your spring selection creates.

[renfield90]

Originally Posted by strat61caster

Most view suspension tuning as a black art and the vast majority confine themselves to buying off the shelf proven setups.

Too true...and that was another motivation for me to run different f/r rates. I don't have the talent to beat people in a spec car, the only way I'll win is by doing something different and better than them.

Also part of the reason I left Street class.

[oinojo]

My co-driver daily drives his car. So I set it up as such. Also setting the car up for flat ride helps with the knarly bumps even though we are using Triple Adjustables. Never had to worry about bumps. Luckily the aftermarket supports a lot of different swaybars so I don't have to heavily rely on spring rates.

[Silver Ignition]

I wanna go slightly off tangent but still on topic here...


Trying to fix a situation; 2013 FR-S STX car KWv3 with off the shelf springs


With no preload the car sits far too low and I feel roll center comes into play. So I adjusted preload to get the car at what I feel is a good ride height but now it rides like shit (duh.)


So my question is; how do I raise the ride height without crushing my springs with preload?

[renfield90]

Originally Posted by Silver Ignition

I wanna go slightly off tangent but still on topic here...


Trying to fix a situation; 2013 FR-S STX car KWv3 with off the shelf springs


With no preload the car sits far too low and I feel roll center comes into play. So I adjusted preload to get the car at what I feel is a good ride height but now it rides like shit (duh.)


So my question is; how do I raise the ride height without crushing my springs with preload?

Pics? Did your kit come with helper springs? You should be able to achieve reasonable ride heights without totally overdoing the spring preload.

[strat61caster]

Originally Posted by Silver Ignition

I wanna go slightly off tangent but still on topic here...


Trying to fix a situation; 2013 FR-S STX car KWv3 with off the shelf springs


With no preload the car sits far too low and I feel roll center comes into play. So I adjusted preload to get the car at what I feel is a good ride height but now it rides like shit (duh.)


So my question is; how do I raise the ride height without crushing my springs with preload?

I'm actually dealing with this on a set of KWs for another car, it's actually coilbinding now. We had to make a spacer between the helper and main and compress the helper all the way to get enough ride height so the tires wouldn't rub. Removing the helpers wasn't an option as there was not enough threads on the strut body for just the main spring. I believe we will be swapping to a stiffer spring from hyperco or eibach that will be longer and have better travel on top of the extra stiffness be compared to the spring kw ships with their kit. Hopefully we can take the spacer out when we do that.

Totally different platform so no specifics that can carry over but we just made the discovery a week or two ago. Typically companies will ship a decent spring to begin with but I haven't been too impressed with KWs off the shelf hardware.

It's similar to the weight of an 86 and the spring is stiffer than the one I ran on my coilovers (7k to 6k) so I'm inclined to believe the spring is of subpar quality, it's obviously not long enough nor does it have enough travel to do what it should.



2020 Edit: KW spring was trash on that chassis, swapped to a ~400# Eibach Race Spring, got the ride height we needed, ride quality improved dramatically with the stiffer spring that wasn't coilbinding, surprise surprise. Looks like KW V3's come with a 6k spring up front, you shouldn't have to preload that spring much at all to get a decent ride height imho since that's what I have on my car we actually have the spring loose at full droop right now.

[smg1138]

Forgive me for reviving my old thread from the dead, but all this self-isolating has got me super bored. Just curious what kind of ride heights are you guys running this season? That is if the season ever gets started

[e1_griego]

I got an event in and a school (and a track day!) before the lockdown started.

I feel like my car is pretty tall compared to most, but it seems to work alright and I'm still rubbing through the fender liners.

[smg1138]

@e1_griego That looks pretty close to what I'm running. I'm measuring about 13.5" from hub center to fender right now. Feel like I'm always trying to find the perfect balance between daily driving and racing. What suspension setup do you have? I've been running RCE T2's with 400/400 springs since 2016.

[e1_griego]

MCS 2WR, 400/400.

I've had it lower in the past but I mostly run in Packwood which isn't particularly smooth.

[Breadman]

Originally Posted by smg1138

@e1_griego That looks pretty close to what I'm running. I'm measuring about 13.5" from hub center to fender right now. Feel like I'm always trying to find the perfect balance between daily driving and racing. What suspension setup do you have? I've been running RCE T2's with 400/400 springs since 2016.

im about 1" lower maybe a hair lower and i already cant get in my friends driveway. i dont know how anyone can daily drive while running lower