Yes, mostly. The bump stops are a very cleverly-designed supplemental spring / bottoming protector that affects vehicle dynamics on a vast majority of vehicles (especially performance cars) almost as soon as the suspension begins to move. The front bump stops are designed to engage sooner than the rear, largely to create 'stable understeer' which more drivers are comfortable with. Many people think of bump stops only being - well, for bumps! The name belies their actual function in a modern suspension. They start to engage very early in the travel, and in some rare cases, the front suspension is literally RESTING on the bump stops (witness the 94-97 Miata R-package, FD Mazda RX-7, and even 1st gen Acura NSX if my memory serves).

Many performance or sports cars only have a few inches of total front shaft travel (best case, assuming non-lowered) and most front bump stops are around 2 - 2.5 inches long. So a 2,5 inch bump stop on a 3-4 inch travel shaft means about 1-1.5 inch best case before the bump stop starts engaging. Usually the situation is under 1 inch before engagement. For example (Porsche 996 analysis back in 2002):Clip from Panorama article, PCA, April 2002, on close proximity of front 996 suspension to bump stop contact. 6mm for RoW, 18mm for USA versions.

The RoW Sport had 6mm of travel before front bump stop contact and the USA suspensions had 18mm. In its initial engagement, a modern MCU bump stop is fairly linear so the rate increase is quite transparent which is why many people don't think it's happening. Then, after it compresses 40% or more of the initial length it'll build much more non-linearly. The exact rate depends on the material density and bump stop profile. The more conical, or the more ridges and segments a bump stop has, the more likely it'll be linear and gradual in its initial spring rate.

Most Porsche bump stops have some degree of this progressive behavior, which is good, although like most OEMs I've investigated, they appear to be a lower-quality (more temperature-sensitive) NDI material which is also lower cost. It will still work, but also tend to degrade faster than a TODI bump stop. You can improve ride quality and reduce understeer by substituting a shorter, softer front bump stop in particular. You could also substitute a different rear stop depending on handling characteristic you might want to emphasize - a firmer rear stop for more oversteer. Tuning suspensions for SCCA Street class autocross has forced me to dive deeper into bump stops than I ever imagined!

One point to keep in mind is that the rebound vs. compression bias of the damper will also affect how much the bump stop is used, and how likely the suspension is to continue pulling down into the bump stops. This behavior is called 'jacking down' or 'packing down' and is very common on most performance cars. It's ubiquitous, but not ideal at the levels I see in many factory and some aftermarket dampers. Track-side data and SOTP assessment confirm that 'rally style' tuning (more balanced compression vs. rebound forces, combined with Flat Ride ride frequency tuning) will keep the suspension more level through repeated rough sections.

If you're dealing with potholes, the last thing you want is a rebound-biased setup, constantly trying to pull you down into every single bump. The damper's jacking-down behavior plus the bump stop stiffness work together to create a variable spring rate early in the travel up to the limit of travel.

A few bump stop characteristic curves: shorter 36 and 46mm Speedthane (TODI) bump stops vs. a factory Miata rubber bump stop (note the abrupt spring rate ramp). Also the Speedthane 58mm red, white, and blue (soft, medium, and hard), and Speedthane 76mm red, white, and blue stops.

 

Great insight. Thanks for taking the time to explain this, as well as to make it easy to understand.

 

Hi Shaikh, it’s nice to see you around these parts. I’m a big fan of your work after reading your posts on Miata forums. I hope you’re doing well!

 

My pleasure, Wayne, and thanks for engaging!

 

Greetings, and thank you!

Wow, yes it's been a while since I've posted on Miataforum - you had one back then? It was a great place to start my suspension explorations and Miatas will always near and dear to me. Regretfully just sold my '99 Sport but his new owner will drive him more than I was. I bought an '08 Kawasaki Versys 650 instead I'm more into tuning BMWs and Porsches these days; the driving dynamics are way more fun and having torque is very nice!

I realized after buying and experimenting on my E46 BMW sedan that MacPherson strut suspensions require a lot more attention to detail to dial in well. The high motion ratios and prevalence of large damper shafts (or simply vendors using a LOT of nitrogen charge pressure) create problems with high gas force / rod force. I've been wanting a P-car for ages but until that time I live vicariously through my customers. I'll be posting a lot more technical details, dyno graphs, and analyses of the various Porsches I've worked on over the years and current projects as well. I've wanted to analyze and improve upon a GT3 so perhaps that'll happen via Rennlist as well! I'm also looking forward to create some free Ride Harmonizer spreadsheets like I did for the Miata community. I hope to call this place home for a while, learning and contributing.

 

No, I’ve never owned a Miata. The Google rabbit hole brought me to your posts and it was a revelation. I also found a spreadsheet by you that detailed the spring rates and ride frequencies of the 996 and 997, and it was very helpful too.

I urge you to get a Porsche. I think you would love owning one, especially if it is a GT3.

 

Your last statement pulled a hearty laugh from me! I know I would love owning a GT3, although being able to wind out a lower-powered motor yet torquey motor is also a pleasure. None of these cars can really be driven properly except in a competition environment. Doesn't mean I don't use on- and off-ramps for (ahem) 'vehicle suspension testing, Officer!'

So my sincere recommendation is to at least borrow or drive a Miata (if you haven't). They are quite special. I'm glad you did discover the posts and also the spreadsheet - thanks for mentioning it. I initially saw the data on Rennlist, decided to calculate frequencies and then made a video about it:

The Porsche 911 GT3 uses Flat Ride...

However when you start to exercise the suspension more and look at the combined spring rates / ride frequencies of "primary springs" PLUS "secondary springs (i.e. bump stops) & damping behavior", the situation with Flat Ride gets more complicated! I'll go into detail more in a separate thread (have hijacked this one a bit) but I've found that a car can start with Flat Ride yet due to the interaction of damper rebound bias with front bump stop, the effective ride frequencies will ALWAYS switch into a higher front vs. rear, causing pitch. The rear effective spring rear will also increase in a dynamic situation, but due to less rebound bias in the rear and a softer rear bump stop (plus more travel before the stop engages), the increase in rear frequency is LESS than the increase in front.

Hence, what may begin as 1.8 Hz front, 2.0 Hz rear (Flat Ride) becomes say 2.3 Hz front, 2.2 Hz rear (pitch). This forward shift in ride frequencies also increases steady-state understeer and causes more unpredictability at the limit, because you have a constantly-changing spring rate. I'm sure this kind of behavior is one reason many racers prefer to NOT use bump stops, but in my experience and opinion, that is throwing the baby out with the bath water. Bump stops are amazing little devices and useful handling aids provided you 'harmonize' the springs, dampers, sta-bars, ride heights, alignment, etc.

This phenomenon of 'Flat Ride turning into pitch when driven hard or on a rough road' is what practically every factory sport suspension does, and a good many aftermarket suspensions as well. I only realized this a few years ago and have yet to make any videos about it.

 

Excellent stuff Shaikh!

Looking forward to a new thread . . . and please don't be shy on the 997 Turbo forum

Dean

 

Back to spring sack ...

The bump stops got me thinking. Could looking at the distance to the bump stop be a more accurate measure of spring sack?

Unfortunately, you need the wheel off to see the bump stop. That means the spring is fully extended. You would need something to lower the car onto to load the spring and get a proper measurement here.

But I got a few pictures anyway ...

It looks like on my car (2010 C4S PDK, 162K miles on original suspension) I've got 3.25" distance from the bump stop to the shock body. Of course, this would reduce with the car on the ground.

Continuing on, I've got 5" from the top of the bump stop to the shock body. That means the bump stop is 1.75" in length. It can't compress to zero, so there isn't a lot of additional travel to be had here.

The top of my bump stop appears to have "some" wear. Bruce mentioned that his was crumbly, or something like that. I don't think mine is far behind.

So when I convert to B16s, will the bump stop on that assembly be tuned to my car? And as I adjust the coil overs for ride height, what does that do as the distance to the bump stop changes? Plus, with the spring constant equation F = -kx, what does the extra loading do as you lower the car?

Hmmmm ... It's easy to start overthinking this stuff!!! Or am I not overthinking?

Oops, off the spring sacking topic, again.

 

So Wayne, if you push on that orange bump stop with your finger, I suspect you will be ordering new ones. Those look shot. Notice on the left side/rear... that thing is perished and not doing its job.

BTW. Anyone reading this, RoW (Rest of World / not USA) suspension and "sport" 030 suspensions are all lowered beyond our S models, which are just a smidge lower than base. Lower suspension will engage the bump stop sooner than normal. I read once a long time ago that US law requires some sort of height... I don't know if it is related to headlights, or ground clearance or what, but that is why RoW is lower. Way way back when I had my 2000 Boxster S, it had standard USA S suspension which I thought it looked a bit odd with so much space over the wheels. I didn't want the harder ride of the 030 Sport, so when I was in Berlin, I stopped into a Porsche dealer there and asked about buying RoW shocks and springs (ich spreche schlecht deutsch). My plan was to put them into my luggage but naively, I didn't realize how big my luggage needed to be until the parts guy pulled out a fully assembled shock/spring. Yikes... I felt like a dope.

Anywho, these bump stops really do deteriorate... I recommend anyone just jab your hand up there and feel around... put those skills you honed when a teenager into practice. if you poke it good, it will crumble away. No, you are not damaging it, it is just done. You can just replace the bump stop, but to do this, you need to remove the shock/spring assembly from the car, and on your bench, remove the top nut, disassemble the sandwich, and then slip on the new bump stop. A new OEM stop costs around $20 per wheel... no biggie. But if you are going to pull all that apart, you may consider refreshing control arms etc. etc.....

Am I just making you paranoid about disintigrating bump stops? Well.... if they really do come into play during normal day-to-day driving, then you need to do this or you are not getting the juice as originally squeezed.

Peace
Bruce in Philly (now Atlanta)

 

I'm beginning to wish I had never read this thread!!!!!

As for ride height, it's my understanding that in the USA, the DOT requires room to add chains for winter driving. That's also why many feel the need to add wheel spacers. We're not only higher, our wheels are also more inboard. That's why I discount the concerns about widening the stance, although I haven't done that myself.

But the lower aspect makes me wonder what else gets changed on RoW cars. We definitely can't lower our cars that way and still adjust camber correctly.

 

I just looked at the parts diagrams downloaded from the Porsche website, and the only mention of ROW in the catalog was relating to some acoustic package. Maybe the RoW stuff is for older models or Porsche just leveled the world.

Peace
Bruce in Philly (now Atlanta)

 

Hi Wayne,

I have a busy day in the shop today but I wanted to make a couple remarks.

Thanks for the pictures, this is exactly why it's so useful to get eyes-on and take some direct measurement! You can see how little free travel there is on an OE suspension, if the suspension isn't lowered to RoW or Euro-spec standards. I'm impressed the bump stops are still present, that's a good sign.

Any Bilstein-based coil-over (B14, B16, or H&R Street Performance which uses Bilstein internals) will have an inverted front strut so the bump stop will be inside the strut housing and not visible / accessible unless you take the strut insert out: see video here on that process. The rear dampers are a hub-mounted, non-inverted shock so that bump stops remains visible / accessible under a dust boot, if provided.

With 162k on your OE suspension, I would love to talk with you about possibilities! Perhaps over the weekend, by phone, etc.?

Cheers,
Shaikh

 

Hi Dean!

Thank you, I hope all is well and you're enjoying your outdoor adventures as well. We'll have to chat about your experiences in the 997 Turbo section.

Shaikh

 

Here are a couple more pictures taken with the car in the air and then on the ground. The tape on the fender gave me a reference so my measurements to the edge of the hubcap are relative to each other.

Wheel movement is greater than shock movement, but the hubcap difference is nearly 2 3/4" which takes up a lot of what was there on the shock travel.

I should have mentioned this before ... my car is PASM.