Questions regarding lowering, bump stops, and Bilstein inserts...

logician · 02-08-2003, 09:30 PM

Is it possible to cut bump stops on Bilstein inserts for a Boge strut? I've read the instructions in the Smart Racing site, but those are for a Bilstein strut. For the Boge strut, I believe the bump stop is inside of the Bilstein insert, and not exposed. Is it possible/recommended to disassemble the insert to access the bump stop. Or do people leave the bump stops uncut for this kind of insert, or is it recommended to get Bilstein struts for a lowered car so that the bump stops are accessible? How much of the front bump stop should be cut?

I'm interested, because I'm planning on replacing the shocks on my recently purchased '73 911s that is lowered to 24 3/4" front fender height, and 23 5/8" rear height (probably not enough rake). I believe that ride height qualifies as an aggressive lowering, and I intend to drive this car on the track, so I am very concerned about having enough suspension travel.

Any help would be greatly appreciated!

-Juan

Chuck Moreland · 02-08-2003, 09:35 PM

I'd say yes, that is aggressive lowering!

As you point out the bump stops are not accessible on Bilstein for Boge. Bilstein recommends the sport shocks for agressively lowered cars. I don't know for fact, but I suspect the sports have a shorter internal stop.

As low as your car is, you might consider raised spindle RSR struts. This restores lost suspension travel and corrects the dynamic geometry with gets hosed with aggressive lowering.

logician · 02-09-2003, 11:41 AM

Hi Chuck,

Thanks for the reply. I was planning on using the sport inserts, but hadn't considered the possibility that the bump stops might already be shortened in them. Can you provide a pointer to Bilstein application information? I couldn't find any specs on their web site, other than a cryptic recommendation of which shock to use.

So anyone out there have experience with Bilstein inserts for Boge struts in competition/track? Are you using the sport inserts, and what is your front ride height set to?

Thanks!
-Juan

old seventy six · 02-09-2003, 04:39 PM

Bilstein Sports all around on my '87. Lowered to 24.25 front/23.75 rear. Steering rack spacers installed. Loads of neg camber.
Aligns great. Drives great.

RallyJon · 02-09-2003, 05:21 PM

Quote:

Aligns great. Drives great.

But how much "up" travel do you have before it hits the bumpstop?

I'd like to know this, too. Every shock absorber made has an ideal ride height. This data is available for all the slammed Hondas--someone must have it for 911s?

old seventy six · 02-09-2003, 05:50 PM

I haven't hit the bumpstops.

Chuck Moreland · 02-09-2003, 07:35 PM

Hi Juan

My information on the sports comes from a discussion we had here awhile ago. As I remember it someone actually contacted Bilstein asking about differences between sport and HD, and mixing the two on the same car, and the value of high pressure shocks.

In fact here is the thread:

Does Ambient temperature affect gas shocks?

jluetjen · jluetjen's Garage

BTW - I don't have specifics on the sizes of the bump stops in question, but be sure to think hard about changes that you make to the bump stops -- they're not there just to protect the shocks/frame.

Well designed bump stops are designed to smoothly increase the spring rate just prior to the car bottoming out. For street use the result is a somewhat more comfortable ride in an overloaded and/or lowered car. Roadcourse (track) use is a different story all together.

Cars' suspensions are designed to "suspend" and absorb irregularities in the road so as to keep a fairly even and predictable force on the tires, and thus traction on the road. A car that bottoms out will suddenly un-weight or over-weight a given contact patch resulting in wildly unpredictable handling. The purpose of bump stops is to smoothly transition the suspension from the ~350 lb/in rate of the springs to the infinate spring rate of a bottomed suspension over about 1 inch of travel. Shortening bump stops to gain suspension travel may work in a pinch, but will result in that transition happening in 1/2 an inch if the bump stop is cut to half height or .01 inch if the bump stop is removed all together. Picture the change in the slope. No imagine if you were to be cornering at 90 MPH at maximum grip when suddenly your outside rear corner went solid. In a 911 that would result in a certain spin.

If your car is "aggressively" lowered I would recommend you take the time to "re-engineer" the suspension settings for the amount of travel that you have. By this I mean take a look at your spring/T-bar rates at both ends as well as your sway bar rates. Using a round figure of 1G for street tires or 1.5 (2?) G's for race tires and figure out how much travel you need or have. If you have only limited travel, you should seriously consider increasing the spring rates or raising the car some. Also take a look at what it would take to bottom the suspension on the bump-stops as well as to bottom the bump stops.

"Tuning" bump stops can be a subject in itself, but in general -- suspensions work best when they have room to move (non-actively sprung ground-effects are accepted). Suspensions where are set lower without corresponding adjustment in travel will drastically reduce that movement.