Bump Stops for Street Class
11-10-2014, 09:15 AM
#1
Burning Brakes
Thread Starter
Bump Stops for Street Class
I remember reading an analysis of the then new 996 suspension in Pano that showed how the bump stops came into play extremely early in the compression stroke. As I remember it, within .9 inches. If this is true, why aren't people using the allowance to replace these tall bump stops in Boxsters, Cayman's & 996's in Stock/Street with something still progressive but much stiffer? Wouldn't this be a huge advantage (the ability to vastly increase spring rate and limit maximum roll) that most other cars don't have, instead of worrying about the slightly stiffer springs with PASM?
Seems like stiff bump stops allied with plenty of low-speed compression and you've got a go-kart that doesn't need a lot of the missing front camber.
Just wondering.
Seems like stiff bump stops allied with plenty of low-speed compression and you've got a go-kart that doesn't need a lot of the missing front camber.
Just wondering.
11-10-2014, 10:46 AM
#3
In case people don't know, this is the kind of thing he's talking about. As long as the bump stops aren't contacted any sooner in the stroke than the factory ones, they're legal.
I have a set of these packed into the rear of my STR car since I already have the biggest feasible torsion bars.
I have a set of these packed into the rear of my STR car since I already have the biggest feasible torsion bars.
11-11-2014, 08:26 AM
#5
Drifting
I had no idea this was even a viable option. Has anyone tried this yet? If not, we may all need to do this at the exact same time, because it will likely be the last year we're classed competitively in street.
How does it affect ride quality in regular driving? I'd imagine it would be better than super-stiff springs because of that first inch of travel, but big bumps may still become uncomfortably harsh.
How does it affect ride quality in regular driving? I'd imagine it would be better than super-stiff springs because of that first inch of travel, but big bumps may still become uncomfortably harsh.
11-11-2014, 08:38 AM
#6
Burning Brakes
Thread Starter
11-11-2014, 08:51 AM
#7
Burning Brakes
Thread Starter
In case people don't know, this is the kind of thing he's talking about. As long as the bump stops aren't contacted any sooner in the stroke than the factory ones, they're legal.
I have a set of these packed into the rear of my STR car since I already have the biggest feasible torsion bars.
I have a set of these packed into the rear of my STR car since I already have the biggest feasible torsion bars.
D. A suspension bump stop is considered to be performing the function
of a spring. Therefore, the compressed length of the shock at the
initial point of contact with the bump stop may not be increased from
the standard part, although the bump stop may be shortened for the
purpose of installing non-standard shocks.
If you have stock shocks, all you need to do is look at the length of the existing bump stop. That's what you have to work with… no longer. If you have aftermarket, which may have different body length vs. overall length, you need to know the compressed length of the stock shock at the point where it contacts the stop and figure out what that means for the aftermarket, i.e. compress the aftermarket to the same length and see what gap you have for a bump stop. This can be a little complicated if the mounting is different and a little difficult to measure if there's a lot of gas pressure. Maybe somebody has an easy way to do this?
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11-11-2014, 09:15 AM
#8
Burning Brakes
Thread Starter
I found a pdf of the Pano article on-line. I just searched for 996 suspension analysis. It doesn't have the figures, however, and the text doesn't say how soon the bumpstops are contacted.
Here is a quote: The front and rear bumpstops or “micro-cellular progressive jounce bumpers” pictured in
Figure 3 are actually auxiliary springs that work in tandem with the steel springs to
provide a very progressive spring rate at the extreme of suspension compression.
Here is a quote: The front and rear bumpstops or “micro-cellular progressive jounce bumpers” pictured in
Figure 3 are actually auxiliary springs that work in tandem with the steel springs to
provide a very progressive spring rate at the extreme of suspension compression.
11-11-2014, 11:57 AM
#10
Many modern cars (not just Porsche) are designed with bumpstops that come into play much earlier. Mazda for example designed the Miata and Mazda2 to actually corner on the bumpstops. Here's a cool thread of a guy that played around a bit.
So while it's probably outside of the initial intent of the rule, I bet if you had a look at any fast ES Miata you would find carefully tuned bump stops. They're not shy about saying they use them. Fat Cat is really on the forefront of this, worth the time to poke around their site a little.
Last edited by burglar; 11-11-2014 at 04:02 PM. Reason: Removed part for clarity.
11-11-2014, 03:55 PM
#11
Burning Brakes
Thread Starter
1. Shocks are free, within stated limitations
2. Bump stops are part of the shock
3. Bump stops are free, within stated limitations
bTW I looked at 986 bump stop on Pelican. Tall mothers. Ripe for replacement.
11-11-2014, 04:02 PM
#12
Oh, rest of the rule:
So yeah, legal.
Bump stops installed externally
and concentric with the shaft of a shock may be drilled out to
fit a larger diameter shock shaft. Bump stops may be substituted for
the purposes of installing non-standard shocks.
and concentric with the shaft of a shock may be drilled out to
fit a larger diameter shock shaft. Bump stops may be substituted for
the purposes of installing non-standard shocks.
11-11-2014, 04:18 PM
#13
Drifting
More like "spec-miata-plunge-cut-legal". Replacing the bump stops not to accommodate a non-standard shock, but just for the sake of changing the spring rate, is definitely not in the spirit of street class and is begging a protest. Just because everyone is doing it doesn't mean it's right.
The bump stop rule is in the shock absorber allowance section, not an independent suspension allowance. In context with the shock facilitation allowances:
If that last line is opening this allowance up for abuse, it needs to be struck.
The bump stop rule is in the shock absorber allowance section, not an independent suspension allowance. In context with the shock facilitation allowances:
C. To facilitate the installation of commonly available aftermarket shock absorbers, struts, or strut inserts whose shaft size is larger than the
center hole of an upper shock mount assembly, that hole may be enlarged by the minimum necessary to accommodate the shock shaft size, provided the following restrictions are met:
(1) the enlarged hole must remain concentric with the original configuration;
(2) the enlargement of the hole does not require modification of a bearing (as opposed to a washer, sleeve, or plate);
(3) neither the hole enlargement nor the location of the shock shaft changes any alignment parameter. Provided these constraints are met, this permits enlarging of the center hole in an upper shock mount with an integrated rubber bushing, where the bushing is integral to the mount and bonded to the plate and the mount is provided by the OEM as an assembly. This includes drilling out and/or removal of the metal sleeve.
D. A suspension bump stop is considered to be performing the function of a spring. Therefore, the compressed length of the shock at the
initial point of contact with the bump stop may not be increased from the standard part, although the bump stop may be shortened for the
purpose of installing non-standard shocks. Bump stops installed externally and concentric with the shaft of a shock may be drilled out to
fit a larger diameter shock shaft. Bump stops may be substituted for the purposes of installing non-standard shocks.
center hole of an upper shock mount assembly, that hole may be enlarged by the minimum necessary to accommodate the shock shaft size, provided the following restrictions are met:
(1) the enlarged hole must remain concentric with the original configuration;
(2) the enlargement of the hole does not require modification of a bearing (as opposed to a washer, sleeve, or plate);
(3) neither the hole enlargement nor the location of the shock shaft changes any alignment parameter. Provided these constraints are met, this permits enlarging of the center hole in an upper shock mount with an integrated rubber bushing, where the bushing is integral to the mount and bonded to the plate and the mount is provided by the OEM as an assembly. This includes drilling out and/or removal of the metal sleeve.
D. A suspension bump stop is considered to be performing the function of a spring. Therefore, the compressed length of the shock at the
initial point of contact with the bump stop may not be increased from the standard part, although the bump stop may be shortened for the
purpose of installing non-standard shocks. Bump stops installed externally and concentric with the shaft of a shock may be drilled out to
fit a larger diameter shock shaft. Bump stops may be substituted for the purposes of installing non-standard shocks.
11-11-2014, 06:18 PM
#14
Burning Brakes
Thread Starter
More like "spec-miata-plunge-cut-legal". Replacing the bump stops not to accommodate a non-standard shock, but just for the sake of changing the spring rate, is definitely not in the spirit of street class and is begging a protest. Just because everyone is doing it doesn't mean it's right.
The bump stop rule is in the shock absorber allowance section, not an independent suspension allowance. In context with the shock facilitation allowances:
If that last line is opening this allowance up for abuse, it needs to be struck.
The bump stop rule is in the shock absorber allowance section, not an independent suspension allowance. In context with the shock facilitation allowances:
If that last line is opening this allowance up for abuse, it needs to be struck.