Bump Stops

[Gary Morrison]

Following Lou's email I have gone through my archives and dug out some
discussion of bump stops and their role in the MX-5's suspension. The three
messages below came out of a protracted discussion set off by several people
who were experimenting with Ground Control coil-over suspension setups and
attempting to find optimum spring rates.

Gary


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IIRC, a stock Miata is designed to ride on its progressive bumpstops while
cornering.
Kinda like a stock 3rd gen RX-7. I've also been told that the 2nd gen Miata
has more
rear suspension travel than the earlier cars. That's probably why the new car
doesn't
snap back as hard and is less twitchy under hard cornering. Don't know fer
sure
though..

shiv

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=====================================================================

On Wed, Jul 28, 1999 at 06:40:19PM -0700, Jyri J. Virkki wrote: >

GOAL: Raise ride height to stock to maximize suspension travel, and
measure how much travel used during track event.
Springs: 450F (6") / 275R (7")

Slightly softer than my CSP setup (550F, 325R), but firmer than my current
street setup (325F, 225R).

Settings: (goal is approx. 12"F/12.5"R which is close to factory ride
height, and within shop manual tolerances).

NL-S : Spring length, No Load on suspension (free hanging, w/tire)
NL-A : Available suspension travel (top of shock to bump stop), No
Load on suspension (free hanging, w/tire)
L-S : Spring length, as above, but with full static load on suspension.
L-A : Available travel, as above, but with full static load on

suspension.

Wh height NL-S NL-A L-S L-A
-----------------------------------------------------
LF 12.0625 6 3.75 4.1875 1.75
RF 12.0625 6 3.625 4.25 1.75
LR 12.5 6.75 4.125 4.75 2.0625
RR 12.5 6.75 4.1875 4.6875 2.125

Additionally, I tightly put a tie wrap on each shock tube to serve as
a record of the highest position the shock moved up to during a
session (the shock pushes up the tie wrap on compression, but it stays
put during rebound).

Time honoured procedure.

The result? No travel was left over! The suspension was bottoming out,
the shocks hitting the bump stops (and the tie wraps rammed into the
bumpstops .

...

The obvious reaction is that I need stiffer springs. However, there
seems to be something odd about this. The car was up to stock ride
height, with springs far stiffer than stock, yet it bottoms out!

But.. how then can a car with stock springs ever hope to not bottom
out, but presumably it doesn't.. or does it?!

It does. The miata bumpstop is a very integral part of the suspension and
serves as a rising rate spring in normal operation. There is well over an
inch
of shock travel (or about 2 inchs of wheel travel) available from first
contact with the bumpstop to where it starts to get really firm.

A stock miata corners with the outside suspension well into the bumpstops.
The body roll on a stock miata is mostly the inside lifting which it can
easily do with the soft springs.

I ran cut down stops (only cut a little) when I had the stiff springs, but
now I run full length stock R package bumpstops (R package rears are
different
than standard rears). Also, the 1.8 cars have longer but softer with more
travel bump stops than the early car.

These cars are designed to use the bumpstops, so one wants to be careful how
much trimming and lowering one does or you end up messing up the balance of
the car or making it squirrelly in bumpy turns.

-dg

==============================================================


==============================================================

Once upon a time Mark Cookson wrote

I don't think that I am riding on the bump stops in the rear, and the car
doesn't feel bad most of the time. Even bumps while cornering don't seem

to

Yes, nobody thinks they're riding them Do the tiewrap test and get
back to us...

The important thing to realize is that the initial spring rate of the
bump stop is very soft, softer than the spring itself. Thus, when the
shock contacts the stop is not really noticeable, nor does it feel
bad, which is why it doesn't seem as if it was riding the bumps,
though it is.

It only becomes bad if the difference is large. Say you cut off 3/4ths
of the bump stop, so that its initial spring rate is somewhere between
600 and 1000lb/in, and you are using soft 200lb/in springs. The
transition will be very abrupt, you'll definitely feel it and the
handling will suffer. This is a pretty extreme example though.

But as I said, without detailed data on the bump stop spring rate
progression, it is hard to say what lenght is ideal. What is clear is
that it should not be cut excessively unless you run very, very stiff
springs.

--
Jyri J. Virkki - Santa Cruz, California - Linux: The Choice of a GNU
Generation

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[Ray]

Thats quite interesting eh Gary,
thats one of the reasons why Koni supply little white disc's in their shock set,
its to stop any rubber from the bumpstop damaging the neck seal at the top of the
shock, so they must have had some knowledge that the bump stops hit the top of the
shocks, not sure of the material, nylon or ptfe, so anybody fitting Koni's should
make sure the kit has these disc's and to fit them,
when i lowered(springs) and fitted my Koni's (top groove) i refused to cut my bumpstop
rubbers like some people said i should, and i have never had a handling problem and
i know i can touch them under very hard cornering.
also you have to remember US cars have different suspension to us, one of the reasons
why i dont take much notice of their "must do" alignment guides.
Ray

Following Lou's email I have gone through my archives and dug out some >discussion

of bump stops and their role in the MX-5's suspension. The >three messages below
came out of a protracted discussion set off by >several people who were experimenting
with Ground Control coil-over >suspension setups and attempting to find optimum spring
rates.
[...]

[Ray]

Thats quite interesting eh Gary,
thats one of the reasons why Koni supply little white disc's in their shock set,
its to stop any rubber from the bumpstop damaging the neck seal at the top of the
shock, so they must have had some knowledge that the bump stops hit the top of the
shocks, not sure of the material, nylon or ptfe, so anybody fitting Koni's should
make sure the kit has these disc's and to fit them,
when i lowered(springs) and fitted my Koni's (top groove) i refused to cut my bumpstop
rubbers like some people said i should, and i have never had a handling problem and
i know i can touch them under very hard cornering.
also you have to remember US cars have different suspension to us, one of the reasons
why i dont take much notice of their "must do" alignment guides.
Ray

Following Lou's email I have gone through my archives and dug out some >discussion

of bump stops and their role in the MX-5's suspension. The >three messages below
came out of a protracted discussion set off by >several people who were experimenting
with Ground Control coil-over >suspension setups and attempting to find optimum spring
rates.
[...]