Dealing
with parabolic springs winding up under torque
Michael Beswick contacted the V8 Register
in 2011 for advice with parabolic springs winding up under torque.
Here he provides useful feedback on how he has got on.
You
highlighted information on your web site and Brost Forge in
Kings Cross. I was a bit cheeky in seeking help from the V8
Register as I do not have an MGV8, but a supercharged 4 pot
MGB Roadster. Still quite torquey though! After a couple of
discussions, I took my bent springs to Brost for them to repair,
re-temper and add a third half leaf (from centre mount forward)
to reduce the stress on the "main" leaf. Time will
be the judge but so far, so good: the ride is the same as before
and so far no sign of winding up. The replacement parabolics
that I had to fit last summer, as we were off on holiday, showed
very slight bending after about three months - less than 900
miles.
I
gently tried to find if there was anything like a "specification"
that Brost could supply but I think they just did it by experience!
They do everything from rocking horse springs to springs for
steam engines.
I
also looked at anti tramp bars, but as my pair of extra leaves
cost £60 it seemed worth a go! The anti -tramp bars also
seem to rely on bolts through the floor around the front hanger
whereas this method still allows for some movement but has the
extra "meat" of the third leaf to help. If you have
a fire-breathing monster of a V8, you may need an alternative
arrangement, but I would think this would be suitable for the
more modest powered versions with a power output similar to
a Factory MGBGTV8. If in doubt Brost are worth a call!
Hopefully all will be well-if not I will update you! If any
of your members want my opinion on my experiences with Brost
and spring repairs, by all means give them my email or phone
number!
By
the way, Brost were helpful and friendly- they took two weeks.
They had an informal approach, but I arrived at 10.30am on both
occasions which is their tea time!
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What
are parabolic springs?
The MGB and derivatives like the MGBGTV8 have multi-leaf rear
springs with the rear axle attached. The rear spring has to
resist the "wind up" effect and with the higher output
with the V8 engine, stiffer rear springs were necessary to deal
with that although the consequence was the ride comfort was
reduced by the firmer response with that rear suspension set
up.
Multi-leaf springs were very common on cars up to the
1970s in Europe and Japan and to the late 1970s in America when
the move to front-wheel drive and more sophisticated suspension
designs saw automobile manufacturers use coil springs instead.
Unlike coil springs, leaf springs also locate the rear axle,
eliminating the need for trailing arms and a Panhard rod, thereby
saving cost and weight in a simple live axle rear suspension.
Parabolic
leaf springs are a more modern application that consists
of two or more leaves. The leaves touch only in the centre where
they are fixed to the axle and at the outer ends where they
are fixed to the vehicle. This design usually has fewer leaves
and each leaf represents a complete spring in itself and will
act as such. To do this the leaf is tapered, from the centre
(thick) to the outer ends (thin). This tapering follows a parabolic
curve - it means that every centimetre (or inch) the thickness
of the leaf decreases in an amount that relates to the square
function of its length. In this design inter-leaf friction is
unwanted so there is only contact between the springs at the
ends and at the centre where the axle is connected. Spacers
prevent contact at other points. Aside from a weight saving,
the main advantage of parabolic springs is their greater flexibility,
which translates into vehicle ride quality that approaches that
of coil springs. There is a trade-off in the form of reduced
load carrying capability, however. The characteristic of parabolic
springs is better riding comfort but not as "stiff"
as conventional "multi-leaf springs".
Typically
when used for automobile suspension, the leaf both supports
the axle and locates or partially locates the axle. This can
lead to handling issues such as 'axle tramp', as the flexible
nature of the spring makes precise control of the unsprung mass
of the axle difficult.
See an expanded note with photographs. More
V8 website resources
Parabolic
springs on an MGBGTV8
V8BB
thread on parabolic springs on an MGBGTV8
Posted: 230312 |
