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Spring rates & damper lengths
My 'day job' involves
analysing different suspension designs using CAE. I mainly use a
piece of software called Adams Chassis to evaluate the performance of a
suspension design, and then change the various tunable items in the
suspension to improve the performance based on various targets. The
tunable items include obvious things such as spring rates and damper tune,
but also tyres, bush rates, the hard points of the suspension, bump stops,
spring aids, rebound stops... the list goes on.
So, it was fairly inevitable
that I would want to start messing around with the suspension of the Mojo!
Damper lengths
One thing that I noticed from
way back when I setup the ride heights was that the Mojo lacked rebound
travel at the rear. I noticed this when I jacked up the rear of the
car, and the wheels only dropped (with respect to the body) by about 15mm
before they lifted off the ground. Generally, about 1/3 to 1/2 of
the total suspension travel should be into rebound, to allow the wheels to
'drop' into dips in the road. Given that the shocks should offer at
least 75mm of travel, 15mm of rebound travel is a little low, and
noticeable when driving the car on bumpy roads in that the rear end can
feel as though it is skipping around, especially into dips in the road.
I mentioned this to Jeremy
Phillips at the Stoneleigh show in May, and he acknowledged that the
Protech shocks I am using seemed to come out slightly shorter (by about
12mm) than the Spax shocks, although they are specced to have the same
open and closed lengths.
I then got chatting with the
owner of Protech, and he seemed to think that they would be able to change
a few bits and pieces around to lengthen my existing shocks at minimal
cost.
So, fast forward to
September, and I was due to spend a week in Wiltshire, close to Protech's
base, so I gave them a call and arranged to drop my 2 rear shocks in to
them (the fronts seem OK, by the way) for modification.
Spring rates
In addition to the lack of
rebound travel, I also felt that the Mojo was overly stiffly sprung,
especially for mainly road use. On the track the car is fantastic,
but bumpy roads can turn into a roller coaster ride. The issue seems
to be predominantly at the rear, with an uncomfortable bouncing feeling.
In technical terms, the ride frequency is too high. The ride
frequency is a measure of how fast the car would bounce up and down on its
springs if you were to remove the dampers. Generally, road cars are
in the 1.1-1.5Hz range, and normally the rear is set to be a slightly
higher frequency than the front.
I did some analysis on the
Mojo (I won't go into the 'ins and outs' here) and came up with ride
frequencies of approx 2.48Hz front and 2.54Hz rear (F-R ratio 1.02).
I then dug out some info on ride frequencies of mid or rear engined sports
cars, and found that a Porsche 911 (996) turbo has a front ride frequency
of 2.08, rear of 1.73 (F-R ratio 0.83). This was interesting in that
the rear was significantly lower than the front (This tallies with
comments often made by road testers about the 911, and the nose 'bobbing'
around more than the rear over bumps).
Some reasons for this are:
- To tame the notoriously
lively back end of the 911 (a lower roll stiffness gives less weight
transfer at the rear, and hence more at the front, which moves the
balance towards understeer)
- To help with traction on
uneven surfaces.
Given that the weight
distribution of the Porsche is pretty much identical to the Mojo at 40/60
F/R, I decided that going for a lower ride frequency at the rear could
work on the Mojo as well. Disadvantages are that it will reduce the
total roll stiffness (so the Mojo will roll a little more) and that there
may be a little more understeer. I don't expect these to be issues
as the Mojo rolls very little at present, and has a propensity to
oversteer (sometimes snappily!).
As a first stab, I did the
analysis again with 300lb/in rear springs (the original setup was 400lb/in
all round) which drops the rear ride frequency to 2.28 (ratio F-R of
0.92), so a reduction of 0.2Hz- surprisingly little! However, not
wishing to make too large a change at first, I have decided to try this
setup, retaining the original front springs for the time being. If I
decide to reduce the rear rates significantly more, I would need to also
reduce the front rates to maintain a reasonable balance, and then the roll
stiffness would really start dropping off. I could counteract this
with anti-roll bars, but things quickly start getting complicated!
I mentioned my plan to
Protech, who may be able to dig out some suitable springs for my visit to
them- otherwise I'll be ordering some.
It will be interesting to
drive the Mojo again after the mods...
Driving Report
Well, my visit to Protech was
very successful. I dropped the rear shocks in on Saturday morning,
and Kevin had a dig around in their supply of springs. All of the
300lb/in springs were hiding, so my choice was 275 or 325, both in 7"
lengths rather than the original 6" springs. I decided to try
the 325s, not wishing to make too big a change in one go.
I returned on Wednesday, and Kevin chatted through what they had done:
1. Fitted 8mm longer damper shafts
2. Updated a few internal components to reflect what is 'current' spec
3. Refilled with new oil
The repercussions of point 2 are that the dampers will be stiffer for a
given setting- Kevin suggested running maybe 3 or 4 clicks lower than
before. (The Protechs are good in this respect in that if you find you are
running on the first click and you still find they are too stiff, you can
reconfigure the adjuster to give a lower range).
I was charged £30 for the work which I thought was very reasonable,
plus the going rate for the springs.
On returning home I eagerly got on with fitting the shocks back on to
the car. At first a mild panic was caused when I realised that the
longer springs meant that the lower spring platform was now looking
perilously close to the outer CV joint boot- yikes! However, once
all fitted in place I had a good 5mm, which I decided was enough. I
backed the damping adjustment all the way back to the softest setting,
dropped the car onto its wheels and had a quick fiddle with the spring
platforms to get the ride height about right (in theory I should now have
to go through the corner-weighting procedure once again. I'll do
this at the earliest opportunity).
Then it was time to go for a drive- and what a difference it had made!
The rear of the car now feels much better matched to the front, with a
firm but flat, well damped ride with none of the previous high frequency
bouncing motion. I did try adding one extra click to the rear
dampers, but have now reverted back to the first click.
Overall, the ride is significantly better. Some bumps on my
journey to and from work that used to really unsettle the back end are now
dealt with much more smoothly. The rear feels well planted over dips
and crests, which gives a much more confident feeling. As a bonus,
I've not really noticed the extra body roll that must be now present.
I'm very pleased with this upgrade, and now wish I had done it months ago.
If anything I think I could now reduce the front rates very slightly, but
maybe only down to 375lb/in. For the time being, I'm not going to
worry.
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