Spring Rates

When I first ordered my shocks and springs I went with the springs rates suggested by NTS:

400lbs/inch front
250lbs/inch rear

Initially these felt ok, but the more I drove, and the more confident I got, the more clearer it became that something wasn't right with the handling...
I found that the ride was pretty poor on bumpy roads (and we have a lot of bumpy roads round here...), and I was getting jolted about all over the place.
More importantly though, the car had disasterous understeer when pushed to the limits of grip.
It was very difficult to get the tail out, and when I did it'd very quickly switch between under and oversteer resulting in some funky handling.
Another indication that something wasn't right was that the tyres squealed like a pig going round tight corners, even when I wasn't pushing it hard.

After spending a while playing with ride heights, damper settings, camber, and toe, and not getting anywhere I decided to do a bit of research on spring rates.

A good starting point seemed to be to calulcate the wheel frequency (using the Allan Staniforth method in his book Competition Car Suspension - great book by the way!).
My calculations were full of measurement errors, estimates, and assumptions, but I figured they'd still give me a reasonably good idea of what was going on.

The calculations suggested my wheel frequencies were around 130cpm on the front and 100cpm on the rear.
Staniforth suggests 80-100 cpm for a sportscar and 100-125 for a race car.  He also suggests that the rear should be slightly higher than the front to enable it to catch up with the front over bumpy surfaces.
So that might explain a few things then!

At that point I decided to work through the calculations the other way to see what sort of spring rates I might actually want to start with.
After playing around I came to the conclusion that the rear needed to be around 100lbs higher than the front and that a good starting point would be 300lbs on the front and 400lbs on the rear, giving 115cpm front, and 130cpm rear.

As a quick and dirty test I decided to swap the front and rear springs giving me 250 on the front and 400 on the rear.
I've had it set like this for a few days and it's much much better!  The fronts does feel a tad soft, but the tyre squeal and understeer have been eliminated, she feels far more composed on bumpy ground, and as an added bonus the front seems far less prone to locking under heavy breaking.
It's probably gone a little far the other way and is slightly tail happy now, but it's more fun like that :)

I still think 300 front / 400 rear will give a good balance and started looking for a new set of springs for the front.
I managed to find a pair of 325lb springs secondhand (but unused) for less than half the price of a pair of new 300lb springs and will hopefully get time to fit them next week.
That should still keep it slightly biassed towards the rear assuming my calculations aren't too far out!
I'll let you know how I get on.

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UPDATE 30/09/2013:

I've been out a few times with the current spring rates (325lbs front / 400lbs rear), and I'm reasonably happy with them now.  I'm certainly in no rush to change them again!
The car feels fairly well balanced front to rear now and goes into a nice controllable oversteer when provoked, which is what I was aiming for :)

The only thing I would say is that the ride is possibly a little too hard for some of the dodgy roads I drive on, and I *might* think about fitting softer springs next year.

Back in one piece - with new diff mounts

Took me a bit longer than planned, but the car's just about back together after my little incident with the diff.

Picked up a very orange replacement diff from Porkchop over on the Haynes forum.
Got some solid ali mounts to replace the rubber ones in the diff arms. These were supplied by Stot on the Haynes forum.
Got my propshaft repaired by Northwest Propshafts.  This is where most of the delay occured.
The rear UJ got quite a battering when the diff broke and needed replacing.  Initially they thought they might be able to repair the UJ, but it turned out to be unrepairable.  So what they did was replace the rear UJ with a stronger, rebuildable one, and replace the whole shaft leaving only the front UJ and splined bit that goes into the gearbox as original.
When it turned up it didn't mate with the diff, as the nut on the end of the diff got in the way...  They arranged for a courrier to pick it up though, and a week later it came back, and thankfully fitted just fine.
The other thing I did was fabricate a new front mount for the diff. It's similar to the old one, but 'chunkier' and solid-mounts to the chassis rather than using the landrover mounts:

I'm not particularly happy with the current diff mounting arrangement, but I'm sure it's better than my last one and will hopefully get me through what's left of the summer!

Just waiting for the roads to dry out now so I can have a bit of a shake down run.

UPDATE:
I've done 50 or so miles over the last couple of days.  It's still on one piece and I can't say I've noticed any additional vibration.
I do miss the old LSD though.  Pulling away doesn't feel quite as brutal with the new open diff!

Teething Troubles...

Well 650 miles in and I've gone and broken it:

Not sure whether the rubber mounts or the diff broke first, but either way it's back to the drawing board...

Road Registered

Didn't mention on my last post, but on the way home from IVA I stopped by Peterborough DVLA office.
A very helpful chap there went through the paperwork with me, took a payment for tax and application fee, and inspected the car.
Two working days later (Monday this week) I got a letter containing my reg number and tax disc!
I got an age-related J-reg plate which was nice.

Took the car out for a damn good thrashing Monday night, and have been commuting in it all this week.  Loving the unusually balmy weather :D

IVA Passed!!!!!!!!

Wooooooooooooooooo! I passed my IVA re-test today!

Here's what I had to do to get through:

1. Replace the slide pins on the dodgy rear caliper.  One of them looked home-made, and not at all suitable...
2. Replace Rally Design's crappy forged coupling (RD820F) with a pressed steel type instead.  Couldn't get the forged one to sit tight on the steering rack spline no matter how much I tightened the bolt up.  I'm not the only person to have this problem either...
3. Delete my Mondeo rear brake bias valve to improve my overall braking efficiency.  The rear brakes are quite weak on the MX5 and you don't need a valve to get trhough IVA.
4. Chop a half inch or so section out of the brackets that hold the end of the handbrake cables to the rear calipers and weld them back together so the cables clear the chassis.  I raised the ride height a bit too just to be on the safe side.
5. Rotate my rear flexi line slightly so it didn't touch the chassis any more.


Though not on my fail sheet I also:

* Made the clutch pedal longer and replaced the master cylinder with a 0.625" one.  The clutch was rediculously heavy and snappy before I did this.  Still not great but vastly improved.

* Re-packed my exhaust with wire wool as the first lot blew out on the way home from my first test!

* Re-work the front wingstays to make them a bit stronger.  One of them sheared off within hours of booking my re-test, and I had two evenings to re-make them!

IVA Fail :(

It was the big day for me yesterday.
Luckily I made it to the test centre in one piece, and with plenty of time to spare (I had my doubts about driving there, but she didn't miss a beat!).



Sadly a first time pass wasn't to be though

The car was tested by the main tester (Doug), his trainee, and a sort of supervisor that goes from centre to centre checking tests are done to the same standard. So it got quite a thorough looking over!

They are all good blokes, very friendly, and generally very positive about the car. The supervisor was a bit of an MX5 enthusiast so we had a good old natter on all things MX5.

First test was emissions, which passed without trouble (they were quite happy with the letter from Mazda for proof of engine age).

Next was interior/exterior projections, and a good look around the car.
They liked the fact I'd stuck lots of bits of rubber on there and couldn't find any nasty edges (unusual for Leighton Buzzard apparently!).

Next they stuck it on the ramps to check underneath. This is where it started going wrong...
They spotted one of the rear brake lines and both handbrake cables were rubbing on the chassis. They weren't last time I checked, so they must have settled/bedded-in during the drive there.
They also spotted there was a bit of play between the UJ and steering rack which they didn't like. I tried tightening it up, but there's still too much play.

The ramps came down and they checked all the lights. Head light aim was way off (and still is), but everything else was fine.
Interestingly they didn't seem to measure any heights or distances of the lights.

After that they weighed it then did the brake and speedo test.
The speedo over-read by quite a bit, but was still within IVA spec.
I'll come on to the brakes later...

Next test was exhaust noise, passed with a comfortable 95dB, followed by rear vision, which passed easily.
They liked my Furore e-marked IVA wing mirrors and the position of them saying a lot of people fail by putting them too low and getting too much rear wing in the way.

Next the trainee tester had a blat round the car park to do some more brake testing and to check self centering. Both were fine.

Finally they took a few measurements with a tape measure before disappearing into the office to enter all the figures into their spreadsheet.

Sometime later they emerged with my fail sheet...
This had the fails I knew about above, along with a fail for brake efficiency, and a fail for a big mismatch between each side on the handbrake.





Not too bad a fail really.
Not quite sure what to do about the brakes yet, but the rest should be easy enough to fix.

IVA Tomorrow!

Less than a day to go now :O
Haven't done very much to the car for the last few weeks, just little tweaks here and there and adding random bits of trim.
I think I've done all I can realistically do to prepare now, so have to wait until tomorrow to see if it's enough.
Not really expecting a first time pass, but hopefully the list of things to fix won't be too long or difficult!

Here's some recent-ish pics: