Front anti roll bar diameters

Thank you all for confirming the ant-roll bar dimensions.
In reading the other detail information I’m not sure I completely agree with it all.
Yes the 2002/2003 cars are “firm” and it’s why the “comfort” package was introduced as far as I can determine.
The comfort pack springs were softer and again it is my understanding that the anti-roll bar was increased in diameter.

Regarding the “bump steer” comment that could be a crucial piece of information.
Im trying to get to the bottom of what a PO did and why.
My car tends to require constant steering input when travelling in straight lines on roads such as dual carriageways.
I have measured between the wheel centres to the bodywork. At the rear it is standard ride height.
At the front it is more like 340 mm.
Measuring from the underside of the spring platform to shock absorber bolt centre I get 79 mm. Now there will be some error in this due to the way the load comes on the suspension components and measuring the other side of the bolt to get an average is not practical.
I have done some sums on spring rates and noting that a 5mm difference in spring platform position between standard and sportpak1 is supposed to lower the car by about 12mm the geometry of the suspension components must give a multiplier of around 2 to 1.
What I can’t work out is how the factory accommodated delivering the same ride height with softer springs unless the shock absorbers were different or the free length of the springs was different.
In any event I think I have a pair of softer springs, or a bespoke pair of springs with the original spring rate.
I need to get it back to standard ride height to give more pot hole clearance and hopefully resolve straight lines characteristics.
And before anyone asks, yes I have had it 4 wheel tracked twice and most of the ball joints are new or in good condition.

It sounds to me like you actually are suffering from bump steer. 

If the car is lowered too much the bottom arms ( or wishbones) where the ball joint attaches to the hub, the angle is flattened  in effect because by shortening the springs you pull the wishbone up to be able to attach the top strut mount. That's probably the easiest way to visualise it.
it means if the wishbone is raised then the angle of the track rod ends is also raised which flattens that angle too. 
If the steering rods to the track rod ends coming from the steering rack are flat it means any suspension movement up or down pulls on the track rod end and "steers the car".
That is why the Roll centre is always lower than the Centre of gravity of the car because it creates that necessary sloping angle to stop bump steer. The angle absorbs it.

Formula One cars have the RC and the CG almost at the same level so that they can get away with 25mm of suspension travel....ie almost no roll. 

Look up Roll Centres on Google...a myriad of examples will come up and you can see how lowering a car makes it actually roll more and... far worse....it induces bump steer. 
Once you lower a car around 20mm you are approaching that territory depending on design....at 40mm it is definitely happening in most cases and it's why many won't admit that their lowered cars handle and steer horribly.

In terms of cornering ability it is better than nything I have had before, but that isn’t saying much. In terms of confidence I’d say the Renault Megan was pretty good but the ability of this TF to turn in on a corner is remarkable.
The steering issue is only noticeable when going in straight lines on good quality straight roads.

Using the kerb weight and guessing 50/50 split front to back and then across the car and comparing spring compression based on the figures from Mike Satur’s website for standard front springs (same as listed in a previous posting above) and then doing some sums I estimate that for a standard free length of spring that there is likely to be a difference in compression that equates to that lower ride height of 340mm as close as I can get it.
If someone can correct me I’d be delighted to hear from them.
If someone has the comfort pack I’d very much like to know what the distance is from the underside of the spring platform to the bolt centre, the thickness of the spring platform and any other relevant dimension such as free length of the spring or whether there is an additional packer at the top of the spring .

Measuring from the centre of the hub to the lower centre of the wing lip how much lower is your car compared to a standard TF? 

Bump steer doesn't show up on corners or is hardly noticeable because your are turning the steering wheel . It shows up on straight smooth roads where the steering wheel moves in your hands as the suspension rides the roads. 

I think I know what you are trying to achieve in that you like the feel of the dampers and shockers together but they just give you a too low ride height? 
If so buy a longer spring. If you can buy a one inch longer spring with a very slightly less spring rate it will work within the shocks travel and valving but raise the height of the car. 
It will also remove or help to remove bump steer.

 

No not quite.
i think that the PO fitted a different spring than standard, or a comfort spring that delivers a lower ride height by virtue of being softer.
So I think that fitting a standard spring will resolve the situation.
What I need to be sure of is that the anti roll bar fitted to the car is the right one for the standard springs, hence my question about diameter.

As to how much lower is it than standard the answer is about 20 mm i.e. an additional 10+ lower than spottpak1

Gotcha.  
Fitting a std spring would cure the ride height and help with the steering as It would help with steering angles.

I wouldn't worry too much about the roll bar. I also have an MX5 and guys fit stiffer bars from an RX8 to their cars with std springs and height.  I would go with the spring first.
Good luck!

When you had the tracking done did you take the correct figures with you or did you say “please do a four wheel alignment”. I did the latter with ATS in Taunton and our TF behaved exactly as you describe. Having looked at the print out it was totally wrong, I took it back and insisted they did it again using the correct figures. There is absolutely no comparison between the before and after, straight line stability is perfect, turn in on corners is truly excellent, no bump steer. ATS told me that they just use the pre programmed settings, no one checks to see if they are the correct ones. 
A local performance workshop who had more than a three week waiting list for tracking told me that it happens all the time. In the good old days a mechanic would look up the data and adjust to that, most of those working in tyre retailers don’t know their a**e from their elbow. 

Hello TA22GT, was your measurement made with the bar coated/painted?
If so that likely confirms a 20 mm bar.

Yes it was. The bar is in good condition so I reckon it gave an acurate reading. Got to be 20mm I think.

Thank you I very much appreciate your help in confirming that.

You are very welcome! 
Everyone here wants you to resolve it.

I might grumble about my car but I love it and come time I will have it sorted. I'm in it for the long haul and I learn from every post.

If it’s any help, the standard blue flash front springs I removed have a free length of 247 mm. Roger Parker’s book states 2002 spec roll bar at 18mm and 2005 spec at 20mm, however this does not tally with my 2002 car which is 20mm.