Chris's new build of an MX5 based car

[CTWV50]

Oh also I forgot to mention I used this website to work out spring rates. Very useful! I worked out sprung and unsprung weight using our weight watchers glass bathroom scales! And they didn't break! Not the most accurate way to work out unsprung weight but it must be close.

http://www.hypercoils.com/spring-rat.../#.UymYXNwwgfp



I also rechecked the brake lines on full lock for clearance etc.





These mazda 323 brake lines with the brackets I've bodged together have worked out really well with the 1.8 brake carriers and the 13" wheels.

I took the car down the neighbours driveway again this morning to check out the suspension difference. It was a limited test but it's not the most level of roads and there is one big bump in the road which I went over with the drivers side wheels fairly fast and I didn't really feel anything through my seat or steering wheel which was surprising. As I say only a limited test but promising. I also tested for steering self centring and it was completely AWOL guess I've lost the negative camber at the new ride height, I'll dial that back in at the weekend.

Quick shot of the car in front of the neighbours greenhouses.



I went as fast as I could in the limited space and stamped on the brakes, slightly locking up the pass side front wheel, but no horrible sensation through the steering wheel or pulling.

I think I can now move on to the 100mm ball test and get the front finished off mechanically.

Is the 100mm ball test only with the wheels in the straight ahead position or do they do the test with full lock on as well? Anyone know?

[skov]

Originally Posted by CTWV50

Oh also I forgot to mention I used this website to work out spring rates. Very useful! I worked out sprung and unsprung weight using our weight watchers glass bathroom scales! And they didn't break! Not the most accurate weigh to work out unsprung weight but it must be close.

http://www.hypercoils.com/spring-rat.../#.UymYXNwwgfp

I've looked at that calculator before and didn't much like it to be quite honest.
Ideally you want to be calculating spring rate based on wheel rate, then adjusting ride height with pre-load and/or spring length.
That calculator seems to do it ar5e about face and calculates spring rate based on ride height, then tells you what wheel rate you'll happen to end up with.
Converting those figures to wheel frequency you'll have 135 cpm by my calculations.
As a guide:
60 – 80 cpm Comfortable road car
80 – 100 cpm Sports road car
100 – 125 cpm Racing cars without wings
200 – 350 cpm Current racing cars with downforce

[CTWV50]

I don't really know enough about it to comment John. So if your using 250lb springs on the front, how much preload in mm have you put into the spring?

[CTWV50]

I'm sure your right John, I just threw some numbers at this calculator and got these results.....



I noticed it didn't take into account damper angle though.

http://www.racingaspirations.com/app...ncy-calculator

This is fairly confusing!

[skov]

Originally Posted by CTWV50

So if your using 250lb springs on the front, how much preload in mm have you put into the spring?

I think I've got 325lbs on the front at the moment, and only have a tiny bit of preload.
With 250 springs I needed lots and lots of preload!

Originally Posted by CTWV50

I noticed it didn't take into account damper angle though.

It can, but not directly. You have to take account of the damper angle in the leverage ratio. Can't remember how to do that off the top of my head though!

[CTWV50]

Originally Posted by skov

I think I've got 325lbs on the front at the moment, and only have a tiny bit of preload.
With 250 springs I needed lots and lots of preload!




It can, but not directly. You have to take account of the damper angle in the leverage ratio. Can't remember how to do that off the top of my head though!

Hmmm, I don't know. It feels ok but only had very limited testing really. Also I'm using the centre of the wheel for the A-B dimension, and using ET15 not standard mx5 ET38 wheels and I've moved the shock wishbone mount out around 20mm, so there really is a lot more leverage on the spring. Also I've only got 1" of shock travel for droop due to the 13" shocks but it gives me 1.5-2" droop at the wheel. I guess like you I'll have to see how it behaves on the road and take it from there.

If I suffer understeer due to the front spring rate, I had the mad idea today of making a rear ARB. But I'm going to forget about that for now, sounds like trouble! I can always try a lower spring rate and try preloading them.

[CTWV50]

OK I've had a bit more of a play with these calculators and as one of them gives me a "motion ratio" of 0.411. see below..

MOTION RATIO
Motion Ratio = (Dimension A / Dimension B) * sin(Spring Angle)

The motion ratio is the mechanical advantage (lever ratio) that the wheel has over the spring in compressing it.

I converted this for the Staniforth calculator to 2.43:1 and then my cpm dropped to 100.74cpm.

I wasn't sure if this was correct but looking at the wheel rate the numbers are very similar. but one is described as "effective wheel rate" so I'm not sure what the difference is there.

WHEEL RATE

Effective Wheel Rate = Spring Rate * (Motion Ratio)2

Wheel Rate is the effective spring rate at the wheel, due to the leverage advantage the wheel has with respect to the spring on the control arm.


Staniforth gives a wheel rate of 76.25lb/in whereas hypercoils gives an effective wheel rate of 72.55lb/in.



Does that look about right? I have no idea!

[CTWV50]

Originally Posted by CTWV50

Staniforth gives a wheel rate of 76.25lb/in whereas hypercoils gives an effective wheel rate of 72.55lb/in.

OK this is because I told one calculator what springs I was using (450lb/in) and one calculator was telling me what springs I should use (430lb/in).

If I put 430lb/in into the Staniforth calculator i get 72.285lb/in. Same as hypercoils pretty much!

[skov]

Hmmm, you've got me doubting my calculations now.
I had been using the eibach equations here:
http://eibach.com/america/en/motorsp...sion-worksheet
However they seem to correct for shock angle slightly differently to others which seems to give very different results

[CTWV50]

Well, I've had a couple of glasses of wine but, I'll have a look at that tomorrow!