Springs

[Ian Williams]

Denis, if you are attempting to use a Big seven front axle with flattened spring and std Big seven radius arms mounting on the front cross member I am afraid it will not work, the radius arms will foul the chassis. It you attach the radius arms to the side of the chassis as many do with this conversion if will handle badly, understeering dreadfully, you also will need to bow the beam a'la Ulster/Nippy if you wish to achieve Austin sports ride height, this is for your Nippy I assume. As far as working out spring rates is concerned I have no idea, in my case it has been trial and error over many years, I finally had my special handling extremely well, a couple of years before the accident. Precise, nimble, quick turn in, little understeer, manageable roil oversteer, but that was all on a track with relatively smooth surface, on the road it would skip on bumps and needed the dampers wound back. A friend drove my car, he was used to having a slow handling Big seven axle on his own special and and mine scared the living crap out of him even though I loved it, so perhaps results are somewhat subjective. To achieve this happy state, ( well for me! ) I had unknown spring rates, certainly not soft rides which is what I started with 35 years ago, but not Ruby springs either, I chopped and changed adding and subtracting leaves altering lengths, adding damper arms, stiffening chassis etc, etc, etc.

[Dennis Nicholas]

Hi Denis,
To be as helpfull as possible more info is required.
Fast road/race suggests a two seater? Big Seven spring suggests you have a wider Big Seven front axle? 
It is possible to work out spring rates for leaf springs by measuring the lbs force deflection per inch by making up a jig. Adjusting the rate by adding and subtracting leaves. Static height and therefore ride height being determined by the initial set which may need altering depending on final rate selection.
For example are you keeping your rear springs located inside the chassis or moving then on top of the chassis. This design choice will effect the static camber you start out with if using the original Austin axle spring locating lugs.
You give no idea as to weight and % f & r with driver. Or the f & r track depending on final wheel offset. Remember spring rate is one thing but wheel rate another and that is determined by the track to spring eye/sliding block if not using shackles, ratio/leverage.
Also please remember that the castor angle may be fine once the final chassis height, not forgetting the driver, front to rear is established.
If you can give us a bit more info I'm sure lots of good advice will be forthcoming.

Regards,

Paul N-M

Nothing so complex Paul.  It is a standard(?) Nippy with a BMC A series engine mounted with a Toyota 5 speed gearbox.  The front axle is a standard big 7 with standard new spring.  If I remember I did shorten the radius arms so they would mount at the cross member. Rear axle is the late A7 1938/9 and mounted with standard springs (from a Ruby if I remember as one proper spring was bent to 90 degrees after previous owner had a big accident with it [with A7 engine]).  I have been running it for many years with this set up but with the BMC engine with a BMC gearbox.  Not too distant past lack of availability of "A" series boxes and a broken top piston ring dictated a change to an A+ engine which just happened to be available locally from a scraped 40,000 mile BMC and the coincidence of the Toyota 5 speed box to A series conversion kit aimed at Morris Minor market.  This would allow less frenetic long distance road cruising at sensible speed (70mph). I previously had not bothered about the thought of the twist and consequent increased speed of wear in the spring/shackles.
I was just hoping out of curiosity for a simpler way to measure the leaf-spring rates.
  I suppose for the front just clamp middle of spring, upside down, in vice and hang weights off end and measure deflection for each weight......but I only have one 56cwt and a smaller weight available.  I suppose if I were not so busy I could find a way of moving my hydraulic press over to the vice and using it to exert known (from the pressure gauge) controlled force and measuring distances moved.
Again from memory I think the front spring was not far off being flat when running with the BMC A and its BMC gearbox.


Ian
as said above it has been running many thousand road miles and MCC trials in its BMC engine/gearbox and big 7 front axle guise (big 7 axle is bowed as standard and seemed to handle quite well (15 inch wheels for road but scary on 19 inch wheels on road between trials hills. 16 inch were preferable for road but demise of easily obtainable tyres at a sensible price and my having some 15 inch wheels and the availability of tyres (Citroen 2CV front and Peugot rears) dictated the change.....had to accept the higher RPM cruising. In the end the body falling to bits and bad rusting dictated a massive very long term rebuild.

Thanks for all ideas and comments...we never stop learning.

Dennis