'Spring is in the mail'

The 'spring assembly' as I am calling it, consists of an eye bolt on the end of an M8 threaded rod (sometimes called 'stud' or 'studding' (as I've learnt it's called)). The rod passes up through a hole in the support piece which is firmly bolted to the main pedal box. The rod carries on up through the centre of the spring, through a large diameter washer sitting on top of the spring before a nut is fitted to the top of the rod thereby sandwiching the spring between the large washer and the main support piece. The eye bolt at the other end is attached to the wire rope which is of course attached to the pedal arm. As the pedal is pressed, the pedal arm rotates causing the wire rope to pull down on the eye bolt. This causes the threaded rod to be pulled down through the inside of the spring, because a large washer secured by a nut is on top of the spring, then the spring is compressed by this action. Sorry this is a bit wordy but I can't show a photo because I haven't built it yet. A some point on the threaded rod inside the spring, a nut will be fitted which stops further compression of  the spring – I'm calling this nut the 'stop nut'.

Rough concept sketch

Ok, so I have been trying to (a) calculate what springs I will need and (b) find somewhere where I can buy these springs.

For the calculations of the brake pedal spring I have made some assumptions about, how high up the pedal arm the pedal will be mounted, how much angle the pedal will move through, how much angle the pedal will have to start with, how much force the pedal will be pressed with etc etc

I determine that if I push the pedal with 300N of force, through an angle of 10 degrees, from a starting angle of 45 degrees then the wire rope, which in turn is compressing the spring, will move 9.95 mm. This means that to go from a 0N load on the load-cell at rest position to 150N (about half rated load) at full pedal down, there should be a spring with spring constant ~49Nmm-1. This is quite a substantial spring, definitely not something I'm going to be able to scanvage out of an old bicycle pump.

Of course, recognising that I have made quite a lot of assumptions here and things could change in the design for any number of reasons, I am hedging my bets and buying four different springs. The most important parameters for my springs are:

1. Spring constant (a measure of how hard to compress the spring is).
   
   
   And
   
   
2. Spring outer and inner diameter. I need it to be big enough inside to fit my shaft and nuts (ooh-err matron). The outside diameter should be small enough so that I can fit some kind of washer on top.

Now for finding somewhere to buy the springs, this has been quite frustrating. I have found plenty of places online that sell compression springs of this type, but a lot of them are totally set up to deal with large volume trade orders and either implicitly state 'minimum order' quotas or don't indicate whether they sell to the hobbyist guy wanting just a couple of springs. Perhaps if spring manufacturers/retailers could be more implicit on their websites about what type of customer they are gracious enough to exchange goods for cash with, then I might've got an order in a little quicker.

I eventually found a place called 'ondrives' which I found through a links page on this site – which has some pretty cool projects on. I have sent in an order to ondrives.

I'm getting four springs – two for each pedal. They are all around 30mm outer diameter and 60-80 mm in length. Spring constants go (approximately) 40Nmm^-1, 25Nmm^-1, 4.7Nmm^-1 and 1.7Nmm^-1.