If you save weight on moving parts what's the equivalent static weight saving ?

[edmandsd]

I understand that there'a a formula to assess the equivalent weight saving on static parts when you save weight in moving/rotating mass (I understand that the static weight savings would have to be higher to equate to the same effective saving ?) I also understood that the equivalent savings are less the further away from the engine the rotational mass savings are made.

 

For example i've gundrilled my driveshafts and lightened the flanges saving about 4kgs and taken the same amount of weight out of the rear brake discs - Would these weight savings of had to be greater on static parts to equate ?

 

That said i've possibly misinterpreted something along the line !

[Peter Carmichael]

Yes. Everything you have heard is right.

 

The moving masses have to be bodily accelerated along with the car, so the weight saving is immediately equivalent to a static weight saving and then there is an extra saving on top.

 

If components rotate at engine revs (higher than post-gearbox revs apart from 1:1 and overdrive gears) then they get store more kinetic energy.

 

Equation for kinetic energy of static components is:

 

K.E. = 1/2 * m * v^2.

 

There is a rotational equivalent:

 

R.K.E = 1/2 * I * w^2

 

Where I is the rotational moment of inertia about the axis in consideration and w is rotational velocity in rad/sec.

 

So what you have to do is work out the ratio between forward motion and the rotational motion for the components under considearation.

 

I can be calculated using simple formulae:

 

I for a disk is 1/2 * m * r^2

 

I for a tube/propshaft (ignoring the UJs) and presuming relatively thin walled is m * r^2

 

IIRC

 

*thumbup*253 bhp, up and running *thumbup*New boingy bits

[edmandsd]

Thanks for this pc - I'll start to try and work it all out.

 

Spool saved a further 3.5kgs (5.1 (lsd) v 1.6) so i'm hoping it will all translate into faster forward motion !

[Tony Martyr]

While the theory is all correct the means by which you can reduce the mass of rotational parts are many and various and some will considerable increase the local stress levels and patterns of the parts in question.

As pc must have said in the past all engineering is about compromise and the balance of two factors: mass, strength with cost as a second order variable.

I would not compromise through material reduction, standard parts of the powertrain unless I was very sure of the effect on fatigue life.

[edmandsd]

You're point is fairly made Tony although I can assure you that I haven't disregarded strength/fatigue life.

I accept that the brakes will need thorough testing although I deliberately chose the 6mm thick centrifugally spun bike rotors not the stainless steel 4.5mm ones.

I'm assured that the gundrilled shafts are in fact stronger (F3 and GT cars use these not only because they're lighter but because they're also stronger with a hole through the middle).

The original Quaife flanges are designed for Mk4 Escort rally cars so completely overkill for a l/weight Caterham.

At 1.6kgs the spool is also much heavier than it really needs to be but i'm happy with the 3.5kg saving it gives me.