Torsion Bar Rates

[EdCherry 34 1 Cars]

Right heres a bit of an update.

 

Some graphs for people to ponder.

 

 

[Rippthrough 98]

Those frequencies look very low for the spring rates, what corner and un-sprung weights are you using for those, I haven't checked the weights but I did a quick calc based on roughly 600kg up front weight, ~40kg unsprung, and it comes out far higher than your figures?

Edited September 6, 2011 by Rippthrough

[Cameron 16]

Are you including the installation ratio in your calcs up front? (both of you)

[Rippthrough 98]

I just guessed at ~0.98, I only did some quick, rough calcs as the cpm values seem much lower than I'd expect.

[EdCherry 34 1 Cars]

Front installation ratio is 0.963

 

Phil my car is 725kgs all in so your weights look like they will differ a fair amount from mine, unsprung weight equates to 35kg a corner on the front and 31kg a corner on the rear on my car again.

 

EDIT: Before it is mentioned for my calcs (which I will dig out the notes) I used weights with me in the car and strategically placed weights to act as the missing body panels...

Edited September 6, 2011 by EdCherry

[Rippthrough 98]

Well, neglecting the tyre I get almost double your rates?

 

Might have fooked up somewhere.

[EdCherry 34 1 Cars]

Want to put up your calcs and when I get my notes out the of the car tomorrow I will put mine up?

[Rippthrough 98]

Well, few assumptions, but I'm going on 725*0.63 for the front weight bias, which gives about 460kg up front, or ~230kg per corner, and 195kg of that then sprung, or ~430lb

 

If we assume a 200lb/inch spring, then the wheel rate will be ~185lb/inch, give or take?

 

Wheel rate over sprung weight gives 185/430 = 0.430

Square root of that = 0.656

IIRC the constant is roughly 190 or something near that, which gives ~125cpm for the front suspension with 200lb springs?

 

That also gives a cpm of around 80 or so for standard road springs, which seems a bit more sensible than ~45

 

Fairly rough, but I don't think my assumption on the weight distribution is far enough out to end up with half those values?!

Edited September 6, 2011 by Rippthrough

[EdCherry 34 1 Cars]

Indeed, after a bit of head scratching with phil I have worked out where I had gone wrong!

 

Will post updated graphs tomorrow much more realistic!

[Cameron 16]

I think it'd be a good idea to put up graphs of wheel centre rate in lbs/in rather than showing cpm, since the ride frequency is going to vary quite a lot for different cars with different weights and weight distribution. Cpm (cycles per minute for anyone wondering) doesn't mean a lot to most people whereas lbs/in is something everyone can relate to.

[Cameron 16]

I come up with very similar results to Ripthrough (see attachment).

 

How did you get to your installation ratio btw? Seems very precise! Just wondering if / how you measured it.

Edited September 7, 2011 by Cameron

[EdCherry 34 1 Cars]

The basic way is using a bit of trig. Once you got the entire front end measured up you just work out the angles at full droop and then full bump. Gives you the angles and in between is the installation ratio. This came out .02 different from actual calculations which are listed below (Gave very minimal difference of wheel rate).

 

 

This works with some graphs which I can't be assed to upload at the minute. So from my notes I have

 

[side B] Loaded coil length / [sin B] sin unit angle (inclination angle of loaded coil length) = Constant K

 

[side C] / [sin C] = Constant K

 

[side C] Fitted coil length / Constant K = [sin C]

 

Angle A = 180* - (B* + C*)

 

[side A] / [sin A] = Constant K

 

Constant K x [sin A] = [side A]

 

This gives you the length of chassis displacement to damper displacement for droop to loaded to full bump.

 

Effective coil rate (static) = Spring rate / [side A] = z1

 

Effective coil rate (full bump) = Spring rate / [side A] = z2

 

z1 - z2 = z3

 

(z3 / 2) + z2 = Fitted Rate

 

You can use that to give you the installation ratio, which im sure you can figure out for youself.

 

Sorry if it doesn't make much sense.

Edited September 7, 2011 by EdCherry

[Cameron 16]

That makes absolutely no sense. Sorry!

 

If you're measuring it on the vehicle, why not just measure wheel displacement and damper displacement and calculate a simple ratio from that? Doing it by trig seems a very long-winded way with lots of potential for mistakes to creep in!

 

I'm not doubting your answer btw.. just wanted to see how you'd got there.

Edited September 7, 2011 by Cameron

[EdCherry 34 1 Cars]

It would make sense with the graph!

 

Point is the car is a good 30 minutes drive from me (both of them) and I dont want to take the time/effort to measure it on the car when I can measure it off the car. Assuming my measurements are accurate, and I carry over 5 decimal places I can see little reason for error. Even if there was error i'd expect it to be pretty small and the overall change to be similar, pretty small.

[EdCherry 34 1 Cars]

Right heres an updated graph showing the comparison between front and rear frequencies against wheel rates.

 

Front Wheel Rates range from 100lbs/in to 500lbs/in springs.

 

Rear Wheel Rates range from 19mm TB's to 27mm TB's.

 

[Henry 1.9GTi 38]

Not to put a dampner on this but isnt it pretty obvious that for the same spring rate the rear will have a higher ride frequency (in Hz god damn it! )

 

Not sure what use the graph is, enlighten me please.

 

Without churning some numbers in a lap time simulator I can't see how this information enables an informed decision to be made on what rates to choose. Going by shopping trolley racing standards with low budgets it's the stop watch that counts as well as driver feel and confidence which will also help said watch. Again expensive to change rear bars and records lap times though. Also without having any tyre data it would be difficult to select spring rates based upon grip and only possible to tune balance which could be done with rear arbs, cheaper and do-able on the day. Make it stiff enough for response time and to negate roll camber then get it driving how you want.

 

Sorry if I have missed the point, I did kind of skim read! All good info anyway and fun as well I would say