Torsion Bar Rates

[EdCherry 34 1 Cars]

Right so doing a few calc's and verifying things. Im not one to take someones word as gospel so did some digging. This graph that everyone is using for torsion bar rates is in my calculations for anti roll bar rates.

 

Here it is:

 

 

And my calculations are:

 

Total Arm Length - Stub Axle to Beam Pin = 245mm / 9.645in

Leverage on Torsion Bar = 195mm / 7.677in

Torsion Bar Effective Length = 933mm / 36.732 in

 

Using the equation:

 

1150000 x (Diameter^4) / (Effective Length (Arm Length^2)

 

I get:

 

19mm Tb = 166lbs/in

25mm Tb = 498lbs/in

 

These seem much more reasonable, can anyone confirm my thinking/calcs are correct? If so will publish my spreadsheet which lists TB rates and ARB rates for the rear.

 

The wheel frequency seems a bit low with those torsion bar rates compared to the front, but its even lower with the ones listed on that other spreadsheet.

[EdCherry 34 1 Cars]

Ignore my comment on the wheel frequency seems I double squared a calculation which messed everything up give me 40 CPM lower than the front suspension! Once reversed it gave me the 15 CPM more than the front wheel frequency I was hoping for.

[Cameron 16]

If you're working this out as a bending beam then stop for a minute and think about how a torsion bar is actually loaded, and the reason they have splines on the end.

[EdCherry 34 1 Cars]

Dont think I am working it out as a bending beam. I think the figures I get are correct just wanted a bit of confirmation as I haven't done this stuff for a while, no real need to recently!

[welshpug 1,646]

where does the "leverage on torsion bar" figure come from?

[EdCherry 34 1 Cars]

Length from the Stub Axle to the Torsion Bar.

[Tom Fenton 1,536]

I did some calcs on a previous discussion some time ago, came up with the bending of the bar accounting for 3% of wheel rate compared to 97% being due to torsion applied to the bar.

[petert 578]

I suggest not worrying about the calcs but rather the lap timer. Record times and mods, make analytical decisions.

[Cameron 16]

I did some calcs on a previous discussion some time ago, came up with the bending of the bar accounting for 3% of wheel rate compared to 97% being due to torsion applied to the bar.

 

This.

 

I'm not sure about your number 115x10^4.. where has that come from?

[EdCherry 34 1 Cars]

Tom - I read that, and decided it was irrelevant that fact there is bending on the bar. Similarly to correction factor for the angle of the front coil, although in the end I decided to use this just for peace of mind.

 

Peter - All very well, but at the end of the day there has to be a starting point.

 

Cam - Errrr id have to get back to you on that. Its somewhere in one of my notes/books/searchs that suggested going do that sort of route, its probably rather pointless of me finding it. Could you suggest a better equation?

[Cameron 16]

I wasn't commenting on whether it was right or wrong, I don't know any equations off the top of my head, like you everything I have is written down somewhere (or hastily googled). Just wanted to know where you got it from as it's an unexplained number.

[EdCherry 34 1 Cars]

Unexplained indeed, and not sure even if I looked at my notes it would explain the number to be honest.

 

Just hoping that my calculations were somewhere close to the right area. Rear CPM is 15 higher than Front CPM, maybe im getting too hung up over wheel frequencies but it gives a nice basepoint as to what the original designers idea/intended balance was for the car and somewhere to work from?

 

I just want to go back to building bloody cars and running them rather than designing them!

[Cameron 16]

For road cars that's pretty normal I think.. it's generally race cars that have higher front ride frequencies.

[EdCherry 34 1 Cars]

Right, did a bit more investigation today to confirm a few things with an ex lecturer of mine.

 

Seems that table of figures is closer to correct, although still need to edit my spreadsheet to accommodate my error which I will do later.

 

Cameron, im not so sure on that but not going to argue as different horses for different courses after all! It seems a fair few have almost identical wheel frequencies within about 3-4 and work pretty well though.

[DamirGTI 340]

http://www.websworld.org/marcel/309/tech/rollrear.php

 

Damir

[Sandy 191]

I thought the table looked pretty good, from the tests we've done, the reality often seems alot softer than the theory.

[Cameron 16]

Cameron, im not so sure on that but not going to argue as different horses for different courses after all! It seems a fair few have almost identical wheel frequencies within about 3-4 and work pretty well though.

 

I did say generally..

[EdCherry 34 1 Cars]

Cameron, soon I will hopefully be able to agree/disagree. After this vast discussions and explorations of different equations for the rear beam we came back to one every time. So lets hope were right! It pretty much tally's with the above table, but also means fitting 24mm rear bars gives a much changed wheel frequency balance for my intended front spring rates.

 

Sandy, are you speaking that through your experience a softer rear works better? Im not a big believer of that, and a pretty big believer that a FWD saloons should have a loose rear end to get it round a circuit quick. Don't want it too loose though, it is me driving after all, already got bets on for how many laps before I end up in the wall.

 

Damir, calculator is going to help me much, but may help some others on here to get a rough idea.

[Sandy 191]

No, what I mean is the torsion bar effective spring rates are softer than theory suggests. Furthermore, I would say aftermarket bars are relatively softer by diameter than OE. I think 15-20cpm higher at the back as has been suggested. Peak rear grip is rarely a problem, but the rear grip progression can be using compromised damper set ups and stiff ARBs and getting that right is important to making the front work, given the heavily skewed roll centre axis these cars have.

Edited August 24, 2011 by Sandy

[Cameron 16]

Cameron, soon I will hopefully be able to agree/disagree. After this vast discussions and explorations of different equations for the rear beam we came back to one every time. So lets hope were right! It pretty much tally's with the above table, but also means fitting 24mm rear bars gives a much changed wheel frequency balance for my intended front spring rates.

 

Sandy, are you speaking that through your experience a softer rear works better? Im not a big believer of that, and a pretty big believer that a FWD saloons should have a loose rear end to get it round a circuit quick. Don't want it too loose though, it is me driving after all, already got bets on for how many laps before I end up in the wall.

 

Damir, calculator is going to help me much, but may help some others on here to get a rough idea.

 

If you come up with some accurate figures then that'll be great! I'd been using that table as a guide because it looked like the right sort of ballpark, but never thought about proving it right / wrong so whatever your outcome is you're doing us all a favour.

 

I'd agree that for a 205 a stiffer rear is better, since the weight is over the front wheels and they have a tough job with driving and steering at the same time it makes sense to give them a break by stiffening up the rear. Hence why I'm deciding to fit a 23mm ARB!

[EdCherry 34 1 Cars]

Sandy, mind expanding on the comment about aftermarket bars?

 

Cam, happy to publish my rates for the rear bars later, but once again they are my calculations so verifying is a worthwhile thing, kind of why I started this! Out of interest what rear bars and front spring rates are you running currently?

[Cameron 16]

What you should do is calculate the wheel rate for your current bars then validate it by measuring the wheel rate on your car, then you can get an idea of whether your calcs are right.

 

Shouldn't be too hard to do either, as long as you disconnect the ARB.

 

I'm running 24mm TB's and 350lb springs, though I reckon it would be more suited to 400lb. I would see what difference a 23mm ARB makes but I have a feeling I won't be out on track again this year.

Edited August 24, 2011 by Cameron

[EdCherry 34 1 Cars]

It depends if I get a chance to borrow the corner weights again as if I can validate it on standard suspension. I guess when the new stuff goes on I could though.

 

Whats the car like in the wet??

[petert 578]

Sandy, mind expanding on the comment about aftermarket bars?

 

Very few heat treat them properly, if at all.

[Cameron 16]

I don't like to push in the wet, I've spent too much time and money building it to risk binning it.

It's strictly a dry weather car!