What Front Rates To Match Rear 26Mm Tb's And 28.5Mm Arb?

[B1ack_Mi16 67]

As topic says.

 

I have a set of torsion bars 26mm and one ARB of 28.5mm lying to no use, while my track car still have standard 309 bars and arb.

 

The front of the track car is set up with 300 lb springs, and I guess this might be on the soft side to match the 26/28.5mm rear setup?

 

Downside is that I'm actually driving the car to and from the events, so I guess it will be even more uncomfortable on the transport stages.

[Cameron 16]

350lbs matches 24mm TB's so you're looking at something like 400-425lbs!

 

Are you on slicks for the events?

Edited May 30, 2011 by Cameron

[B1ack_Mi16 67]

I'm running A048's at the moment, so semi slicks that is.

 

Maybe I'm better off selling those TB's and ARB and getting some slightly smaller ones?

 

However a guy at the event this week were running GAZ Golds on the hardest setting with 500lb springs at the front and said it was working good. However I need to ask him what size bars he is using.

He was running R888s.

[rallyeash 155]

26mm tb's!! unless its a full out and out circuit racer id say they could be to much?

 

im running 23mm tb's now and a 24mm arb which i mite upgrade in the near future, but thats stiff enough for track days and the odd weekend blast.

[Cameron 16]

400lbs up front will be fine with A048's. A 26mm torsion bar works out at 360lb/in wheel rate, so the corresponding front spring rate would be something like 450lb/in assuming a 0.9:1 installation ratio (as you want the front to be slightly stiffer then the rear). It'll be great for circuit use but will be very firm on the roads!

 

I'm running 350lb/in springs with 24mm torsion bars on A048's and IMO it's a bit on the soft side, I'd be happier with 400lb / 25mm bars but it's an expensive decision!

[Cameron 16]

Have a look at the chart in this post if you want to know torsion bar spring rates - Click!

 

To find what the front rate should be, divide the rear spring rate by 0.8 then add about 10% and round up to the closest commonly available spring rate.

 

e.g. 26mm TB's = 358lb/in wheel rate.. so 358/0.8 = 447lb/in, +10% = 492.25 or 500lbs!

 

Ps.. the reason you want a difference in front / rear spring rates is to stop the car pitching when you go over bumps, and having the front end stiffer will improve turn-in.

Edited May 30, 2011 by Cameron

[B1ack_Mi16 67]

Ok, nice table.

 

Is it basicly the torsional spring stiffness from the TB superpositioned to the stub axle location?

 

I see 23/24mm rear bars seem to be better in that case..

 

I will actually have to take a look at some other bars I have which might be 24 so that might be a good solution.

However I still only have 21mm or 28.5mm ARB avaliable. Will the 28.5mm one be too stiff for a 24mm TB setup?

 

I just so wish someone someday would come up with a precise CAD geometry of the whole 205 shell, and subframes, at least frone one, as the rear one is easier to just model a little bit by simple measurements.

Would be good to get the whole suspension setup into multibody dynamics simulations software.

 

But when it comes to the front / rear spring rate ratio that the front ratio / weight approximately equals the rear ratio / rate?

 

So if a car has 70/30 weight distibution.. and 300lb/in springs at front, use (300/70 * 30) = 142 lb/in at the rear? Or is this just not a correct way of thinking of this... (I guess it is not, but I can't really see why).

[allanallen 528 2 Cars]

I'm using 25tb/28 arb on my 309 with 500lb springs on 888s. It works well and in all honesty isn't ridiculous on the road. I may drop down to 450 springs as I will shortly have a lot more negative camber at both ends and my wishbones will be running at a more reasonable angle.

As a starting point I'd say 500 though.

[Henry 1.9GTi 36]

I think you actually want the rear stiffer to stop pitching and get a common recovery after a bump. But its down to ride frequency and the rear is alot lighter so will have a higher ride frequency anyway.

 

Also are you saying a stiffer front via spring rates due to the ARB at the rear being so big? As a stiffer front will just make it understeer given its front heavy. Clio cup cars run something like 400lb front 700lb rear last time I looked. (may be completely and utterly wrong there)

[B1ack_Mi16 67]

I guess in my little calculation at the end of the last point was trying to match the frequencies of the front and the rear.

 

Front for example (if each front corner is 275kg):

f = (1/2pi) * (275kg/53N/mm)^0.5 = 0.36 Hz

 

To get a rear frequency of 0.36 Hz then (if 150kg corner weight):

k = (1/2pi)^2 * 150kg/0.36^2 = 29.3 N/mm = 167 lb/in

 

The latter would equal approx 21.5mm torsion bars.

 

But that's just if to match the natural frequency of each corner, and that might not be the point at all afaik

Edited May 31, 2011 by B1ack_Mi16

[Cameron 16]

You don't want to be matching ride frequencies front and rear as it'll want to see-saw down the road after you hit any bump. The idea is to have around 10% difference in ride frequency, and generally you have the front end stiffer to improve turn-in.

 

You're right though, ride rates are only half the story (unless you have perfect weight distribution) so you would need some very stiff springs to get the front end to have a higher frequency than the rear, especially with 26mm TB's. 0.36Hz is fairly low actually, that's with 300lb springs? (good to reference spring weights in "old money" as most people still talk in lbs/in rather than N/mm)

[B1ack_Mi16 67]

You don't want to be matching ride frequencies front and rear as it'll want to see-saw down the road after you hit any bump. The idea is to have around 10% difference in ride frequency, and generally you have the front end stiffer to improve turn-in.

 

You're right though, ride rates are only half the story (unless you have perfect weight distribution) so you would need some very stiff springs to get the front end to have a higher frequency than the rear, especially with 26mm TB's. 0.36Hz is fairly low actually, that's with 300lb springs? (good to reference spring weights in "old money" as most people still talk in lbs/in rather than N/mm)

 

Yes that would be with 300lb/in springs afaik. Just translated approximately from the table in the other post you made yesterday and found that to be in the region of 52-53N/mm.

 

Well. I'll start by taking a look in the shed, if there are 24mm bars there too I think I'll start by trying those first.

[Cameron 16]

24mm definitely feels good on my car, I'm running 350lb/in springs up front but I find them to be a little soft.. I think 400 or even 425 would be better.

 

Henry brought up a good point about stiffer springs making it understeer because of more load transfer, but IMO you get a bigger loss on a 205 from gaining camber in body roll.. a proper back-to-back comparison is definitely needed!

[Henry 1.9GTi 36]

fair point about the camber gain.

 

But you definitely want the rear to have a slightly higher ride frequency than the front to reduce pitching. After all the front hits the bump first, or should atleast

 

Those numbers in Hz are too low, think some error with your calcs.

 

for natural frequency = SQRT(K/M) where k = spring rate in N/m and M is sprung mass in Kg. Gives an answer in rad/s

 

so for front: (SQRT(53000/275))/2Pi = 2.21Hz

 

If the rear is 150kg and you want a 2.5Hz frequency to reduce pitch motion:

 

((2.5*2Pi)^2)*150 = 37011N/m = roughly 23mm TB

 

May want to look at dampers natural frequnecy when calculating these ride numbers. Simply use Damped natural freq = (SQRT(1-(DampRatio^2)))*natural freq

 

Where dampratio = Actual/Crit damping

 

Crit damping = 2*SQRT(KM)

[B1ack_Mi16 67]

Early in the morning..

Was using N/mm instead of N/m..

 

Certainly makes more sense with your updated numbers.

 

Is there readily avaliable replacement bearings to replace the bushes in the dampers? I guess the bushes will distrub the damping a bit..

[Henry 1.9GTi 36]

Also just to build on the camber issue. From my limited knowlege from uni, when the rear wheel lifts, no additional weight transfer can occur at this axel and so for total weight transfer to remain the same it will then occur at the front. So if the car is understeering you want the stiffest setting at the front that still alows maximum weight transfer at the rear i.e. inside wheel kissing the tarmac. But this stiffest front setting will give you the best camber control at the front.

 

Or could just go silly stiff at the rear and proportionally so at the front to get camber control and max weight transfer at the rear. All assuming our nose heavy beasts will understeer. Correct camber is worth ~10% extra grip so a very good point not to be ignored.

 

and when it starts oversteering static camber on the rear should bring it back into line may need 4 deg or more though!

Edited May 31, 2011 by Henry 1.9GTi

[Cameron 16]

Having the rear frequency higher than the front is a road car thing, everything I've been told / read while at uni suggests the front end to be higher to improve turn-in response. I don't know where you're getting the opposite from.

[Cameron 16]

Ok.. had a little play with the figures, to have the front end 10% stiffer than the rear when using 26mm torsion bars, and with a 62.5% front weight distribution (800kg car) you would need 500lb/in front springs.

 

To match 24mm torsion bars with the same assumptions as above, you'd need 375lb/in springs.

[feb 47]

Does anyone know the weight distribution of a standard 205, are the above weight figures real or a randon figure?

 

The weights 275kg per corner front and 150kg per corner rear are equivalent to 73/27 weight distribution which seems too nose heavy IMHO.

Edited May 31, 2011 by feb

[Rippthrough 98]

Having the rear frequency higher than the front is a road car thing, everything I've been told / read while at uni suggests the front end to be higher to improve turn-in response. I don't know where you're getting the opposite from.

 

'cause when you have to deal with bumps having the rear rate higher settles the chassis faster.

The actual different depends on the average speeds, for a track car 5-10% would fine, road work is slower so at least 10% is needed, start forest rallying and you may well find you need more than that.

Edited May 31, 2011 by Rippthrough

[Cameron 16]

I think as standard it's something like 62% front..

 

Lots of people seem to remove a lot of weight from the rear and stick heavy iron block 16v engines in the front, I think this is where some of the very front-biased WD figures are coming from.

 

I'd love to get my 205 on some corner weights, with the weight reductions I've done I'd hope it was something around the standard figure.. can't be sure though!

[feb 47]

I would be curious to know what the spring rates of the SBC springs are (phase I kit) as with them fitted together with the phase II package (21mm torsion bars) the car was fantastically balanced (for my taste anyway and I think others who have driven it share the same opinion).

[Henry 1.9GTi 36]

Its difficult to take weight away from the front unless chaning panels is an option. My car is 64% front with me in it. And still have standard panels, heater and leccy windows. Mi16 engine. Its actually 65% front without driver interestingly.

 

Im just going by what Iv'e learnt at uni Cam. Also when talking stiffness in terms of springs we always stuck to steady state stuff. It was only an undergrad course and we only touched on transient stuff. Which was pages and pages of equations regarding yaw response/velocity and how it varied with speed. Moments of interia and such like. I honestly struggle to understand why a sitffer front will aid turn in. You want to maximise front end grip so slowing down the weight transfer will enable to car to turn in; softer dampers. Or stiffer rear dampers. However, stiffer certainly adds feel and quickens response times. But may be hiding other issues. When my car snapped its rear ARB, I was effectively on sprintlines. so? 150-200lb? front and 19mm TBs with no ARB. Alot stiffer at the front then. It was truely awful eveywhere. Wouldn't turn in, understeer mid corner, and wheel spin in 4th with a meager 160bhp. No diff ofcourse at that point. Just felt like the rear was being dragged around and at no point did the rear feel like it was aiding by getting the nose in.

 

I think when trying to understand the transient response things just get mind boggling and require alot of base data about the car to make any real interpretation. Data which from a 205 can't really be got that easily. Best to just pick a rear setup and then go out and drive the thing with varying spring rates at the front seeing as they are cheap and easy to change. On coilovers anyway.

[calvinhorse 870]

id like to be the first to say you need 500lb springs

[Cameron 16]

Well, turn-in response and ride frequency pretty much go hand-in-hand.. so a stiffer front spring rate will mean a sharper turn-in. What that means once you get into "steady-state" i.e. mid-corner I can't be sure as there are so many factors to consider, but I have a hunch that you gain more from keeping good geometry than you lose from increasing load transfer.

 

I'm going to agree with Calvin, 500lbs with 26mm TB's sounds right - just very very firm for road use!

 

I agree with matching front springs to whatever you have at the rear, unless of course you're made of money! I think next time I go out on track I'm going to go armed with a set of 400lb springs and swap them over with the current 350's over the lunch break, see whether it does improve things!

Edited May 31, 2011 by Cameron