Spring rates on 30mm torsion bars
11-17-2009, 10:33 PM
#31
Rennlist Member
Absolutely right Patrick. There are times where some body roll, and an alignment to compensate for that, can actually yield faster time - you're getting the same contact patch, but more consistent grip in bumpy corners because of the softer suspension.
I was reading a book the other day that said your spring rates should be the softest you can get away with.
Like everything in racing, it's a double, or even multi-edged sword - you give a little here to take a little there and try to find the best compromise.
I was reading a book the other day that said your spring rates should be the softest you can get away with.
Like everything in racing, it's a double, or even multi-edged sword - you give a little here to take a little there and try to find the best compromise.
11-18-2009, 02:23 AM
#32
Rennlist Member
I guess that's where some of these more modern shock absorbers come in with their high and low speed release valves. I found with the KWs that they were more compliant than my old modified Mo30s, yet I had much stiffer spring rates. Is that the basic premise of variable rate springs in that they allow a certain compression value but get stiffer the more you apply pressure?
11-18-2009, 07:10 AM
#33
Three Wheelin'
I find that 944's spend more time on the bump stops than most people realise. Getting the ride height correct makes a big difference to understeer/oversteer balance and, IMHO, is just as important than spring rate. My Bilstein Escort rear c/overs still spend time on the (shock) bump stops even with 850lb/in springs and a decent ride height. H&R are very specific with ride height settings in their RSS kit which is based on the Bilsteins. Use the bump stops too much and you'll get oversteer and run the very real risk of breaking the c/over mount bolt. RS Barn modify the bump stops to get more travel.
My $0.02...
Mike
My $0.02...
Mike
11-18-2009, 12:17 PM
#34
Nordschleife Master
I've heard of a few theories on front vs rear wheel rates (suspension frequencies) - the "primary" theory is that you want a slightly higher suspension frequency in the rear because, as you drive over a bump, the front wheels hit the bump first, so the rear frequency needs to be a little higher (more cycles per minute) so the oscillation of the rear "catches up" with the front.
This may be true for true purpose built race cars that have double-wishbone suspensions at all 4 corners, but I haven't found it to be true for 944s.
As a general rule of thumb, the "stiffer" a suspension, the "less" grip it'll make. (When the car is in mid corner, the more supple the suspension, the more the tires' contact patch will stay in contact with the road.) This is why, if you're feeling a little over steer, you soften your rear sway bar, or stiffen your front sway bar, to compensate and balance the car.
Making the rear of the car too stiff will make the rear "loose" and promote oversteer. Going a little stiffer in the front will promote understeer - which most people like more.
Does that answer your question Alan? The best thing to do is to corner balance your car once the engine is in, then select a "target" suspension frequency. Get springs to match that, then evaluate how it feels while you drive it and see if you want to fine-tune by adjusting one of the rates a little bit.
This may be true for true purpose built race cars that have double-wishbone suspensions at all 4 corners, but I haven't found it to be true for 944s.
As a general rule of thumb, the "stiffer" a suspension, the "less" grip it'll make. (When the car is in mid corner, the more supple the suspension, the more the tires' contact patch will stay in contact with the road.) This is why, if you're feeling a little over steer, you soften your rear sway bar, or stiffen your front sway bar, to compensate and balance the car.
Making the rear of the car too stiff will make the rear "loose" and promote oversteer. Going a little stiffer in the front will promote understeer - which most people like more.
Does that answer your question Alan? The best thing to do is to corner balance your car once the engine is in, then select a "target" suspension frequency. Get springs to match that, then evaluate how it feels while you drive it and see if you want to fine-tune by adjusting one of the rates a little bit.
Stiffer springs (up to a point of course) give more grip. The full race purpose built 944's have 400# springs or higher (for grip) and the street 944s have 250# springs (for less grip but more comfort).
I do agree its all a big compromise of what you want, but if a softer spring gave more grip, why not keep the stock springs, and use a piece of 2" schedule 40 pipe for a sway bar to prevent body roll (exaggeration used to illustrate my point)?
NASCAR cars ride on the bump stop in the turns, thus eliminating the spring all together, to make the most grip, and to make the car rotate. This means that the front (with essentially no spring) is much more stiff than the rear, so the front has grip and the softer rear has less grip.
A car with a mid or rear engine setup will have stiffer springs in the rear, but that's just to compensate for the added rear engine weight.
I think our cars (with 50/50 weight) should have a balanced setup front to rear, but to make more oversteer, I think you should use less sway bar, or SOFTER t-bars.
11-18-2009, 05:59 PM
#35
Rennlist Member
I've always been told the opposite is true.
Stiffer springs (up to a point of course) give more grip. The full race purpose built 944's have 400# springs or higher (for grip) and the street 944s have 250# springs (for less grip but more comfort).
I do agree its all a big compromise of what you want, but if a softer spring gave more grip, why not keep the stock springs, and use a piece of 2" schedule 40 pipe for a sway bar to prevent body roll (exaggeration used to illustrate my point)?
NASCAR cars ride on the bump stop in the turns, thus eliminating the spring all together, to make the most grip, and to make the car rotate. This means that the front (with essentially no spring) is much more stiff than the rear, so the front has grip and the softer rear has less grip.
A car with a mid or rear engine setup will have stiffer springs in the rear, but that's just to compensate for the added rear engine weight.
I think our cars (with 50/50 weight) should have a balanced setup front to rear, but to make more oversteer, I think you should use less sway bar, or SOFTER t-bars.
Stiffer springs (up to a point of course) give more grip. The full race purpose built 944's have 400# springs or higher (for grip) and the street 944s have 250# springs (for less grip but more comfort).
I do agree its all a big compromise of what you want, but if a softer spring gave more grip, why not keep the stock springs, and use a piece of 2" schedule 40 pipe for a sway bar to prevent body roll (exaggeration used to illustrate my point)?
NASCAR cars ride on the bump stop in the turns, thus eliminating the spring all together, to make the most grip, and to make the car rotate. This means that the front (with essentially no spring) is much more stiff than the rear, so the front has grip and the softer rear has less grip.
A car with a mid or rear engine setup will have stiffer springs in the rear, but that's just to compensate for the added rear engine weight.
I think our cars (with 50/50 weight) should have a balanced setup front to rear, but to make more oversteer, I think you should use less sway bar, or SOFTER t-bars.
No, I think you definitely understood something wrong there.
When you compare stiffness front to back, the stiff end will definitely have less grip (that's the end that will end up doing most of the work to resist body roll).
You never want to ride on the bump stops, because that will give that end infinite stiffness and a major loss of grip.
And I can't see that Nascar ever wants their cars to rotate.
However, totalling up the front and rear stiffness and assuming good balance, it would be hard to say that a stiff car (as a whole) would have less grip than a softer car. The stiffer car resists body roll, which keeps a better alignment, which manifests itself to better contact patches, which would be more grip. There's also a better relationship between feedback to the driver and driver input, in a stiffer car. However the weight transfer is not quite as harsh and the suspension loads a little more gradual in a softer sprung car, which can equal more grip, especially when it's a little more bumpy. So, that's where there's the comprimise that others are saying.
11-18-2009, 07:34 PM
#36
Rennlist Member
Your competitors don't want you to be very fast. 
Being on the bump stops isn't always a "bad" thing. Many people will put in custom or modified rubber bump stops (those foam parts around the shock shafts) to become a "second spring rate". Effectively running a "dual rate" setup.
I know this is the case in NASCAR.
Being on the bump stops isn't always a "bad" thing. Many people will put in custom or modified rubber bump stops (those foam parts around the shock shafts) to become a "second spring rate". Effectively running a "dual rate" setup.
I know this is the case in NASCAR.
11-18-2009, 07:42 PM
#37
Nordschleife Master
So, soft springs = grip ..... while everyone would give away the stock setup on our cars for the 250# welts, at the least?
And, stiff springs = less grip .... but bump stop good for a super stiff spring rate for grip?
Why isn't my question valid then? Every supercar and race car should have 100# springs and a 3" sway bar to control body roll.... (again, exaggeration to show point) I know this is all very complex and all must work together .... I'm just not sure I agree with "softer is better".
And, stiff springs = less grip .... but bump stop good for a super stiff spring rate for grip?
Why isn't my question valid then? Every supercar and race car should have 100# springs and a 3" sway bar to control body roll.... (again, exaggeration to show point) I know this is all very complex and all must work together .... I'm just not sure I agree with "softer is better".
11-18-2009, 10:14 PM
#38
Rennlist Member
Mark, it's not just as simple as "softer springs are better" - because, softer springs give you more body roll, and body roll changes your contact patch with the road.
But, there is a happy medium, of course. Remember, you want the softest "practical" spring rate. Anyone running 600+ spring rates on the front of a 944 is probably running too much spring - anyone running less than 250, not enough. (This is for a race/track car.)
To answer your question more specifically, about the light springs and the heavy sway bars, you're going to get a lot of "fore/aft" pitching (dive under braking, squat under acceleration) - characteristics supercars shouldn't have. Plus, not only does a swaybar prevent body roll, but it also "couples" the left and right suspension together. If the outside wheel hits a bump, you want it to move independently from the inside wheel.
Regarding bump stops / dual rate springs, when the car is putting equal weight on all 4 wheels, it needs less spring rate to counter act that weight. When a car is loaded on one side, that's more weight pushing down on the spring - so, to get the same "suspension frequency", the spring should have an increased weight. The bouncing/dampening factors are different when your car is traveling straight, versus when your car is cornering.
But, there is a happy medium, of course. Remember, you want the softest "practical" spring rate. Anyone running 600+ spring rates on the front of a 944 is probably running too much spring - anyone running less than 250, not enough. (This is for a race/track car.)
To answer your question more specifically, about the light springs and the heavy sway bars, you're going to get a lot of "fore/aft" pitching (dive under braking, squat under acceleration) - characteristics supercars shouldn't have. Plus, not only does a swaybar prevent body roll, but it also "couples" the left and right suspension together. If the outside wheel hits a bump, you want it to move independently from the inside wheel.
Regarding bump stops / dual rate springs, when the car is putting equal weight on all 4 wheels, it needs less spring rate to counter act that weight. When a car is loaded on one side, that's more weight pushing down on the spring - so, to get the same "suspension frequency", the spring should have an increased weight. The bouncing/dampening factors are different when your car is traveling straight, versus when your car is cornering.
11-19-2009, 12:12 AM
#39
Nordschleife Master
Again, I agree with everything your saying. except for the spring part. My example was exaggerated to illustrate what I'm trying to point out.
I just had always heard that stiffer springs were better for grip (though I do agree that 600# springs on our cars is to much).
I'm not dis-agreeing with you either. I have not tracked my 944 yet, but I plan to someday. It seems I'll need to read up a bit more on suspension setups before I buy anything for my car.
I just had always heard that stiffer springs were better for grip (though I do agree that 600# springs on our cars is to much).
I'm not dis-agreeing with you either. I have not tracked my 944 yet, but I plan to someday. It seems I'll need to read up a bit more on suspension setups before I buy anything for my car.
11-19-2009, 12:29 AM
#40
Rennlist Member
The pavement on tracks is never perfectly smooth. Let's say you're cornering and you hit a little bump. Your very light spring allows the body (chassis) of the car to stay "still" while your wheel bounces up - the only mass that has to change directions from the wheel bouncing up is the "unsprung weight". The faster this hunk of mass can react and get back down to the road, the better "traction", or "grip", you'll have.
Now, with a very stiff spring, when that same wheel hits that same bump, and it tries to move up, it also has to lift part of the car up (because the spring rate is so high). Now, in addition to the mass of the unsprung components, you also have some of the mass of the chassis that's moving up - and then that mass has to move back down. This weight imbalance causes fluctuations in traction which make it harder for the tires to grip at a constant, maximum force.
Of course dampening (shocks) play a big roll in this too. That's why having adjustable compression and rebound usually let you run higher spring rates.
Here's wheel movement in action:
Now take a moment to envision what a car might look like if it had no suspension (a spring with an infinitely high rate - a solid spring) - think about how much those tires would be bouncing away from the track surface and compromising their contact patch.
Now, with a very stiff spring, when that same wheel hits that same bump, and it tries to move up, it also has to lift part of the car up (because the spring rate is so high). Now, in addition to the mass of the unsprung components, you also have some of the mass of the chassis that's moving up - and then that mass has to move back down. This weight imbalance causes fluctuations in traction which make it harder for the tires to grip at a constant, maximum force.
Of course dampening (shocks) play a big roll in this too. That's why having adjustable compression and rebound usually let you run higher spring rates.
Here's wheel movement in action:
Now take a moment to envision what a car might look like if it had no suspension (a spring with an infinitely high rate - a solid spring) - think about how much those tires would be bouncing away from the track surface and compromising their contact patch.
11-19-2009, 04:46 AM
#41
Burning Brakes
van, you had me until the whole softer spring is better. what were the reasons they gave for that? i just ask because i have found exactly the opposite. well, to a point. and sorry, but i admit that most of my testing has been with the scirocco simply because the parts are so inexpensive. but here's what i've learned:
1) konis are garbage. adjustable or not, complete crap. they blow out. they leak. they continually just self destruct. bilsteins are sooooooo much more durable and effective. they tend to be stiffer all around than konis but ride better. they seem to be more sensitive to small bumps and just absorb them while being stiff enough to control high spring rates.
2) progressive springs are the konis of the spring world. as much as i hate konis, i feel more hatred toward progressive springs. maybe on a car with 20 inches of wheel travel they could be worthwhile, but when wheel travel is limited (as in most european sports cars) use linear. i've seen linear springs transform cars which jounced around on bumpstops into friggin absolute go karts. and i'm assuming we all want cars that go around corners fast enough to make noses bleed.
3) get coilovers and resist the urge to lower it. there's a fine line between lowering to get a lowered center of gravity and lowering too m uch so you have no travel left. on my scirocco, i was amazed how much a quarter of an inch made in the ride quality. in fact i kept raising the car to find more impressive car handling abilities. as it sits, it looks a bit too high for my tastes but you can't argue with the results.
4) ride isn't always best with softer springs. i've found wheel travel to be so much more important than spring rate. in fact, stick with a stiff spring rate and don't lower the car too much and i'll guarantee that you will be able to run way higher rates than you would expect.
ok, now i know some of this doesn't apply as much to the 944 chassis since there is much more wheel travel. however, i've noticed the last few posts have been talking about bumpstops. in my experience, if you're running off the bumpstops, your car will never feel right. i guess it's possible to design a suspension to run off the bumpstops but why the hell would you want to? it seems to me that you've so messed up the spring rate and wheel travel that you're lost. plus, riding on a bumpstop in a curve and hit a big bump and you're headed straight off the road and so fast that you won't even know what happened. with that said, i don't think i've ever hit the bumpstops on my 951--again, does anyone know the full amount of 994 wheel travel? are you guys saying that with 1-2" lowering, it runs out of travel?
and, van, thanks for that last answer to my question. didn't expect the pics but that actually made the whole thing so m uch easier to understand exactly what you did. i'm going to try that on my scirocco as i simply can't believe it's much different from the 944. one more question on that same idea: so you figured the motion ratio as 91%. and then where does the .828 come from? what exactly is the equation for motion rate to wheel rate?
and BTW so sorry to hijack but i figured it was a little too late for that apology!
1) konis are garbage. adjustable or not, complete crap. they blow out. they leak. they continually just self destruct. bilsteins are sooooooo much more durable and effective. they tend to be stiffer all around than konis but ride better. they seem to be more sensitive to small bumps and just absorb them while being stiff enough to control high spring rates.
2) progressive springs are the konis of the spring world. as much as i hate konis, i feel more hatred toward progressive springs. maybe on a car with 20 inches of wheel travel they could be worthwhile, but when wheel travel is limited (as in most european sports cars) use linear. i've seen linear springs transform cars which jounced around on bumpstops into friggin absolute go karts. and i'm assuming we all want cars that go around corners fast enough to make noses bleed.
3) get coilovers and resist the urge to lower it. there's a fine line between lowering to get a lowered center of gravity and lowering too m uch so you have no travel left. on my scirocco, i was amazed how much a quarter of an inch made in the ride quality. in fact i kept raising the car to find more impressive car handling abilities. as it sits, it looks a bit too high for my tastes but you can't argue with the results.
4) ride isn't always best with softer springs. i've found wheel travel to be so much more important than spring rate. in fact, stick with a stiff spring rate and don't lower the car too much and i'll guarantee that you will be able to run way higher rates than you would expect.
ok, now i know some of this doesn't apply as much to the 944 chassis since there is much more wheel travel. however, i've noticed the last few posts have been talking about bumpstops. in my experience, if you're running off the bumpstops, your car will never feel right. i guess it's possible to design a suspension to run off the bumpstops but why the hell would you want to? it seems to me that you've so messed up the spring rate and wheel travel that you're lost. plus, riding on a bumpstop in a curve and hit a big bump and you're headed straight off the road and so fast that you won't even know what happened. with that said, i don't think i've ever hit the bumpstops on my 951--again, does anyone know the full amount of 994 wheel travel? are you guys saying that with 1-2" lowering, it runs out of travel?
and, van, thanks for that last answer to my question. didn't expect the pics but that actually made the whole thing so m uch easier to understand exactly what you did. i'm going to try that on my scirocco as i simply can't believe it's much different from the 944. one more question on that same idea: so you figured the motion ratio as 91%. and then where does the .828 come from? what exactly is the equation for motion rate to wheel rate?
and BTW so sorry to hijack but i figured it was a little too late for that apology!
11-19-2009, 07:26 AM
#42
Rennlist Member
I find that 944's spend more time on the bump stops than most people realise. Getting the ride height correct makes a big difference to understeer/oversteer balance and, IMHO, is just as important than spring rate. My Bilstein Escort rear c/overs still spend time on the (shock) bump stops even with 850lb/in springs and a decent ride height. H&R are very specific with ride height settings in their RSS kit which is based on the Bilsteins. Use the bump stops too much and you'll get oversteer and run the very real risk of breaking the c/over mount bolt. RS Barn modify the bump stops to get more travel.
My $0.02...
Mike
My $0.02...
Mike
Many of us lower our cars too much mainly because it looks sharper, but this is where the problems all start. Doesn't show up on the street, but on the track I was bouncing and bumping from one corner to the next. Really crap and slow. Raised the ride height and presto, it pretty much fixed everything. Free travel of the springs is the key.
11-19-2009, 07:32 AM
#43
Rennlist Member
There is no absolute regarding springs in terms of softer or harder are better. They are just part of the total package that makes up your suspension system. As Van has ably described, both soft and stiff springs have their advantages. Totally depends on so many variables. Plus most setups offer a spectrum of tunability and then each car is different. In the end there is a lot of time and money to go into setting a car up properly. Add driver, track, track surface, temps, tyres, tyre pressure, wind, and what you had for breakfast....it never ends.
11-19-2009, 09:38 AM
#44
Rennlist Member
The equation was back a few posts:
A 91% motion ratio will become .828 when you square it. Someone else asked about the squaring part - here's an explanation: Squaring the ratio is because the ratio has two effects on the wheel rate. The ratio applies to both the force and distance traveled.
Now, to clarify, I'm not saying softer springs are better - but rather I'm trying to clarify, through extreme examples, how softer springs increase traction by keeping the tire in contact with the road better.
Some of the downsides to soft springs: responsiveness goes down - as the car becomes softer and there is more body roll / chassis movement due to transitions, there becomes a greater time delay to initiate your weigh transfer. This is why in a Cadillac, you turn the wheel a little bit, you wait for the car to lean, then it starts to turn.
Another one, of course is consistency of contact patch. As the tire leans in relationship to the road (camber change) the amount of rubber touching the road changes. Less rubber contacting the road = less traction.
All of this brings us back to "suspension frequencies" - the relationship between wheel rates and the weight of the vehicle that the springs hold up. Street cars have a "target" frequency. And race cars have an "optimum" frequency range.
Progressive springs are great in street cars - not as good in race cars, because the "spring rate range" you want it pretty small. And who wants to buy a set of progressive springs that go from 425 to 475 lbs/in?
Again, depending on your setup, on the track, the sweet spot appears to be somewhere between 350 and 550 lbs/inch.
What I mean isn't that 350 is better because it's "softer", but rather, while you're tuning, if you have 475 lb/in springs in the front, and your car is experience a little "push" - a little understeer - and you want to give the front end a little more traction to get rid of the understeer, put in 450 lb/in springs and see what that's like.
11-19-2009, 09:51 AM
#45
Rennlist Member
There is no absolute regarding springs in terms of softer or harder are better. They are just part of the total package that makes up your suspension system...... In the end there is a lot of time and money to go into setting a car up properly. Add driver, track, track surface, temps, tyres, tyre pressure, wind, and what you had for breakfast....it never ends.