Spring Rates

[Whiskey11]

And the one that doesn't happens to be one of the largest markets for high-performance aftermarket equipment...

"Down with Metrics, we don't want no foreign rulers!"

The bottom line here is that whether you measure in kg or Lbs, you need to use the system that MOST of the people you're talking to understand...

So, I was approaching a standard 1.5705 radian right-hand corner the other day... Or maybe was it closer to 350,000 arc-seconds... ;-)

I like it when you talk dirty (nerdy?) to me!

 

[Pentalab]

And the one that doesn't happens to be one of the largest markets for high-performance aftermarket equipment...

"Down with Metrics, we don't want no foreign rulers!"

The bottom line here is that whether you measure in kg or Lbs, you need to use the system that MOST of the people you're talking to understand...

So, I was approaching a standard 1.5705 radian right-hand corner the other day... Or maybe was it closer to 350,000 arc-seconds... ;-)

Last time I checked, all the tools required for the car were metric. The tires are in metric...and the 2015 car is now a global platform. I use both....so no big deal. I do see a lot of errors when reviewing written material from europe. Europeans can't do imperial / SAE....and americans can't do metric.

My 30 yr old son doesn't understand sae at all..only metric. "get me a chunk of 2x4. He comes back with a small sheet of plywood ! "grab a 1/2" bolt over there". "how many cm is that dad " ? WTF ? "1.25cm"

Metric does have advantages when building stuff though. I'd rather deal with mm... vs screwing around with 64ths of an inch...esp when spacing stuff like meters and gauges across a panel.

Most of the metric stuff I still can't relate to at all....I have to convert it 1st. Still..... if you are going to export products and services....at least do it in both.

 

[sheizasosay]

I will use English sockets on metric nuts out of spite.

 

[frank s]

I will use English sockets on metric nuts out of spite.

I have a dedicated set of Whitworth wrenches for just that purpose.

 

[sheizasosay]

I have a dedicated set of Whitworth wrenches for just that purpose.

Well I just enjoyed myself a nice history lesson.

 

[leviathon]

now this thread has me thinking about adjusting my saleen coilover spring rates

 

[Pentalab]

I don't understand something. Why does the Boss 302 have heavier springs on the rear...and lighter ones on the front ?? The oem GT cars are the same way too.
I have no clue what the spring rates on my Roush "street-race" suspension are... but they are a helluva lot stiffer vs oem GT.

 

[Whiskey11]

I don't understand something. Why does the Boss 302 have heavier springs on the rear...and lighter ones on the front ?? The oem GT cars are the same way too.
I have no clue what the spring rates on my Roush "street-race" suspension are... but they are a helluva lot stiffer vs oem GT.

Because motion ratio and decent front roll center height. The motion ratio of the rear springs in roll is awful in comparison to the front and if the springs were the same the rear end would flop around worse than it does stock.

 

[sheizasosay]

Because motion ratio and decent front roll center height. The motion ratio of the rear springs in roll is awful in comparison to the front and if the springs were the same the rear end would flop around worse than it does stock.

The boss has larger rear tires. Aren't GT's square?

 

[Sky Render]

The boss has larger rear tires. Aren't GT's square.

Yep, GTs are square. 235s on non-Brembo cars and 255s on Brembo cars. At least that's '11+ GTs.

Sent from my toilet using Tapatalk

 

[Pentalab]

Because motion ratio and decent front roll center height. The motion ratio of the rear springs in roll is awful in comparison to the front and if the springs were the same the rear end would flop around worse than it does stock.

Most road course folks have heavier springs on the FRONT....and lighter ones on the rear. The Boss is reversed from that. Most versions of the boss have 285mm rear tires and 255 mm front tires. I think the track version of the boss uses a square setup? The story I got with the 285 /255 setup is the car will want to oversteer even more with any staggered setup....so the fix for that is a thicker rear sway bar. I think the boss uses a 25-26-27mm rear sway bar.

None of that explains why the boss uses heavier rear springs vs the typ road course setups you folks use..with the heavier front springs. What am I missing here ?

 

[sheizasosay]

Most road course folks have heavier springs on the FRONT....and lighter ones on the rear. The Boss is reversed from that. Most versions of the boss have 285mm rear tires and 255 mm front tires. I think the track version of the boss uses a square setup? The story I got with the 285 /255 setup is the car will want to oversteer even more with any staggered setup....so the fix for that is a thicker rear sway bar. I think the boss uses a 25-26-27mm rear sway bar.

None of that explains why the boss uses heavier rear springs vs the typ road course setups you folks use..with the heavier front springs. What am I missing here ?

I agree you always want more front spring than rear. But your comment about any staggered setup will want to oversteer doesn't make sense. So if I had 255's on all four tires and the car understeered and then I changed the rear from 255s to 285s, I would expect greater understeer. Now add more rear bar or spring to get back to previous handling bias.

 

[csamsh]

Most road course folks have heavier springs on the FRONT....and lighter ones on the rear. The Boss is reversed from that. Most versions of the boss have 285mm rear tires and 255 mm front tires. I think the track version of the boss uses a square setup? The story I got with the 285 /255 setup is the car will want to oversteer even more with any staggered setup....so the fix for that is a thicker rear sway bar. I think the boss uses a 25-26-27mm rear sway bar.

None of that explains why the boss uses heavier rear springs vs the typ road course setups you folks use..with the heavier front springs. What am I missing here ?

You're missing a couple things.

Yes, it's a 255/285 stagger. No, a stiffer rear bar makes you oversteer more- stiffer bar=less net grip at that end of the car. No, a stagger makes understeer, not oversteer.

My guess: Ford wanted a staggered setup for the looks (let's face it, most bosses are garage/show queens) so they stiffened the rear springs and bar (25mm IIRC) to make it feel more like a regular GT- more or less removing the extra bias that would have been gained from the larger rear tires.

Why didn't Ford put a 285 on the front? My guess: they didn't want people bringing the cars in, complaining about tramlining. There was a video out about the Shelby that said as much. Also, understeer is good for lawyers. Ford doesn't want stories about "that Mustang that spins for no reason" when inexperience/unskilled people decide to hit that traction control button.

 

[Whiskey11]

Most road course folks have heavier springs on the FRONT....and lighter ones on the rear. The Boss is reversed from that. Most versions of the boss have 285mm rear tires and 255 mm front tires. I think the track version of the boss uses a square setup? The story I got with the 285 /255 setup is the car will want to oversteer even more with any staggered setup....so the fix for that is a thicker rear sway bar. I think the boss uses a 25-26-27mm rear sway bar.

None of that explains why the boss uses heavier rear springs vs the typ road course setups you folks use..with the heavier front springs. What am I missing here ?

No kidding?

What you are missing here is the fact that our front roll centers are below the ground because of how low the car is. You are also leaving out the fact that strut cars have piss poor camber curves when lowered so stiffer springs are necessary to combat those two effects to keep the front tires happier. The Boss 302's are not immune from positive camber and piss poor roll center location. A stock Boss 302 wallows and rolls like a tug boat in a tsunami compared to an S197 with a coilover setup with mild springs and the added front spring rate helps keep the tires happier by keeping the front suspension in a better part of the camber curve. Think Chapman's Law when it comes to the front suspension.

The old adage that stiffer front = understeer is ONLY true when your overall roll stiffness is sufficient to keep the tires happy. None of the stock springs on ANY of the S197's are sufficient to satisfy that need. I predict that the actual spring rate necessary to accomplish something similar to that is somewhere around 300-350lbs/in up front and 225-250lbs/in out back. I don't have any data to back that up so don't ask for it, it's an educated guess. Beyond that, added front spring rate relative to the rear will create understeer if not compensated for.

There are a multitude of reasons why the Boss 302 and GT springs are "reverse" from our springs and it has to do with roll center location, ride quality, camber curve, and driving dynamics. When I talked about the rear springs having a worse motion ratio than the fronts in roll I was not joking. I don't have the numbers here but it's something like .6:1 and the resulting wheel rates are pretty damn low compared to the front. In bump those spring rates are added together and I suspect there is a healthy dosage of "flat ride" theory going on there to hit their ride quality objective.

The staggered wheels/tires certainly plays into it because a wider contact patch at the rear increases understeer, so more rear spring rate is needed to counteract that tendency (which never really goes away with a staggered setup) and it comes in the form of a little more rear spring rate and a lot more rear bar.

If you want a better answer than that then bug the Ford engineers who worked on the cars and see why they did it.

 

[kcbrown]

No kidding?

What you are missing here is the fact that our front roll centers are below the ground because of how low the car is.

But that's only because you've lowered the car.

It seems the roll center will remain above ground so long as you don't lower it by much (half an inch or so).

So I guess that raises the question: how much of an advantage do you really get by lowering the car, as opposed to merely increasing the spring rates while keeping the ride height constant?

 

[sheizasosay]

Somebody pick up the phone and call Vorshlag and tell them their front roll center is underground. Hurry before the RC migrates from China to Bangcock!

 

[Whiskey11]

But that's only because you've lowered the car.

It seems the roll center will remain above ground so long as you don't lower it by much (half an inch or so).

So I guess that raises the question: how much of an advantage do you really get by lowering the car, as opposed to merely increasing the spring rates while keeping the ride height constant?

Reducing the height of the CG is pretty important too. Even with the added body roll associated with messed up Roll Centers, the reduction in weight transfer from lowering the CG which keeps the tires more evenly loaded.

 

[SoundGuyDave]

All things in balance, though! Let it ride too high and you have a LONG lever (polar moment), and excellent geometry. Drop it too much, and the reduced travel will hurt you more than you gain from the reduced polar moment. Additionally, if you lower it too much, you wind up compromising the geometry so much that you really need to compensate for it (ball joints, arm mount relocation) or you wind up with less net grip. There IS a sweet spot, but it's also something that will vary with track type. Rough, choppy track with lots of berms to run over? Travel is more important than roll mitigation. Smooth track? Keep the roll down. Personally, making changes from track to track outside of damper settings and tire pressures is a bit much for my crew of three (me, myself, and I), so I found what I think is a happy medium. How many inches down from stock? No clue. I set the car up where it "feels right" on a variety of tracks, then did the corner-balance thing. Done.

I haven't changed gross ride height since, but I have tweaked some of the suspension angles. Iterative changes. I've gotten to the point where I need more front grip, but am out of camber, and a little afraid of adding more spring, but that's really the next thing on the list. Yes, more spring up front means more understeer, but I'm hoping the increased contact patch under lateral load will give me a net grip increase. I'm hoping to keep the rear grip the same, since it's perfect under track-out. Now I just need to work on the grip up front at turn-in.

 

[Pentalab]

Another method to lower the CG is to use smaller diameter tires...like 25" tall tires...which may well not look too good. Another method would be to lower the eng and tranny. When my LT's were installed, I replaced the oem motor mounts with the steeda heavy duty type. The steeda eng mounts allow for a stock height..or a 1/4" - 1/2" - 3/4" drop. I used the 1/4" drop...so my steeda stb clears my roush air intake tube a little better. If you go too low... I think the steering shaft gets too close to the LT's. It's a simple way to lower the CG (at the font of the car).

The stiffer bushings used on the steeda eng mounts will mitigate the eng osc back and forth vs oem hydra mounts. (IE: if you had a shaker hood..it won't shake).

Since the eng + tranny are heavy items.... dropping em down .25-.5-.75" would lower the CG a bit...without having to use lowering springs. Of course lowering springs + reduced eng /tranny height can both be done at the same time. A 1" drop via front springs + an additional .25 - .5" eng /tranny drop via new eng mounts should make for a substantial redux in CG.

 

[Pentalab]

You're missing a couple things.

Yes, it's a 255/285 stagger. No, a stiffer rear bar makes you oversteer more- stiffer bar=less net grip at that end of the car. No, a stagger makes understeer, not oversteer.

My guess: Ford wanted a staggered setup for the looks (let's face it, most bosses are garage/show queens) so they stiffened the rear springs and bar (25mm IIRC) to make it feel more like a regular GT- more or less removing the extra bias that would have been gained from the larger rear tires.

Why didn't Ford put a 285 on the front? My guess: they didn't want people bringing the cars in, complaining about tramlining. There was a video out about the Shelby that said as much. Also, understeer is good for lawyers. Ford doesn't want stories about "that Mustang that spins for no reason" when inexperience/unskilled people decide to hit that traction control button.

Must have been brain dead. Yes, a staggered setup results in more understeer....and the fix for that is a bigger rear bar.

OK, at what point does tramlining occur ? I don't get tramlining with my 255 fronts + 285 rear setup. I thought tramlining occurs when 275 fronts are used... (or bigger) .

Does tramlining occur from the use of wider fronts..or does the rear width play any part at all ?

I never hear the Roush folks complaining of tramlining with their square 275mm setups. ( 10" rims front + back). I have often thought that a square 285mm setup would be optimum..and 285's would easily fit a 10" rim.

I thought for sure in the summer of 2011, that I had tramline issues. Turns out the toe was outa whack. Then I found out their is no camber adjustment on these cars..so in went the steeda mounts..so at least I could get the desired camber I wanted. Stock, it was -1.1 on the pass side..and -1.4 on the driver's side. Within ford specs... but at least let's make em both the same.