BMR SP083 and roll center, bump steer

[meursiicc]

Hi Everyone,

I have a 2021 Ecoboost with FP Track dampers currently on stock springs and I'm researching a set of springs for them. Heavily leaning towards the BMR SP083 springs, as they achieve a pretty desirable ride frequency for me (1.53Hz front, 1.72Hz rear, calculated using motion ratios and Ecoboost weight distribution data from this forum and using my best estimate of unsprung weight). Intended purpose is 50/50 street and local autocross in CAM-C.

My question: will the drop in ride height with the BMR SP083 springs be significant enough to need roll center and bump steer correction?

Thanks,
Eric
P.S., already have Steeda front and rear sway bars, SKU 555 1017.

Sponsored

 

[S550 Recon]

No it will not.

Enjoy

 

[Nagare]

I'd agree with above, I had a greater drop and "lesser" springs (still thought they were awesome - Steeda put together a great combo in my book) and never felt too concerned about needing anything else up there.

I'll be putting the combination in your post to the test soon enough though, my SP083 springs just got delivered and I already had some FP Track shocks/struts waiting to replace on my V6 before I switched to my new Eco. Not doing sway bars yet though.

 

[meursiicc]

Thanks for the replies! Curious if anyone has used the Steeda roll center/bump steer stuff with the BMR springs.

 

[TeeLew]

If you know enough to know what a ride frequency is, then please explain why you chose those. I'm curious.

 

[meursiicc]

If you know enough to know what a ride frequency is, then please explain why you chose those. I'm curious.

https://www.yourdatadriven.com/race-engineering-racing-car-ride-frequency-ranges/

Rough ranges, but the BMR springs seems to put the ride frequency close enough to dedicated race car stiffness but still decent on the road. The mustang is not a dedicated autocross car for me. It has a toddler car seat in the back too.

 

[TeeLew]

https://www.yourdatadriven.com/race-engineering-racing-car-ride-frequency-ranges/

Rough ranges, but the BMR springs seems to put the ride frequency close enough to dedicated race car stiffness but still decent on the road. The mustang is not a dedicated autocross car for me. It has a toddler car seat in the back too.

If performance is even a consideration, the rear should be softer than the front, period. I'm willing to die on this hill. The whole 'flat-ride' thing is WAYYYYYYY oversold. It's practically meaningless and has zero use in our application. People talk about it because it sounds cool. If you want something that doesn't jar the kid's teeth loose and is a hell of a lot better to drive, go with a softer rear spring, like the FP kit.

Anyone who cares enough to change springs in their car doesn't want it with a Flat-ride spring selection.

 

[Egparson202]

FWIW, I’m running 350F/800R. It’s fine on the street and neutral on the road course. Minimal understeer and much easier to put down power from the apex to the track out.

 

[S550 Recon]

I loved my 900+lb/in factory stock rear GT500 springs so much I went to the BMR 980lbin rears. Ride quality is amazing. With readily available dampers/damper technology, 300F and 1000R rates are fine and won't rattle anyone's teeth out.

The BMR SP083 are incredible springs on the street and have won A LOT of races, definitely one of the more proven offerings for the S550 platform. Leave the 800lb/in and less springs for the straight line street folks who love their cars to squat like crazy.

 

[TeeLew]

I loved my 900+lb/in factory stock rear GT500 springs so much I went to the BMR 980lbin rears. Ride quality is amazing. With readily available dampers/damper technology, 300F and 1000R rates are fine and won't rattle anyone's teeth out.

The BMR SP083 are incredible springs on the street and have won A LOT of races, definitely one of the more proven offerings for the S550 platform. Leave the 800lb/in and less springs for the straight line street folks who love their cars to squat like crazy.

A car is like a primate; It has to squat to go. Go with 10,000's for all I care, but it's slow. For some reason people use rear spring as a d!ck measuring contest. It's like everyone wants to prove they're not afraid of oversteer. To be completely honest, I am afraid of O/S for a couple reasons, but the biggest is that is sucks to drive when you're trying to keep the rear under you in every brake zone or every crack of the wheel. Then when you're on the gas, it skips and hops on the bumps and if it does happen to actually break grip, the recovery is sh!t. Why go out of your way to make more life more difficult than it needs to be?

The deal is, most people will hustle an on-ramp from time to time and that's about the extent of their sporty driving. You don't load the front at all on entry, and you're carrying maintainance-throttle the entire time. In this scenario, you've probably got a completely reasonable balance. But, if you drive in a way where you *do* load the front to carry entry speed in a more track-type of a corner and you're scrapping for every bit of grip you can get on exit because that's where the laptime is, you might not want that. The tire and track grip is going to be a big factor in that decision. Streets have no grip, so spring accordingly, or end up like 90% of car meet YouTube videos.

 

[meursiicc]

Love a good debate and seeing the perspectives.

Fact of the matter is, I have a 310hp ecoboost on 10in wheels and 200tw tires. On the autocross course, I have turn in understeer due to too much pitch and roll causing too much camber loss in the front. Evidenced by tire temp and wear. Mid corner is balanced with slight tendency to understeer (which is fine for track, slower for autocross). Never broken traction on corner exit, in face I can't even get the car to rotate out of tight corners with throttle with the sticky tires. Stiffer front springs and more neg camber is the answer for entry understeer, which needs to be balanced with stiffer rear springs.

That said, spring rates and ride frequency are a known thing in race engineering. Along with bars, you can reasonably estimate your lateral load transfer distribution based on front and rear wheel rates to somewhat match your weight distribution.

In fact, for a target ride frequency of 1.8 Hz, which is reasonable, springs come out to 346fr, 1071rr. For a Nationals prepped car of 2.3hz, we're looking at 565fr, 1748rr. That's why I like the BRM SP083 rates. I'd be okay with softer rears, maybe from GT350R.

So the original question of whether or not the the BMR 1.2in advertised drop in the front will lower the roll center too much...I would need to model my front suspension and find roll center/CoG height which I don't really have the tools for, so hoping someone on this forum already did .

Btw on the street, I stick to 5 over the speed limit or less since my blue mustang is a cop magnet.

 

[meursiicc]

I'd agree with above, I had a greater drop and "lesser" springs (still thought they were awesome - Steeda put together a great combo in my book) and never felt too concerned about needing anything else up there.

I'll be putting the combination in your post to the test soon enough though, my SP083 springs just got delivered and I already had some FP Track shocks/struts waiting to replace on my V6 before I switched to my new Eco. Not doing sway bars yet though.

Seems like a proven combo for our cars. What Steeda package did you run before?

 

[TeeLew]

Love a good debate and seeing the perspectives.

Fact of the matter is, I have a 310hp ecoboost on 10in wheels and 200tw tires. On the autocross course, I have turn in understeer due to too much pitch and roll causing too much camber loss in the front. Evidenced by tire temp and wear. Mid corner is balanced with slight tendency to understeer (which is fine for track, slower for autocross). Never broken traction on corner exit, in face I can't even get the car to rotate out of tight corners with throttle with the sticky tires. Stiffer front springs and more neg camber is the answer for entry understeer, which needs to be balanced with stiffer rear springs.

That said, spring rates and ride frequency are a known thing in race engineering. Along with bars, you can reasonably estimate your lateral load transfer distribution based on front and rear wheel rates to somewhat match your weight distribution.

In fact, for a target ride frequency of 1.8 Hz, which is reasonable, springs come out to 346fr, 1071rr. For a Nationals prepped car of 2.3hz, we're looking at 565fr, 1748rr. That's why I like the BRM SP083 rates. I'd be okay with softer rears, maybe from GT350R.

So the original question of whether or not the the BMR 1.2in advertised drop in the front will lower the roll center too much...I would need to model my front suspension and find roll center/CoG height which I don't really have the tools for, so hoping someone on this forum already did .

On the stock front spring, I had two issues. If I really tried to load the front end, then I'd have so much pitch in the car that I'd be entry loose. If I tried to be more gentle on the entry and keep the car more flat, then I'd have an understeer and lack of response. It was difficult to strike the happy medium, if there even was one. What cures this issue is increasing front spring rates. When you increase the front rate, then you'll be able to load the front appropriately on entry. The front end response improves significantly and you don't overcome the camber in the tire so quickly. While it does increase front roll couple distribution, it also *increases* front steady-state grip, presumably by controlling camber better.

I promise you, you're not talking above me with respect to ride frequencies.

There's nothing wrong with your 1.8 Hz overall heave target. The problem is that you're stuck on Flat-Ride and the distribution from front to rear is completely wrong. I don't know how many times or ways I can say this, but Flat-Ride has *NOTHING* to do with performance driving or on-track behavior. It *DOES NOT* control pitch due to braking, which is far and away the most important behavior to address. It was developed by a very clever post-WW2 engineer at GM who wanted the big boats they making at that time to drive comfortably down Route 66 with dampers that were little more than decoration. Having given it its due, it paints you in a corner when trying to apply it racing or even spirited performance driving, because it dictates the car have a VERY loose spring balance. Because of the distribution it dictates, you end up with an undersprung front and an oversprung rear.

-------------------------------------------------------
Side Note:
Flat-Ride is mentioned in RCVD, but even the Milliken's weren't using it in their examples. It's not a racecar thing, but it's not even a performance car thing. Racecar tuning isn't about getting a perfect 4-wheel drift everywhere. A novice driver might pronounce the car balanced, but I guarantee it will be slow. An experienced driver will tell you it's too loose. The car has to brake and carry entry speed. It has to put power down on corner exit. If you have understeer in the middle, then it's up to the driver to address that with his inputs. There will always be imbalances, but tune to optimize the variables which contribute the greatest to lap time. It's a very, very rare thing for pure mid-corner balance to be the stumbling block.

American OEM's and OEM's who tune chassis' in the US (most of them) have accumlated chassis engineers over the last 40 years that all went through the exact same education and all built FSAE cars against each other in school. BMac, who used to hang around here, was a perfect example. We all read the same books and we all learned about Flat-Ride from either Gillespie or Millikan (they learned about it from Olley). We all know that we can hit flat ride springs and reasonable roll couple distributions, so why not do it? Because we just can't control front ride height dynamics and entry stability while arbitrarily limiting front spring rate to produce an unrelated ride quality. Most refuse to step outside the Flat-ride box, and it has made an entire generation of bad handling performance cars, IMO.

Ford seems to be cluing in just a tick with the Mach 1 and FP spring rates. When I see these clueless dudes in Mustangs losing the rear end and stuffing their cars into guardrails, I feel bad for them. 1/2 their problem is car setup. It comes from the factory with sh!t rear grip and that's just reality. When stock, I had mine snap on me several times. It's not a particularly easy car to drive fast when stock, IMO. My car now isn't only faster than stock, it's much easier and more composed getting the speed than with the stock spring rates.
--------------------------------------------------------------------

1.8 Hz is fine, but, if anything, you need to start with the rear heave mode at 85-90% of the front, rather than the other way around. Ohlins was the first to figure this out with their damper package. They ship with a 515#/in front spring and an 800#/in rear. It's not an accident and they are plenty aware of how to set a car up with Flat-Ride. They know what they're doing for the track, though, so they go the other way. I think that's a pretty good front rate for the track with 200tw tires, but it's probably a bit much for the street. Somewhere from 350-450 #/in on the front is completely, fine for the street, though, if you have reasonable rear compliance.

In terms of the ride height changes with these springs, again, I'm more at odds with the distribution than magnitude. The car *DOES NOT* need more static rake. If anything, less seems to be beneficial. There's a really nasty do-loop that BMR and Steeda both have gotten into with their spring rate/drops which is not correct. The first thing is that they've both chased Flat-Ride to some extent or another, so right away, they've gone with less front spring that optimum. Because they don't have the necessary front spring rate, they need another way to get the front to point into the corner. To this end, they stick *A TON* of static rake in the car. This combo drops the front, what, 3/4" more than the rear? That's something like 8 steps less entry stability just because of the new front to rear ride height difference. So now they've figured out they still don't have enough front stiffness, so what do they do? Both of them sell a K-member with a raised front roll-center and Steeda sells arms with extended ball-joints to do the same. They raise the front roll center to regain the entry response they could have gotten front spring. Since they've jacked the tail in the air and undersprung the front, it has too much dive and roll on brakes/entry and you don't have the necessary stiffness to address camber control and keep the tire planted mid-corner. You've got less entry stability and less front grip than you would have with a stiffer front spring. The added jacking forces in the front end give it back response, but it's bad for grip, so we might cause an issue if there are ripple bumps or something of that nature in the corner. Now we've chased our tails back around in a circle because of front spring choice only to produce a car which is unstable on braking/entry and has a mid-corner U/S.

When we finally get to the throttle side of the corner, it's difficult to control a stiff rear spring. As you increase the rate, the ride and traction over bumps gets incrementally worse and the break-away characteristics get increasingly abrupt. It can't be a complete wet-noodle, but it's definitely the case of applying the minimum effective dose. I'm much happier to have the rear of the car squat in a single motion and gradually saturate the tire grip rather than have the rear end stay stiff or skip-bounce and have more abrupt behavior. I personally like the have the rear of the car as stable as I can and then manipulate the balance of the car with my feet. I'll carry the brake as far as I need to into the corner to get it to tuck the nose. I'll be "both feet up" in longer duration corners for a moment to let the car rotate around the corner before cracking throttle. I'll drive with more throttle discipline to make sure not to induce understeer. Even if you do give up a fraction of time in mid-corner rolling speed, what you make on entry and exit will more than pay you back.




(Getting up early and doing work means you get to play mid-morning, I guess1)

 

[WItoTX]

Yes, you should get the bump steer kit, but it is not required. Pull the fuse and drive your car without power steering. You will feel it the first corner.

FWIW, I’m running 350F/800R. It’s fine on the street and neutral on the road course. Minimal understeer and much easier to put down power from the apex to the track out.

OP, this ^^^^ is roughly where I am at too. I have the FP handling pack rear springs, which I have read are around 600. I don't know this to be true, as the only dyno information I ever found was one page on TMO, and FP never published their numbers. Just riding around, it feels stiffer than 600.

The combination of 350 front, 600 rear has been awesome. Tons of grip on corner exit, and with the 350's up front, turn in is light years ahead of what the factory was. So much grip on exit, I finally put the rear FP sway bar in to reduce understeer on exit (I may go heavier springs like EG has, and back to the stock sway bar if I don't like the feel. But I digress). It also rides decently, but I only drive the car to and from local events.

I say all that to say, don't hang your hat on a frequency, or what anyone else tells you. What feels good to me in a car might not feel good to you. Just plan on needing to buy several sets of springs. AJ Hartman sells a mag ride and (i think) non mag ride coil over conversion kit for your car, which will make front spring rate changes dang near painless.

I don't totally understand ride frequency, but I do know that linked webpage is way too simplistic to use to determine the correct springs for what you are looking to do, without a ton of other research. And TeeLew pretty much covered every engineering base on the subject.

 

[meursiicc]

@TeeLew THAT was the context i was looking for. I think we're in complete agreement on prioritizing stiffer fronts for better turn-in, especially compared to stock. I misunderstood your earlier post as meaning the SPRING rate in the rear should be lower in the rear than front. Now I understand you meant rear wheel rate/ride frequency softer in rear than in front. Makes a lot more sense.

As for target ride frequency...I'm using calculates spring rates from 1.8Hz as a baseline. Which gave me a front spring rate of around 350. I have not found a spring package anywhere that has a 350 front without going into coilover territory.

So, say I'm convinced to go 350/800 or somewhere in that range. Where are people finding 350 fronts? If I don't have to lower the ride height much and change geometry, even better.

FWIW, Bilstein's B16 coilover set also has softer rear. 375/500-725. But, that's $3k with camber plates, and I'd prefer to keep my FP Track dampers. I could just keep stock EB rear springs (668 as I've read, not measured) and put stiffer fronts in. But then I'll need ~300 fronts without lowering the ride height so I don't end up with a silly rake.

Sponsored