BMR Handling springs installed (revised version)

BmacIL said:

It is additive but you have to keep in mind the motion ratio. The front is essentially the same as the front strut. The rear is pretty much at the shock mount, so it has a larger motion ratio (approximately 0.75) than the spring. Then you have to apply that torsional stiffness of the bar with those motion ratios to the wheel rate. There's no true optimal rate, as it depends on what the balance of the car you want. There are optimal ranges for them, but not specific points. I'd need more data about the body/chassis of the car to answer that question.

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Thanks BmacIL. I will just have to do some experimenting, as I was going to do anyways. I would think though, the rear spring rate so high, and the rear sway bar rate being proportionally higher than the front bar rate in comparison to the stock bars, that my feeling of oversteer is likely correct.

I found an article on MotoIQ re: suspension tuning and it mentions several ways to influence oversteer/understeer balance by way of either spring rate, sway bar rate, struts/shocks damping and also, believe it or not, tire pressure. And of course, a combination of all of the above. It goes on to explain that increasing the rear tire pressure can help to reduce the oversteer so I'm going to try that as well.

http://www.motoiq.com/MagazineArtic...-and-Handling-Part-3-Balance-the-chassis.aspx

Bluemustang said:

Thanks BmacIL. I will just have to do some experimenting, as I was going to do anyways. I would think though, the rear spring rate so high, and the rear sway bar rate being proportionally higher than the front bar rate in comparison to the stock bars, that my feeling of oversteer is likely correct.

I found an article on MotoIQ re: suspension tuning and it mentions several ways to influence oversteer/understeer balance by way of either spring rate, sway bar rate, struts/shocks damping and also, believe it or not, tire pressure. And of course, a combination of all of the above. It goes on to explain that increasing the rear tire pressure can help to reduce the oversteer so I'm going to try that as well.

http://www.motoiq.com/MagazineArtic...-and-Handling-Part-3-Balance-the-chassis.aspx

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I wouldn't try the pressure as a first option (not even last, perhaps). In practice, I think the most feasible way is to adjust the sways, until full stiff on the front and full soft on the rear. If there's still too much oversteer, I would replace the rear with the stock one if you still have it around and try again with full soft on the front.

One overlooked aspect in this situation is the relationship of front/rear center of gravity and roll, it will affect how much more sway stiffness will you want on either side. In other words, it ain't as obvious as adding X% roll resistance to front and back and it will understeer the same as stock...the spring/sway rates shouldn't be looked in isolation, geometry needs to be considered as well. I found out the hard way :frusty:

I think it'll be hard for a regular guy like you and me to theoretically crunch out all of these chassis parameters, the best bet is to try in practice.

Rebellion said:

I wouldn't try the pressure as a first option (not even last, perhaps). In practice, I think the most feasible way is to adjust the sways, until full stiff on the front and full soft on the rear. If there's still too much oversteer, I would replace the rear with the stock one if you still have it around and try again with full soft on the front.

One overlooked aspect in this situation is the relationship of front/rear center of gravity and roll, it will affect how much more sway stiffness will you want on either side. In other words, it ain't as obvious as adding X% roll resistance to front and back and it will understeer the same as stock...the spring/sway rates shouldn't be looked in isolation, geometry needs to be considered as well. I found out the hard way :frusty:

I think it'll be hard for a regular guy like you and me to theoretically crunch out all of these chassis parameters, the best bet is to try in practice.

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Good points, and duly noted. Good advice. I’m going to continue to drive it and then in a couple weeks go full stiff on the front bar. I need more seat time with it as is. It’s a warm day here, in the 70s. And much dryer on the road. The warmer temperatures should help my grip and will be a better representation of what it actually feels like.

Had my first long trip in the car this weekend. It was a 3 hour drive to my brothers place so good representation. It felt really good. My goal in switching springs was to achieve a more consistent, predictable ride and handling feel.

On the highway it feels smoother because the ride is more consistent and predictable. Driving at higher speeds the car feels great and lost that floaty feeling in the rear. Bumpsteer appears to be minimal or unnoticeable. It does still tramline but I attribute that to the higher negative camber and my tires have been rotated and have some wear in them. Over harsher expansion joints you feel it a lot, but not anymore so than before. And also I have aftermarket IRS parts including the cradle lockout which is where a lot that comes from in my opinion. Steering in turns feels also more direct and predictable.

Over back roads (Pennsylvania which suck) you do feel it more. Whenever you have a stiff suspension like mine, the very bumpy roads aren’t going to be fun and that’s the case here. But no more than before and may even be slightly better. But it’s not a drastic downgrade in this area at least which I was afraid of. As the back roads get better it becomes very fun to drive.

Reactions of the chassis are noticable and felt. Which can lead to a bounciness at times when the road isn’t good. But that’s just because my IRS has very little deflection and is basically solid mounted to chassis so- I’m going to feel the road changes in my seat. It does seem like the faster I go the smoother it becomes also.

Overall a very stiff, sporty, yet predictable ride and handling now. I’m very pleased with my choice so far and can’t wait to drive it more. Even the stance- the car looks better now. The shocks seem well suited to these springs IMO. Not rattling teeth stiff but very responsive and reactive. This is probably on the upper end of what is acceptable for a street car. This would probably be good for someone who wants the responsiveness on a track but is willing to accept a stiffer feeling for street. Stiff - but not like a car you wouldn’t want to drive. For me it feels like a good balance. The harshness of the bad roads is tolerable for the gain you feel when you want it for smoother surfaces and just predictability overall.

[MENTION=19599]Bluemustang[/MENTION] thank you for such detailed feedback.

Your description of the characteristics of our SP083 makes me grin, we got exactly what we wanted from these springs my friend!

BMR Tech said:

[MENTION=19599]Bluemustang[/MENTION] thank you for such detailed feedback.

Your description of the characteristics of our SP083 makes me grin, we got exactly what we wanted from these springs my friend!

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Thanks Kelly! I really appreciate your advice as well. It was very helpful in making this decision. It’s been a long time in the making.

In doing my research I think these are best combination of rates and drop for a street handling focused car. I’m sure I will have more feedback to give and I’ll be doing tweaking with the sway bars. First, I will try full stiff on the front bar. I am strongly considering the GT350 sway bars though as I have a feeling that those will be perfect for the rates I have now.

I’m thrilled to see BMR continuously improving and I respect the engineering that goes into their products. Had I another chance at it I’d have more BMR parts on my car.