PSA: Think twice before lowering your car

[K_M3]

As a takeaway from this thread, is there a bump-stop we could use with Koni/Dinan to help alleviate this problem? Or is this lack of travel linked solely to the strut?

[Bry5on]

As a takeaway from this thread, is there a bump-stop we could use with Koni/Dinan to help alleviate this problem? Or is this lack of travel linked solely to the strut?

One way to combat this would actually be to raise the upper perch Point and run a longer and softer bump stop so abrupt bumps are smoothed more progressively. We’ve been talking about one axle but the important thing is that you need to balance the axles together, this is where it gets most difficult when you’re talking bump stops for extra spring progression. You can relatively easily put yourself in a place where relative front spring rate ramps up too fast on corner entry, causing understeer, while relative rear spring rate ramps up too fast on corner exit, causing oversteer. In general, the shorter your bump stop is, the faster your spring rate ramps up.

As an example, the original Tesla model S (it may have changed since) ran very long front bump stops, and the bump stops were slightly compressed at static ride height. This allowed the team to use linear springs but still achieve a progressive spring rate in bump travel. Because the bump stops were so long, the ramp up of spring rate was progressive enough to manage reasonably well on the road.

[cobra]

I love that you did this!! How do you know you're at 1.8Hz?

Just mathematically. 300/500 spring rates equates to around 1.8Hz.

[cobra]

I think Bry5on implies that because of imperfect streets (pot holes, cracks, light up road dividers) that more travel for street driving is recommended vs the track where he said we can get away with 10mm travel and thus like lowering the car to prevent overall travel to retain correct negative camber during turn in with stiffer springs will be better.

Correct me if I misunderstood.

So this begs the question - what benefits come from running stiffer springs on the track? Is it more for response and control? Would it have less grip than a softer sprung car, but be so unwieldy it would be hard to go fast?

[BlueBimmers]

Kind of on thread - this soon to be record setting ZCP on BaT looks to be riding high to me, assume it's stock: https://bringatrailer.com/listing/2006-bmw-m3-coupe-93/

I remember a Bimmer magazine article that claimed ZCPs rode higher than earlier cars, but nothing official from BMW. Anyone know?

[Bry5on]

So this begs the question - what benefits come from running stiffer springs on the track? Is it more for response and control? Would it have less grip than a softer sprung car, but be so unwieldy it would be hard to go fast?

A few things:
1) car is more responsive to inputs (keep in mind, the faster your car is traveling, the faster your inputs will be for the same track. So the more grip you have, the more responsiveness you’ll need)
2) tires are grippier, assuming you’re running track tires, so you need more roll stiffness to manage how much the car rolls and keep geometry in a stable, controllable place
3) tires can handle more heat to cope with the higher frequency ride rates

I’m probably missing something from memory but it’s all about managing the tire, then tweak to driver preference from there. In theory, you’ll get the most grip from the softest spring that you can run, as you’ll have the most consistent load on the tire, but given that you need to manage the car moving around, with associated geometry issues, and especially manage ride height with aero, soft springs are not always practical.

[T.J.]

Just for other readers here who might have read this as factual engineering or physics, roll stiffness is not what determines weight transfer in steady state cornering, cornering load and suspension geometry is the primary determinant of weight transfer, or more specifically, load at each tire in steady state.

You correctly picked up on the inaccurate comment “The stock suspension rolls too much which equals more weight transfer” but I think you tripped up a little in your correction.
Roll stiffness absolutely effects the distribution weight transfer, along with CG and track width. The roll stiffness depends on the springs and sway bar but also the suspension arms which is where the geometry and roll centers come into play.

[Bry5on]

You correctly picked up on the inaccurate comment “The stock suspension rolls too much which equals more weight transfer” but I think you tripped up a little in your correction.
Roll stiffness absolutely effects the distribution weight transfer, along with CG and track width. The roll stiffness depends on the springs and sway bar but also the suspension arms which is where the geometry and roll centers come into play.

I think I failed to accurately simplify. Thanks for the add, this is correct.

[Mayan-Viking]

I removed the rear bar when I built the roll cage but before I put any aero devices on. It adds a lot of rear end grip. A sway bar makes an axle less independent and more of a solid axle...like a Mustang . The sway bar picks up the inside rear wheel when loaded up in a turn so the tire has less grip. No rear sway bar allows the inside wheel to droop.

Some might not like it, might feel lazy or not responsive. You have to be willing to step on the throttle.

I love it! The rear end feels much less skittish. With a ZF LSD I can get on the throttle, get the rear sliding and then it just hooks up and goes.

The flat ride guy has an interesting video on sway bars. I think he mentioned driving with no front bar either on his Miata.

This is an interesting comment and I'm thinking I should go take my rear sway bar off to see what the difference feels like. What does conventional wisdom say about the cons of performing this mod?

[cobra]

This is an interesting comment and I'm thinking I should go take my rear sway bar off to see what the difference feels like. What does conventional wisdom say about the cons of performing this mod?

I tried researching and honestly didn't find many negatives, other than it makes a car less "playful" and less likely to drift. My experience yesterday showed that it improved cornering grip noticeably and maybe made it slightly less responsive in cornering response, but a tradeoff I would accept.

It's very easy to remove, give it a shot. Honestly the bar is pretty small and attached very inward so it doesn't contribute a huge amount of stiffness.

[eacmen]

This is an interesting comment and I'm thinking I should go take my rear sway bar off to see what the difference feels like. What does conventional wisdom say about the cons of performing this mod?

no need to remove the bar completely to test how you like it. Just disconnect one of the end links

[FBloggs]

Shorter, stiffer springs combat that macstrut body roll, which peels the tire tread off the road surface.
When I autocrossed Front drive VW, It was de rigeur to use a massive front sway bar to prevent the macstrut body roll from trashing the front contact patches.
This would off course combat understeer.

When I tracked my E30/36, a softer or removed rear swaybar would increase the vertical load on the outside rear tire, ergo more traction to exploit.
It was the fashion in the 90's-00's.
The downside was a little more instability under braking for the corners.

[Wernd]

This is an interesting comment and I'm thinking I should go take my rear sway bar off to see what the difference feels like. What does conventional wisdom say about the cons of performing this mod?

More body roll which also means a longer time for the weight to finish transferring or as stated above a slower transition. Those are the only two negatives which can be mitigated with an adjustment to a set of coilovers. The positive is more rear grip

[bigjae46]

You correctly picked up on the inaccurate comment “The stock suspension rolls too much which equals more weight transfer” but I think you tripped up a little in your correction.
Roll stiffness absolutely effects the distribution weight transfer, along with CG and track width. The roll stiffness depends on the springs and sway bar but also the suspension arms which is where the geometry and roll centers come into play.

How would my comment be fixed to be accurate?

[bigjae46]

Your thoughts are on the right track. Extra progressiveness in the springs (or bump stops, as they are also springs) and generally high spring and roll rates combined will be able to combat much of this.

If you’re not driving on a flat road however, you’ll still need suspension travel to allow the car’s wheels to track the road while the car carries its inertia and doesn’t follow the road exactly. The spring is really there to allow the car and the wheel to track different paths (the tire does some of this too). A softer spring rate helps the car keep a more consistent load in the tire contact patch as the wheels move up and down with road imperfections. This ultimately results in more grip on imperfect roads, which all roads are imperfect to some degree. If you have a track only setup, your imperfections will be small, less than 10mm or so, so you can get away with a stiff spring (also, tire choice is very different here). As the spring compresses or unloads with a stiff spring, the force at the tire changes greatly, ultimately not providing a steady load for your tire to resist, which is what you’ll want if you want to be fast.

edit, add: So now we have roads that aren’t perfect, and we must have some soft enough springs, and consequently need suspension travel to allow those springs to do their job. You now have a lower roll rate because of the softer springs, so when you lower your car, even if you start at parallel, your suspension will roll enough to put you into the bad geometry scenario where your control arms are inverting and your roll center starts moving wildly. You basically always end up in a place where you need the ride height if you’re driving on streets that are not perfect. Unless you’re changing control arm and tie rod pickup points…

If I'm understanding what you're saying...then I lowering the control arm mounting points would help with a loss of negative camber and maintain more optimal suspension geometry?

I run -4.1 camber in the front right now which gets even wear. I mostly use tire wear to set camber because tires ain't free! I also don't have the time jump out of the car and take tire temps...I'm usually jumping right into another car.