THSCC Forums

Todd's right. I also consider rear coil springs to be unproven technology. And disc brakes are a risky proposition at best.

However, I was joking about adding a big dose of rear toe-out. Just learn to use the hand-brake.

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Art McDonald
Premier Amateur #518
2008 Dishman Cup
Pivot Cone Snob

Rodney is a waxer (but in a good way)

Michael a rule of thumb is to soften the end that understeers and/ or stiffen the other end. These two things either separately or together will help eliminate understeer.

So, NO front strut tower brace. Softer or NO front sway bar (have to balance this against possibility of loosing traction at the front because one side will no become unweighted more and without a limited slip that tire could just spin).

At the back end BIGGER sway bar. And add as much stiffness as possible.

In my track CRX the front sway bar is still attached because these cars can be a bit tail happy. In the rear an adjustable bar set to medium stiffness.

Shocks in the front full soft. In the rear, depending on weather, typically medium or more stiff.

an easy check to see if the front shock towers are flexing is take a look at the paint around them. If its cracked and such then indeed something is flexing and you need to address that (perhaps add the dreaded tower brace). My guess though is that they are fine. Adding the front brace will increase understeer.
Hope this helps.

Ron

Why would adding a strut tower brace do the same thing as adding front swaybar rate?!?

Seems to me that it *could* do the opposite. What you're after to reduce understeer in the front is better contact patch management. Seems to me that if the top of your strut is able to "wander" around that you just might not have your front suspension working properly at all (ie. camber loss could be happening just as easy as camber gain, toe changes are probably bad, etc).

I don't know the vagaries of older FWD cars, so if someone has anecdotal evidence then I'll bow to that, but the issue is perplexing me. But to me a strut tower brace is stiffening the box that the suspension lives in, which could in turn "stiffen" the suspension, but perhaps in a good way. Ie. if it does stiffen the effective rates, perhaps you'd be better off going softer with your rates now and have a box that the suspension can work properly in.


--Donnie

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That's what I'm talking about-- the way complex mechanical devices work is often non-intuitive. I still don't know if the above is true, but it does make sense.

In any event I was considering adding a rear strut tower brace, since stiffening up the back end is supposed to help with understeer.

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"Build a man a fire and he'll stay warm for a day. Set a man on fire and he'll stay warm the rest of his life."
'93 Subaru Legacy DIRTBOMB
'98 BMW M3
2013 Rallyx Co-VP

you can try the ones off my forester see if they will fit. If so i will make you a real good deal on them.

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I believe Donnie is spot on here in that you can't make a broad-brush assumption about the specific handling impact due to adding a strut bar in terms of over/understeer. Having a stiff structure for the suspension to transmit its loads with minimal deflection of the suspension geometry (mounting points) is the goal. Actually having the lowest possible, within reason, deflection of the whole chassis between each side and front to rear is what everyone wants (and gets with the modern roll cages constructed these days).

I'm most familiar with BMW sedans over the past 35 years of driving/autocrossing/tracking/etc, and prior to the late 90's, strut bars were useful in stiffening up the front structure -- especially on cars from the 1970s-1980s. However, on "modern" BMWs (which for the purposes of structure design I'm referring to the E39 5-series and newer since that was the model they leaped forward on with body structure design) I remain skeptical of the benefit to the average user, if any, of a strut brace for the vast majority of those purchased (i.e. typical street driving and even track events). These body structures are so stiff, in both torsion and bending, that I'm guessing (gut feel) that the addition of an expensive strut bar is of little measurable performance benefit.

I've challenged a number of the makers of these things to show real engineering data (i.e. Dinan for one since Steve has the knowledge and ability to instrument a car and interpret the data) on a strut bar in a modern BMW chassis, but NOBODY in the industry will respond. The process wouldn't be too hard: strain gages on the strut bar for example could be used to measure loads in real-time for example, and even an optical system could be rigged perhaps to measure tower deflection w/o a bar in place, etc.

The net of it is that nobody has any real engineering data showing the usefulness, or lack thereof, of a strut bar on modern BMW (and other makes as far as I know) structure. There is no reason for them to do this I suppose since if the data proves there is no benefit to handling, their whole marketing plan goes out the window since "everyone" flocks to these things like crack in the current market.

If anyone is interested in structure stiffness (torsion and bending) of BMW 5-series over the past 4 generations, check out this link: http://www.autosteel.org/AM/Template.cfm?Section=PDFs&TEMPLATE=/CM/ContentDisplay.cfm&CONTENTFILEID=3632
Page 6 shows the significant leap in structure stiffness at the E39 point in time (due design, steel used, and manufacturing technique improvement allowing these to be implemented)...it is shown in terms of the body structure natural frequency increase in bending and torsion.

OT, but page 12 has an interesting look at the difference in high strength steel used between the E46 3-series on the far left and the E90 3-series on the far right.

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
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1975 CanAm 125MX2

Thanks so much for the engineering analysis. Numbers don't mean you understand a problem, but its a heck of a lot easier with numbers than without. I've read similar opinions about the front of Subarus: the design of the engine compartment is such that its already as stiff as you can get.

What I don't have is similar information as to wether adding a bar to the strut towers in the rear of the car is going to do anything.

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"Build a man a fire and he'll stay warm for a day. Set a man on fire and he'll stay warm the rest of his life."
'93 Subaru Legacy DIRTBOMB
'98 BMW M3
2013 Rallyx Co-VP

to be honest, probably not. this is your second rallyX correct?

drive more races so you can actually see if the bar makes a difference. (to you at least)..

i threw a WRX swaybar on my 2.5, hell i cant even tell. but if would have added more than just the swaybar, then i probably could have told a difference in the back end sliding out more.

id say experiment more. take jeremy up on his offer to see if they will really help. yeah, technically they will, but you may not have a feel for it.

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'11 RallyX VP
1998 Integra Type R - track fun
1999 Subaru 2.5RS - dirt fun

I too used to believe as Donnie and some others that adding a front strut tower brace would reduce dynamic camber changes and actually reduce understeer. That was until I took the one off my old Talon awd.

I had put one on with the assumption that its already difficult to get enough negative camber in front so I wanted it to STAY set. However driving with it attached didn't seem to make things better.

Oscar White suggested I disconnect it. However I still wasn't convinced it would have the desired effect. But disconnecting it was easy (just remove the bolt for the bar on one side) so I did it.

I almost spun the car in the first turn! It was an amazing difference. From that point on I was convinced that adding a front strut brace to a AWD or FWD car would increase understeer.

As for making sweeping assumptions, I thought it was relatively well known that to reduce understeer you soften the front or tighten the back. I didn't think I was treading any new ground on that. Of course there may be variations from car to car but as a starting point I thought this was pretty well known.

If still not convinced disconnect your rear sway bar and see how it affects handling. Of course then you run into the risk of unloading one side so much that, without a limited slip, you loose traction.

I believe the only reason folks keep front sway bars is to assure the contact patches of the front tires do not change as much as they would without the bar while in corners. An exception I can think of is the S2000. Karl Shultz says he keeps his on because the car already has a LOT of oversteer.

As for the strut towers actually moving, I really don't think this is an issue most of the time. If it were think how unsafe that would be? A tell tale sign that they were moving would be cracked and/ or flaking paint around the towers.

Ron

Ron,

It sounds like we're talking about two different things here? If you simply took off a strut tower side-to-side top brace and almost spun in the first corner, someone was seriously amiss? Bizarre. If so, then the Talon structure (likely designed in the 80s) is probably a flexi-flyer for sure. It likely doesn't have a torsional natural frequency of 29Hz.

The whole subject of "stiffening the front/rear", etc, to influence balance (i.e. front/rear weight transfer side-to-side weight transfer) is of course well understood and accepted. We're aren't arguing that subject here at all. [As a huge off topic though I will admit to being surprised at how stiffening the front swaybar on a stock class BMW sedan actually can reduce understeer when no other changes are allowed because you get less roll, therefore less loss of camber.]

However, making the leap to say that stiffening the body structure on one end will significantly influence understeer/oversteer is simply too big of a jump to claim it applies across the board to all modern cars. If a body structure is already "stiff enough" such that adding a strut bar yields no measurable significant benefit (likely on some modern chassis designs), then its removal/addition will have no bearing on the handling of the car.

The net is that assuming across the board of all modern cars that adding a strut brace to one end will significantly influence the under/oversteer balance of the chassis is not as clear-cut as the old days imo.

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

I've always viewed those guidelines that Ron quoted, as a tuning guide for a car that's already setup and is already mostly balanced. I don't think they were really meant to be used as a guideline for making mods to your car.

They should definitely be used with caution on a mostly stock car that comes from the factory with a pleasant ride.

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In need of car.

Yes, I've only done 1 rallycross, and so don't have a good feel for the car. Its just Jeremy is unloading his old forester mods, and I have a chance to pick one up used.

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"Build a man a fire and he'll stay warm for a day. Set a man on fire and he'll stay warm the rest of his life."
'93 Subaru Legacy DIRTBOMB
'98 BMW M3
2013 Rallyx Co-VP

Chuck there's stiff and then there's STIFF. as an example my C4 corvette track car has a full roll cage and bars that penetrate the fire wall and connect to the chassis, forward of the front suspension, My garage floor is perfectly level, I insured this when it was poured. I cannot get the C4 to sit on 4 jack stands, just the machining differences in the jack stands makes the car rock back and forth, albiet it is a minute amount but it still rocks back and forth. If I jack up the C5 in the same sport it sits on all four stands like it was glued. The 1987 BMW LeMons car was a similar situation, when you jacked it up the doors would jam until I added more door bars and strut tower braces to the car. once that was done there was no place on a turn or curve on the track that you could not steer the car to. Stiffening the car made a tremendous difference.

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Donnie I agree with what you just posted. If the car is stiff enough already then adding additional stiffness should have no effect. However, how do you know if its stiff enough already?

I'd think that any car that is of unibody construction has more flex then we want.....for track use. But, even if its stiff enough already there is no downside, that I can think of, in adding stiffness. Whether it be a some brace or tying the roll cage to the "frame".

I'm actually surprised the BMW needs the stiffer front bar to keep the contact patches right. Being marketed as a performance car I'd have thought BMW would have done the job at the factory. Wonder if the EU versions have the stiffer bar and BMW softened things up for us sloppy Americans?

R

Not at all. The EU versus American market cars are essentially identical in suspension specs for most BMWs. Welcome to the world of stock class autocross where the only suspension mod you can make is a front sway bar and shocks.

Stock BMWs handle wonderfully for 9.5/10ths driving. However, like many strut-based front suspension street cars, when punished in a track or autocross environment, the lack of sufficient negative camber under max loading is apparent. Of course this also applies to many other makes than just BMW. Very few factory stock cars have optimum front tire camber under full load (i.e. at the limit of adhesion still having -1.0 to-1.5 camber relative to the road surface). I think the vast majority of factory stock cars, even the highest performance versions, all have sub-optimal camber under full suspension loading, and many have horrible camber actually due to: body roll, not enough camber gain on compression (most strut suspensions), and required reasonable static camber for typical street driving.

_________________
Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2