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JonRB

Original Poster:

43,892 posts

159 months

[news] 
Saturday 23rd July 2005 quote quote all
I know that they reduce body flex, or more specifically, prevent the suspension towers from moving relative to each other, but what does this actually achieve?

I've heard that it gives slightly sharper turn-in, but that's about all I've been able to ascertain.

I'd also like to know what the disadvantages are, because surely if there are none then all performance cars would have them as standard surely?

DavidCane

764 posts

128 months

[news] 
Saturday 23rd July 2005 quote quote all
When car engineers deisng the suspension geometry, thier design has to assume that the suspension mount positions remain constant. It's the chassis's job to keep the mounts for the suspension components in their desired location.

When you have a big hole in the chassis between the left hand side and right hand side suspension (your engine bay) you're gonna get some flex of the body (albeit a mm or 2).
The brace keeps the struts in the position that they're supposed to be in.

I'm sure the top sports cars are designed with less compromise to ensure that the chassis is rigid enough at the front and rear to keep the suspension mounts in place.

GreenV8S

23,471 posts

171 months

[news] 
Saturday 23rd July 2005 quote quote all
By stopping the shell from flexing sideways around the top mounts, they stiffen the mounts up so that the dampers react solidly against the mass of the car rather than just distorting the shell - this means the dampers are more effective. This only really matters if you have relatively stiff suspension with minimal compliance (so you've ruled out most production cars) and are more concerned about handling than ride comfort. Stopping the top of the strut moving sideways also means the geometry will remain [more] consistent under load, which might be very important if you're racing on a billiard table with rock hard suspension and grippy tyres. I suspect that for many people who buy them the most important thing is that when you open the bonnet it makes your car look as if it has been 'race prepared' or souped up. You can get them anodized, covered in fake carbon film, you name it ...

Downside apart from the cost and effort to fit, is that it obstructs access to the engine.

JonRB

Original Poster:

43,892 posts

159 months

[news] 
Saturday 23rd July 2005 quote quote all
Hmmm. Thanks Pete.

Are you saying that by stiffening the shell they impact ride quality?

The reason I ask is that I have the opportunity to buy second-hand front and rear braces for my Corrado. My Corrado is already running fairly stiff suspension and ride quality has been impacted as a result and it is only just acceptable at the moment, to be honest. Are you saying that ride quality would be further impaired?

I've noticed that my Chimaera has far more compliance in its suspension setup and I put that down to the stiffer chassis allowing softer springs and dampers. I was therefore planning on fitting front and rear strut braces to the Corrado and then softening up the dampers by a few clicks. I take it this could well be a BlackAdder-style "brilliant plan, but with one small flaw - it's bollox"?

>> Edited by JonRB on Saturday 23 July 10:23

GreenV8S

23,471 posts

171 months

[news] 
Saturday 23rd July 2005 quote quote all
JonRB said:
Are you saying that by stiffening the shell they impact ride quality?


It might impact the ride quality slightly because it will effectively remove some compliance from the top mounts, but the important thing is that it connects the dampers rigidly to the mass of the vehicle which gives them something solid to react against. But there's no point doing this if you have big squishy bushes on the top mounts or soft squidgy suspension, it cures a problem that is more important when you have a hard setup.
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HiRich

3,337 posts

149 months

[news] 
Saturday 23rd July 2005 quote quote all
Basic theory on any remotely modern car is:
- Use the chassis/body to locate everything where it is supposed to be.
- Use the suspension to deal with all the uppy-downy stuff, and control movement.
(pre-war, there was some degree of using a 'whippy' chassis)

Logically, you want to separate components to perform separate jobs. Therefore, you want the stiffest chassis possible. The most notable parameters are:
- Full bump on both wheels of an axle
- Full roll (one spring compressed, one released)
- Torsion (Front axle twisted one way, rear axle twisted the other).
The potential benefits are:
- The suspension should be based on a more stable platform, and therefore more capable of doing what it's supposed to.
- Chassis strength & durability
Potential problems are:
- Extra weight
- Stiffening one part of the chassis may expose a weakness (of stiffness or strength) elsewhere in the chassis.
- Any noise or harshness that gets through the suspension and bushes is less likely to be attenuated in the chassis (ie you might get more vibration in the steering wheel, or more scuttle shake, etc.)
- The designers will (to some degree) have factored in a lack of chassis stiffness, whether locally or generally. So in an ideal world, you would want to play with the suspension settings.

Now the practical (though I'm winging this, not knowing the layout specifically). Pop the bonnet of your Corrado and take a look. Imagine that you have just crash landed over a hump-backed bridge, so both wheels are in full bump.
- With struts pointing inwards slightly, and transferring huge vertical forces, the chassis around the strut mounts is going to want to fold in slightly - the top mounting for the suspension is moving.
- Get under the car: The odds are that the lower mounts (for the wishbones) are trying to pull apart as well.
- Back above: Since Isaac Newton said that every action must have a reaction, the load going into one wheel must be countered by the load going into another. Look at how that would happen. Odds are it's through the bulkhead. Every spotweld, every hole, every stress concentrator is being worked - over the long term welds with fatigue, and cracks will form. Though probably not yet visible, it is probably already happening on a 10 year old Corrado. Though not necessarily a big problem, your chassis is weakening, getting a bit sloppy, and rust is getting into those newly exposed crack faces.

Now imagine that you have rigid bars between the across the upper and lower mounts. They would now be rigidly fixed relative to each other. Also, loads will be going straight from point-to-point, not through the bulkhead. Stiffer, stronger, and more durable. Although we've only looked at full bump front, similar processes will happen with all the other scenarios.

Now the tricky bit. To decide whether you should fit braces, which ones to fit, and the effects you can expect, you really need to know:
- How good your chassis is already
- How good the proposed braces are
Since I am assuming that you don't intend to send it to McLaren International for rig analysis*, you'll need to wing it a bit, and/or gain recommendations from a reputable, knowledgeable person who has already experimented. Unfortunately, that's not me.
(* If it's possible and you're feeling brave, tension a piece of thin garden wire between the mounts. Jack the car on a front point and see how much wire you can pinch. Repeat on a rear point. If you're really brave, get two big fellas to climb onto the engine or into the boot. Any slack in the wire indicates flex at about 1/4 of the potential torsion or bump loading)

Overall, I would suggest:
- On an FWD, the most likely sequence of benefit would be front upper, front lower, rear upper, rear lower.
- When you look at load paths above and below, you can make a fair guess. If the bulkhead is very close to mounting points, it is likely to be fairly stiff already. If it's a foot away, the benefit of a brace will be more apparent.
- Look at your proposed brace, particularly how it will react in compression. Ideally, you want a dead straight bar. The more bent it is (to clear the engine), the less stiff. Even an inch out of straight will significantly reduce its effectiveness.
- Whilst you might find a slight change in noise, ride and vibration, it is unlikely to be more than one damper click soft.
- Braces are often pretty cheap. If the full set you describe are at a reasonable price (> 200?) it's probably worth a punt. You can always take them off.

The first thing you are likely to notice should be a crisper, tighter turn in and reaction to steering inputs. On a Corrado, this is likely to be a very pleasant experience. You may also feel it much better 'planted' under heavy braking. And similarly, if the car gets disturbed over dodgy surfaces (washboard effect, cat's eyes, tramlining) you may get a feeling that it is just more stable underneath you. Then, working on the assumption that with adjustable dampers and stiff springe you go on track, you should notice this stability and responsiveness more there.

So that's all the theory. Braces won't give you as obvious a benefit as stiffer springs or dampers. But it's about getting the basics right, getting a more stable platform on which to build, and I would always recommend doing it. Think of it this way - race an rally teams will spend thousands, millions on suspension design & setup. But the very first thing they do when a new car arrives in the workshop is work out how stiff a roll cage they can fit in there. On a smaller scale, you're doing the same thing.

GarryM

1,112 posts

170 months

[news] 
Saturday 23rd July 2005 quote quote all
Wow! That's what you call a comprehensive answer! A friend of mine was asking me about fitting one to his BMW Z3. If you don't mind, I think I'll just give him a copy of your post.

GravelBen

11,394 posts

117 months

[news] 
Monday 25th July 2005 quote quote all
so for my 1990 4wd Subaru wagon, with 185/65/14" tyres, pathetically soft springs and shocks that are pretty much knackered, I take it there'd be no point in a strut brace then? This whole understeer business gets a little annoying but the wallet doesnt want me to do much...

dilbert

7,738 posts

118 months

[news] 
Monday 25th July 2005 quote quote all
Surely though the brace is only really effective in tension.

Most of the braces that I have seen are so flimsy, that if you put them in compression they're just going to bend.

I'd have thought that the best approach to this problem is to weld in a full on gusset. If you have an engine block in the way then the answer is to have a stout bracket that bolts to either side of the engine block, making the block a structural component.

JonRB

Original Poster:

43,892 posts

159 months

[news] 
Monday 25th July 2005 quote quote all
dilbert said:
If you have an engine block in the way then the answer is to have a stout bracket that bolts to either side of the engine block, making the block a structural component.
Thus completely negating all the work the designers of your car did with NVH (Noise, Vibration and Harshness) and rendering your engine mounts fairly useless.

GreenV8S

23,471 posts

171 months

[news] 
Monday 25th July 2005 quote quote all
[quote=dilbert]Surely though the brace is only really effective in tension. [quote]

Not at all, you are after stiffness not strength. A thin metal strip preloaded in tension can stiffen things up usefully.

HiRich

3,337 posts

149 months

[news] 
Tuesday 26th July 2005 quote quote all
dilbert said:
Surely though the brace is only really effective in tension.

Most of the braces that I have seen are so flimsy, that if you put them in compression they're just going to bend.

I'd have thought that the best approach to this problem is to weld in a full on gusset. If you have an engine block in the way then the answer is to have a stout bracket that bolts to either side of the engine block, making the block a structural component.


The top brace brings home the bacon when in compression, and the lower brace normally delivers in tension. So you are sort of correct - the upper brace needs to be stiff in compression and ideally straight (and many after-market are pretty pathetic. Without going into buckling theory, straightness is very desirable.

Using the engine a a stressed member raises three issues:
- As Jon syas, NVH goes out the window. The penalty is severe.
- You would have to re-engineer the layout, including rigid engine mounts. If you go that far, you may as well cut out the front end and start again.
- There is also the question of whether the engine is stiff enough. When, about 5 years ago, when they restored the ATS F1 car from the '60s, they realised that the reason it was so unsuccessful in period was that the engine twisted and seized the pistons. A pair of roll hoop braces directly onto the rear suspension mounts solved the problem (nice, because it is a truly beautiful car)

GreenV8S

23,471 posts

171 months

[news] 
Tuesday 26th July 2005 quote quote all
HiRich said:
the upper brace needs to be stiff in compression


Needs to be stiff, but not necessarily in compression. For example if you imagine a wire that has infinite stiffness in tension and no stiffness in compression, put a 100 lb tension preload on it and now your engine bay is rigid for the first 100 lbs. The important thing is stiffness, not strength.

dilbert

7,738 posts

118 months

[news] 
Tuesday 26th July 2005 quote quote all
Well, I suppose that the brace could be cruciform in cross section, or maybe even a large diameter thin walled tube, but I've certainly not seen any of those.

If commercially available braces actually do do something useful, of which I'm not certain, there must be a market for something with a decent cross section. Maybe even a composite tube filled with a light alloy foam.

A bit like aero chocolate.

dilbert

7,738 posts

118 months

[news] 
Tuesday 26th July 2005 quote quote all
GreenV8S said:

HiRich said:
the upper brace needs to be stiff in compression



Needs to be stiff, but not necessarily in compression. For example if you imagine a wire that has infinite stiffness in tension and no stiffness in compression, put a 100 lb tension preload on it and now your engine bay is rigid for the first 100 lbs. The important thing is stiffness, not strength.


Don't the rubber bushes at the top of the struts suffer if you have a preload?

HiRich

3,337 posts

149 months

[news] 
Wednesday 27th July 2005 quote quote all
GreenV8S said:
Needs to be stiff, but not necessarily in compression. For example if you imagine a wire that has infinite stiffness in tension and no stiffness in compression, put a 100 lb tension preload on it and now your engine bay is rigid for the first 100 lbs. The important thing is stiffness, not strength.

Good in theory, but pointless in practice. In every practical suspension system that I can think of (and I can think of many) the upper brace will need to be in compression (that is, the distance between the upper mounts wants to decrease from the static condition), at least when the brace is doing something worthwhile. Yes you could apply pretension, but by definition you are also applying loads to the suspension mounts/strut tops and therefore distorting them. You will have improved dimensional stability, but to the wrong dimensions...

Dilbert,
The best sections for compression will be tube (square or round) or a forged I-section. Most that I know of are tube, though whether diameter and wall thickness have been optimised is certainly up for debate.

GreenV8S

23,471 posts

171 months

[news] 
Wednesday 27th July 2005 quote quote all
No, the loads are applied to the shell not the damper mounts. I don't know anyone who is using this as a strut brace, but I know a couple of people who are using this to pre-load engine mounts to move the roll axis of the engine/box assembly within the chassis.

Mr Whippy

19,147 posts

128 months

[news] 
Friday 5th August 2005 quote quote all
It's a bloody road car with a natural frequency setup of around 1-1.5 hz at the front.

Unless they are running silly stiff front dampers the body will twist along it's length after you take away any movement in the front of the car.

Ie, make the front solid, then the next weakest link will twist instead, probably in a manner that wasn't engineered into the design.

Some cars are even assumed to distort when designing the setup.

Add less weight is a better idea if he want's a crisper turn in. Unsprung first, then sprung!

Dave
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