Coupe vs Roadster - chassis stiffness

[Ewazix]

[
I suppose this comment and few others beg the question is there a point where say a sub 1,500kg roadster/coupe is stiff enough and that anything extra bears no fruit. .

That's an interesting point, and the theory of diminishing returns makes sense here.
Relatively the Z4 roadster shell is stiff, even in comparison to some fixed roofed cars, does increasing the stiffness even further have any tangible benefit?
From my brief experience of both cars back to back on the same track, I'd say no. There's of course caveats to that statement. Firstly I probably wasn't driving dave's car quite as close to the limit as my car, secondly I'm probably not experienced enough to notice the (I gusess small) chassis flex influencing handling characteristics. Thirdly, the cars are not in entirely the same spec, there's a difference in our tyre widths.
I'm sure Dave might chime in at some point on this, but he also drove my car (pretty bloody quickly I might add!) and I don't think the additional chassis stiffness made any difference to him, and this is a guy who has done a fair bit of track driving/racing.

Interesting points on weight Vs stiffness and the need for some controlled compliance to assist with tyre contact (and comfort) which is why we have suspension and air filled tyres I suppose. This paper gets very geeky but seems to agree with your thoughts and interestingly notes that so long as limited flex isn't altering wheel geometry it's OK, and that excessive bush compliance has far more detrimental effect on geometry. I think that's what it's saying anyway
see CONSIDERATION OF CHASSIS STIFFNESS FOR
OPTIMAL SUSPENSION OPERATION
http://users.telenet.be/AudiR8/Chassis%202002-01-3300Design,%20Analysis%20And%20Testing%20Of%20A%20Formula%20Sae%20Car%20Chassis.pdf

 

[TomK]

see CONSIDERATION OF CHASSIS STIFFNESS FOR
OPTIMAL SUSPENSION OPERATION

Ooh, thanks for that! Good bed time reading lol...

In reference to the excessive bush compliance results, I have to agree, in particular with our rtabs. The limiter kit is a very important upgrade imo if you're driving on track.

 

[Ewazix]

see CONSIDERATION OF CHASSIS STIFFNESS FOR
OPTIMAL SUSPENSION OPERATION

Ooh, thanks for that! Good bed time reading lol...

In reference to the excessive bush compliance results, I have to agree, in particular with our rtabs. The limiter kit is a very important upgrade imo if you're driving on track.

Chassis stiffness and bush compliance are always worth studying before bedtime

 

[Fishy Dave]

Currently working at Spa, will post when back next week. The circuit not the corner shop btw

 

[Anonymous]

My 2ps worth

As per most cars, most drivers/owners try to knock roadsters saying oh the scuttle etc etc

Whereas in fact the speed and forces needed to be exerted to find these is out of the talent range of said drivers

For me, when I bought my MR, as others have found it floaty on the front end with slight movement over the front wheels into high speed corners

These both together made me think wtf have I bought?!??
As my driving style is not racing line but more granny line, and the car needs to hit the apex just right for this

But I for love nor money couldn't hit the sweet spot carrying high speed into the corner without out being in fear of what the front was up to and lost confidence though not so scuttle based it certainly was a flex issue and lift over front

My route
H&R arb/OEM strut brace/H&R lowering springs and recently OEM CSLs

My drive now......
Hubba hubba corners at any speed flatter than a witches :titshake:
I'm in my normal relaxed driving style and position
Ask Bmw addict (Dan) as he has the3.0si and finds it twitchy and scary as my original issue, and was shocked how flat and composed mine was and at 130 on bend that we were still casually having a chat

So definitely do upgrades and only sign of scuttle is when I go into a twisted carpark ramp
Worth every penny !
And these upgrades aren't massive money bar wheels but result is a car I can out corner most and the love of no brakes at roundabouts to scare the chav corsa racing me
Loveeeee my car .com haha
Stiffness does affect some though

 

[TomK]

My 2ps worth

So you've driven both have you? Would be good to hear about those thoughts rather than the 'facts' that H&R arbs & springs + OEM CSL wheels is the answer to life the universe and everything for the 50th time :roll:

 

[Anonymous]

My 2ps worth

So you've driven both have you? Would be good to hear about those thoughts rather than the 'facts' that H&R arbs & springs + OEM CSL wheels is the answer to life the universe and everything for the 50th time :roll:

Haha I'll ignore the sarky reply as you mean that aren't the answer?

If read through my input
It states how my car was and is now
And that Dan with his coupe having been in my car says that his feels how mine used to be before I upgraded, as at high speed we were both sat chatting, whereas in his he'd be glued to wheel concentrating, and the front lift and loftiness as well as front end scuttle looses his confidence

Hence he's now going for brace and ARB to help stiffen but no springs as drives in London everyday and doesn't want to bottom out

So there's the comparison

 

[TomK]

well that clears that up then.

 

[Anonymous]

well that clears that up then.

Haha you boy
I will hopefully be meeting up soon with redun
With his coupe as we want to compare exhausts
As he's got cats and I've got decat yet same race cans
So we may try others cars and I'll see if any difference to be found both to exhaust and scuttle

Ps my reply wasn't being sarky buddy
I'm just singing praises upon transformation that the H&R bits do to the car

 

[TomK]

well that clears that up then.

Haha you boy
I will hopefully be meeting up soon with redun
With his coupe as we want to compare exhausts
As he's got cats and I've got decat yet same race cans
So we may try others cars and I'll see if any difference to be found both to exhaust and scuttle

Ps my reply wasn't being sarky buddy
I'm just singing praises upon transformation that the H&R bits do to the car

Sorry, maybe I've got out the wrong side of bed today. I don't really get it though, you're saying add a thicker ARB (which does precisely 0 for chassis rigidity) and add a strut brace (which does add a small amount of rigidity) and then your mates coupe will feel like your roadster?? Am I missing something here?

 

[goldbcfc]

i suppose stiffer is better than floppy go ask your mrs see what she likes..............

id still have a roadster though thats like unsafe protected sex bare back dangerous but feels good

 

[Beedub]

well that clears that up then.

Haha you boy
I will hopefully be meeting up soon with redun
With his coupe as we want to compare exhausts
As he's got cats and I've got decat yet same race cans
So we may try others cars and I'll see if any difference to be found both to exhaust and scuttle

Ps my reply wasn't being sarky buddy
I'm just singing praises upon transformation that the H&R bits do to the car

Sorry, maybe I've got out the wrong side of bed today. I don't really get it though, you're saying add a thicker ARB (which does precisely 0 for chassis rigidity) and add a strut brace (which
does add a small amount of rigidity) and then your mates coupe will feel like your roadster?? Am I missing something here?

yeah totally lost here too I'm afraid, and if you felt flex in your car to the degree you did with that level of stiffness id suggest it may need checking out, from what your describing you seem to be talking about a high degree of body roll.... ARB will do absolutely nothing for chassis stiffness

I've driven both and they felt identical, i was also driving them both very hard... in the end i went for the one that gave me more smiles....

 

[Anonymous]

It's all good buddy
But the arbs stop the car from moving
Ie pitching and rolling
And strut stops the front from flexing
Combined both give a flatter car

Mine doesn't lift under acceleration
Nor does it scuttle as arbs don't allow the flexibility

 

[TomK]

It's all good buddy
But the arbs stop the car from moving
Ie pitching and rolling
And strut stops the front from flexing
Combined both give a flatter car

Mine doesn't lift under acceleration
Nor does it scuttle as arbs don't allow the flexibility

Mate, the ARBs are attached to the chassis with rubber! The do not stop the chassis flexing, they stop it rolling. 2 entirely different things.

 

[Beedub]

ok ill have to agree to disagree here D4... but your NOT talking about chassis flex... at all.

 

[Ewazix]

I may be being thick here (quite possible ) but surely if suspension bushes and springs are less compliant then more of the forces acting on the car are transferred to the chassis which will make the chassis flex more. By stiffening the suspension you have controlled pitch and roll of the chassis ON the suspension and but made the chassis bend more, not less.

This is the balance the racing chassis boffin talks about in the paper I posted, he suggests that the optimum chassis to suspension stiffness ratio is about 10 - 1. As I say I'm no expert but if a boffin produces a lot of physics and maths that say over-stiffening suspension or chassis can lead to undesirable characteristics and a slower unit then that's surely a good rule of thumb.

 

[TomK]

I may be being thick here (quite possible ) but surely if suspension bushes and springs are less compliant then more of the forces acting on the car are transferred to the chassis which will make the chassis flex more. By stiffening the suspension you have controlled pitch and roll of the chassis ON the suspension and but made the chassis bend more, not less.

Correct!

eta: hence one of the reasons why the roadster M has slightly softer damping than the M coupe. It's common for cabriolets to follow this rule, in order as you say to give the chassis an easier time as it were.

 

[Fishy Dave]

I recently drove fishydave's M for a few laps round castle combe. It's a track I know well now in my coupe and I found no noticeable difference between the two, certainly not in terms of rigidity.
Dave has driven my car previously and came to the same conclusion. Of course a track is smooth so perhaps there would be a noticeable difference out on the roads/pot holes but I doubt it, the cab is stiff as it is.

driven both didn't notice any difference what so ever. as above.. this is a roadster built from day one that way, and its apparent... choose which ever one looks the best to you.... my roadster feels as stiff as my new fiesta ST and much stiffer than my macan, that actually flex's lol.

Tom's is the only Coupe I've driven so far but would agree that at Castle Combe (smooth but still a bit undulating) I couldn't feel any difference between the two in terms of chassis flex or stiffness. Differences came down to sound :wub: and the race seat. The 986 Boxster S and Caterham we owned in the past had more noticeable flex than the Z4.
I added a front strut brace to my M but noticed no difference at all.

 

[Fishy Dave]

eta: hence one of the reasons why the roadster M has slightly softer damping than the M coupe. It's common for cabriolets to follow this rule, in order as you say to give the chassis an easier time as it were.

I'm wondering if BMW choose slightly softer damping for the Roadster from a marketing pov, perhaps assuming that more hard core driver would choose the Coupe whilst the sun loving open top driver might appreciate a slightly softer ride? Rather than any sort of chassis dynamic reason?

 

[Fanatic]

BMW gives a different part number for the chassis of the M and the non-M cars. Is it possible that the M roadster is stiffer??