SFC question for the smart guys :)
#1
SFC question for the smart guys :)
As I was looking at my car and my SFCs, I began to wonder how they could add tortional stiffness to the car. I mean, I understand how they prevent Mopar body wrinkle, and I understand how a roll cage will provide roll stiffness, but here is a piece of box tubing that is only 2" in cross section and a car that is maybe 60" in cross section. I can't find my dang ME Handbook, but I remember that tortional rigidity is proportional to the forth power of the diameter. Based on that, a puny 2" bar is not going to add mesurable tortional rigidity. Yet I read here where people install them and feel the difference. What are they feeling, if not tortional rigidity? Or is my math wrong?
#2
Re: SFC question for the smart guys :)
SFC question for the smart guys
Which subframe connectors do you have? If it is a 2 point design like BMR and others, they leave a considerable amount to be desired (in my opinion). If you can find your book before I can (I'm moving this weekend and am about to take apart my network and drag it to the new place...where I'll be waiting for cable service until Tuesday), you can likley calculate the torsional stiffness of a subframe connector (made of round material) by considering it to be a Anti-roll bar that is "X" long and has arms on it that are 50+ inches. I suspect that math will not yield a very favorable number for torsional strength. Boxed tubing (or mild steel round stock) is a lousy torsion bar (unless it is tempered spring steel and being used as an actual torsen bar, such as Mopar, but we'll leave that for another discussion) as I'm sure you already have figured out. One of the best ways to add torsional strength to any rectangular object (like a chassis) is through diagonal bracing (or thicker material, etc). I prefer the SLP style of subframe connector, it is still not a perfect design, but by attempting to tie the chassis together diagonally, they are attempting to address the problem.
I suspect that the best design (if it would fit the chassis) would simply to take a standard 2 point design and add an "X" brace from the 4 points of the connectors to the opposite diagonal side of the car. The problem is going to be clearance. You may need to move the "X" down away from the chassis a considerable distance to clear the exhaust and the driveshaft at full droop, this makes the resulting trade off between chassis stiffness and ground clearance a losing battle.
However, GM designed the chassis to what they considered to be "an acceptable level of torsional rigidity". It passed crash testing and they sent it to market. By adding at least "some" extra stiffness to something that was considered by many to be "good enough" will likley have at least some degree of impact on the package as a whole. This, I suspect, is what everyone is "feeling". The only other possibility is the "my wallet is lighter, so the car must be faster/stiffer, etc" feeling....one of the two will have to get my vote for the answer.
Just my thoughts....(and opinions).
Kevin
P.S. I don't think your math is wrong. But I'll try to recheck it once I find that box.....
#3
Re: SFC question for the smart guys :)
My roll cage adds more rigidity than the SFCs do (my opinion).
However, the boxed SFCs make a great place to jack the car up NASCAR style. Plenty of room to get jackstands underneath.
However, the boxed SFCs make a great place to jack the car up NASCAR style. Plenty of room to get jackstands underneath.
#5
Re: SFC question for the smart guys :)
The Double Diamonds are better than many of the designs that are out there (at least in my opinion). I am not a fan of Kenny Brown engineering, but that is also a personal opinion (that has been discussed on a couple other recent subframe connector posts). Even so, I still consider them to be better than many (or all) of the 2 point designs that I have seen on the market.
As a side note, they are cadmium plated. Welding Cad plated steel produces a gas (as a byproduct) that is poisionous. If you do a set of those, be careful, or have a professional that has welded cad plated steel do the work.
Just my thoughts....
As a side note, they are cadmium plated. Welding Cad plated steel produces a gas (as a byproduct) that is poisionous. If you do a set of those, be careful, or have a professional that has welded cad plated steel do the work.
Just my thoughts....
#6
Re: SFC question for the smart guys :)
From the "other" discussion forum ...
Just some more data ....
And to give you an opinion as to what people are feeling, is more than likely a weight reduction in their wallet.
Originally posted by L G Motorsports
the main purpose of the sub frame connectors (are you ready for this), is to "connect the sub frames". There is not much torsional rigidity to be gained by a tube welded at each end ( and in some cases bolted). The rear pickup points flex under acceleration, and braking. That is what caused the floor of an f body to flex, not the cornering forces that act upon the spring perches. sub frame connectors do not eliminate that.
the main purpose of the sub frame connectors (are you ready for this), is to "connect the sub frames". There is not much torsional rigidity to be gained by a tube welded at each end ( and in some cases bolted). The rear pickup points flex under acceleration, and braking. That is what caused the floor of an f body to flex, not the cornering forces that act upon the spring perches. sub frame connectors do not eliminate that.
And to give you an opinion as to what people are feeling, is more than likely a weight reduction in their wallet.
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#9
Re: SFC question for the smart guys :)
My roll cage adds more rigidity than the SFCs do (my opinion).
However, the boxed SFCs make a great place to jack the car up NASCAR style. Plenty of room to get jackstands underneath.
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Hadn't thought of that. Great idea.
#10
Re: SFC question for the smart guys :)
From the "other" discussion forum ...
Just some more data ....
And to give you an opinion as to what people are feeling, is more than likely a weight reduction in their wallet.
Originally posted by L G Motorsports
the main purpose of the sub frame connectors (are you ready for this), is to "connect the sub frames". There is not much torsional rigidity to be gained by a tube welded at each end ( and in some cases bolted). The rear pickup points flex under acceleration, and braking. That is what caused the floor of an f body to flex, not the cornering forces that act upon the spring perches. sub frame connectors do not eliminate that.
the main purpose of the sub frame connectors (are you ready for this), is to "connect the sub frames". There is not much torsional rigidity to be gained by a tube welded at each end ( and in some cases bolted). The rear pickup points flex under acceleration, and braking. That is what caused the floor of an f body to flex, not the cornering forces that act upon the spring perches. sub frame connectors do not eliminate that.
And to give you an opinion as to what people are feeling, is more than likely a weight reduction in their wallet.
#11
Re: SFC question for the smart guys :)
SFC question for the smart guys
Which subframe connectors do you have? If it is a 2 point design like BMR and others, they leave a considerable amount to be desired (in my opinion).
If you can find your book before I can (I'm moving this weekend and am about to take apart my network and drag it to the new place...where I'll be waiting for cable service until Tuesday), you can likley calculate the torsional stiffness of a subframe connector (made of round material) by considering it to be a Anti-roll bar that is "X" long and has arms on it that are 50+ inches. I suspect that math will not yield a very favorable number for torsional strength. Boxed tubing (or mild steel round stock) is a lousy torsion bar (unless it is tempered spring steel and being used as an actual torsen bar, such as Mopar, but we'll leave that for another discussion) as I'm sure you already have figured out.
Oh, I thought temper moved the yield point out, but the stiffness is controlled by the modulus of elasticity, which is related to composition. But, as I said, I can't find my ME handbook. Actually, these days I write embedded software so don't use the handbook much. Recently for things like the two satellite receiver boxes, although I've done a lot of machine control too.
One of the best ways to add torsional strength to any rectangular object (like a chassis) is through diagonal bracing (or thicker material, etc).
I prefer the SLP style of subframe connector, it is still not a perfect design, but by attempting to tie the chassis together diagonally, they are attempting to address the problem.
I suspect that the best design (if it would fit the chassis) would simply to take a standard 2 point design and add an "X" brace from the 4 points of the connectors to the opposite diagonal side of the car. The problem is going to be clearance. You may need to move the "X" down away from the chassis a considerable distance to clear the exhaust and the driveshaft at full droop, this makes the resulting trade off between chassis stiffness and ground clearance a losing battle.
The only other possibility is the "my wallet is lighter, so the car must be faster/stiffer, etc" feeling....one of the two will have to get my vote for the answer.
Just my thoughts....(and opinions).
#12
Re: SFC question for the smart guys :)
Boxed tubing (or mild steel round stock) is a lousy torsion bar (unless it is tempered spring steel and being used as an actual torsen bar, such as Mopar, but we'll leave that for another discussion)
I'm not sure that's any better.....
Kevin
PS I'll always take a cage for stiffness over connectors....that 3rd dimention helps alot....
#13
Re: SFC question for the smart guys :)
I just read all the SFC threads and see that a lot of guys like SLP or LG with the crossbraces. I hope they will do a decent job keeping my T tops from leaking or getting loose. I wouldn't mind at least a 4 point roll bar. Who makes the best and easiest to install roll bars? Having easy access to the rear seats would be nice but if that would make it weaker than a different style bar I guess that won't matter much. I'd like to get decent SFC and a roll bar installed at the same time since I will need to remove the carpets to prevent fires from welding and to get the roll bar in.