Aluminum Main Girdle Support

[swolds]

A friend and I are designing a main support girdle made of T7075 Aircraft grade aluminum........the girdle will overlap billet steel 4140 main caps 1 through 5..
It will sraddle all 5 main caps and will have 2 sections..... 2 pan rail pieces and 5 main cap sections that will straddle exsisting billet caps that it will crossbolt to the sides of the billet caps via cap screws...
Once bolted together on the block we will weld it ......so it will be one piece....being aluminum we are wondering what preload we should set for this as it is aluminum and not steel...which would be .002 -.003 on the pan rails...being aluminum I figured it would be more...and had the same thoughts from others......what should the air gap be set to ?


the girdle will look like this but will not have it's own integral main caps as such


http://photobucket.com/albums/v237/70w3 ... girdle.jpg

Thanks Ron

[Mark Workman]

One thing you might consider is the weldability of 7075. The exact reasons why are in my reference material at the shop, but here is a link that may be of interest. Also look at the data on this link regarding loss of strength in the welded area.

http://www.lincolnelectric.com/knowledg ... #question1

Assuming that the weldability issues are resolved, another thing that concerns me would be the different rate of expansion of aluminum vs the iron or steel of the block and caps over that large of a span. It would be significant.

Mark

[swolds]

One thing you might consider is the weldability of 7075. The exact reasons why are in my reference material at the shop, but here is a link that may be of interest. Also look at the data on this link regarding loss of strength in the welded area.

http://www.lincolnelectric.com/knowledg ... #question1

Assuming that the weldability issues are resolved, another thing that concerns me would be the different rate of expansion of aluminum vs the iron or steel of the block and caps over that large of a span. It would be significant.

Mark

Thanks for the info.....I never realized this about 7075 Al.....so I guess it will be a bolt together girdle..we need to utilize the strongest grade aluminum.
As far as expansion goes another friend has a girdle with the same design....and it is bolt together, same principle though....a girdle overlaying exsisting billet 4140 main caps....heis seeing 1100 plus hp with a factory cast block with weak mains so it does work it has been together for 15yrs same block.
My thoughts were to make a one piece design, so there are no points that will have a chance to move due to fasteners incorperated into the design and having 10 attachment points

As far as expansion rates of different materials ...what is everyones opinion as to how huch heat is generated in this area of the motor to really have an affect in this aspect



Thanks Ron

[MadBill]

I'd be more concerned about the stiffness of aluminum. Its Modulus of Elasticity is barely one third that of steel, so you would need 3X the thickness to gain the same effect, completely cancelling out any weight advantage in most loading conditions.

[Silverback]

One thing you might consider is the weldability of 7075. The exact reasons why are in my reference material at the shop, but here is a link that may be of interest. Also look at the data on this link regarding loss of strength in the welded area.

http://www.lincolnelectric.com/knowledg ... #question1

yea, that link pretty much covered what I was thinking was going to be a bigger issue with aluminum in general (outside of picking a weldable alloy), that typically what you’re left with are welds that are 1/3 the strength of the base metal (of course, you could do better, but it’s not easy). Done correctly at that point you could re-heat treat assuming that you welded it with a smiliar alloy but then you’d have to machine again afterwards.

I'd be more concerned about the stiffness of aluminum. Its Modulus of Elasticity is barely one third that of steel, so you would need 3X the thickness to gain the same effect, completely cancelling out any weight advantage in most loading conditions.

half the weight of steel, but you need 2x as much for the same ultimate strength (in general better for his aerospace alloy ) and 3x as much of the stuff for the same stiffness…

Steel starts looking really good, I wonder if there would be a point in something like 4340 or something (to get something that could be consistently heat treated and machined OK), but if you’re going that far how about some 300M (leaving out the real exhotics).

ACTUALLY Ti welds well and would be kind of slick…

[swolds]

One thing you might consider is the weldability of 7075. The exact reasons why are in my reference material at the shop, but here is a link that may be of interest. Also look at the data on this link regarding loss of strength in the welded area.

http://www.lincolnelectric.com/knowledg ... #question1

yea, that link pretty much covered what I was thinking was going to be a bigger issue with aluminum in general (outside of picking a weldable alloy), that typically what you’re left with are welds that are 1/3 the strength of the base metal (of course, you could do better, but it’s not easy). Done correctly at that point you could re-heat treat assuming that you welded it with a smiliar alloy but then you’d have to machine again afterwards.

I'd be more concerned about the stiffness of aluminum. Its Modulus of Elasticity is barely one third that of steel, so you would need 3X the thickness to gain the same effect, completely cancelling out any weight advantage in most loading conditions.

half the weight of steel, but you need 2x as much for the same ultimate strength (in general better for his aerospace alloy ) and 3x as much of the stuff for the same stiffness…

Steel starts looking really good, I wonder if there would be a point in something like 4340 or something (to get something that could be consistently heat treated and machined OK), but if you’re going that far how about some 300M (leaving out the real exhotics).

ACTUALLY Ti welds well and would be kind of slick…

We have done a 4140- 5 main girdle already....just looking for some input on an aluminum piece......bolt together it will be and yes it will be bulkier due to a lower tensile strength.....I never realized 7075 couldn't be welded....good thing someone pointed that out here
My buddy is insisting on doing it in aluminum....

[MadBill]

It may be counterintuitive, but plain old cold rolled steel is probably the best choice. All you get with 4130, etc. is greater ultimate strength, which doesn't come in to play unless you're trying to restrict how far the fragments of a shattered block fly. It doesn't add a bit to the stiffness since all steel alloys have essentially the same modulus.

[swolds]

It may be counterintuitive, but plain old cold rolled steel is probably the best choice. All you get with 4130, etc. is greater ultimate strength, which doesn't come in to play unless you're trying to restrict how far the fragments of a shattered block fly. It doesn't add a bit to the stiffness since all steel alloys have essentially the same modulus.

Well Program Engineering makes all their main caps and support girdle are made of 4140 or 7075 ...all they do is this type of stuff for nascar and aftermarket main cap replacement for factory blocks..
I would agree making the girdle similar material to the block would be a good idea.....have you ever seen the main webs in a 455 Oldsmobile pretty damn flimsy...couple that with huge 3" mains ...and large rod journals and big stroke....it has to be better than nothing....

Again I do not like the idea of using aluminum....it is something he wants to do.....but it seems to be working in a factory 455 block with a 4.800 srtoke @ 1100 plus hp in a 3000lb car......so it has to work......just looking if anyone has done such a thing in this material.....

[highVE]

I used a girdle that looks just like that design, made of steel and it went on a 500ci buick. stock block, filled , studed mains with girdle. made 876Hp.

Mike Theroux

[Silverback]

It may be counterintuitive, but plain old cold rolled steel is probably the best choice. All you get with 4130, etc. is greater ultimate strength, which doesn't come in to play unless you're trying to restrict how far the fragments of a shattered block fly. It doesn't add a bit to the stiffness since all steel alloys have essentially the same modulus.

OK, then you have me stumped.

I’ve always been highly opposed to using 4130 in roll cages and similar applications (as have a lot of people). Normalized 4130 is barely stronger on average then just plain 1018 DOM (their possible ranges overlap so it is possible to get a piece of 4130 with a lower yield strength, but generally we’re talking about 70Ksi for 1018 and about 73Ksi for 4130), and the issue is that 4130 hardens with heat so when you weld it you get very hard areas around the weld where the stuff self quenched, and those also tend to be brittle, so in a hard impact they crack and break leaving gagged points rather then absorbing the impact like a piece of 10xx steel would have. If you could heat treat the whole thing after you put it in the car then you could potentially have a safer cage, but real world, that’s impossible. I believe that is why mild steel is used in nascar and others, and can’t for the life of me understand why the NHRA, IHRA and I believe most road racing sanctioning bodies (SCCA) requires 4130 in faster cars AND allows you to use thinner/smaller diameter tubing. It doesn’t make sense.

The only conclusion that I could come up with (based on something that someone told me, that I never bothered to look up/confirm) for the proliferation of non heat treated, welded 4130 parts in assorted racing was that the 4130 stiffer then a 10xx steel. Now you’re suggesting that is not the case, and in that case I’m lost. I can’t imaging why you’d use 4130 in anything that isn’t going to be heat treated.

[MadBill]

You're exactly right in everything except your final conclusion, Silverback. The error is in assuming there actually is some good reason to use high strength alloys in a chassis! The only ones I know of are :
A.: In components such as the roll cage, where strength counts more than stiffness and it's paramount that it not collapse on the driver. (Although for a few pound more, a mild steel cage can take the same loads without any heat treat issues)
B: If you are using really thin wall tubing, say 0.040" or less, a fully heat treated alloy structure will be less susceptible to accidental denting and the resulting loss of stiffness (A pop can with no dents is hard to crush. A flick of a finger on it's side will cause it to crumple when being lightly squeezed by a can crusher.)
C: If drivers are a dime a dozen, but you really need the car to survive The Big One with minimal damage.

It's fairly easy to verify the equal stiffness of steel alloys. Just take two similar samples... no, make it three: one mild steel, one normalized 4130 chromoloy and one 4130 heat treated to a fare- thee-well. Make them of say, 1/8" x 1" flat stock 24" long, clamp them side by side overhanging the edge of a bench, and attach a one pound weight near the free end of each.

The deflections will be identical. Now replace each with two pound weights. Again, they will all deflect the same amount. Continue adding weights. Eventually, the mild steel strip will be permanently deformed by the load and will not return to flat, while the chromoloy ones will, continuing to deflect equally. With (as you say) only slightly more load, the normalized 4130 will also be permanently deformed, while the fully heat-treaded specimen will not.
Therefore, since a race car chassis (and many other applications) requires stiffness (resistance to twist, bend, etc.) rather than strength (resistance to crash damage), there seems to be few good arguments beyond the "wow" factor for the fancy alloys..