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Old 07-10-2003, 10:03 AM   #1
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Chrome Moly Rear End or Mild Steel?

This question is aimed at anyone with EXPERIENCE of running a chrome moly chassis and/or rear end on the street...

I'm building a 600hp 383 with 250 shot of n2o pro street car, I've been advised to get an Alston Fab9 rear end, Chris Alston says I am borderline for the mild steel housing, n if I twist it, the housing is junk. I've been advised to get the chrome moly rear end, but someone told me that chrome moly is too rigid for street use, and that it'll crack - just as a chrome moly chassis would crack on the street which is why he's running a CDS subframe with a chrome moly cage.

According to the Moser Axle Tech section on their site, with 800ft lbs of torque and a 4.88 rear gear with a 2.75 first gear I would need 35 spline axles - which would mean (according to Strange Engineering) that I'd need to run a 3.250" axle tube housing. So the price has just gone from $400 to $750 on that rear housing.

However, I've known racers to run full slicks on bigger motors than mine with a 300 shot of n2o using only 31 spline axles and a mild steel standard Ford 9" housing.

So my question is - do I NEED 35 spline and chrome moly this n thats? Or would a mild steel housing with the brace-kit and a 33 spline axle be suitable enough?

I'm starting to add up the costs for my drag car project n its hitting the $13,000 mark just on parts, I'm trying to save some money by not buying things I dont NEED.
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Old 07-10-2003, 10:43 AM   #2
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Thoughts:

1) DISCLAIMER: I've not run any of the equipment you are considering. Stop reading here if you wish.

2) As an engineer, and parts manufacturer, I would advise taking the advice of your axle manufacturer. Yes 4130 (Cr-Mo) is somewhat stronger than low carbon steel (aka CDS or mild steel), but Cr-Mo is usually used when saving a few pounds is critical, or extreme strength at the same weight is critical. The 3.25 mild steel tubes will probably be stronger overall than the 3 inch Cr-Mo. If Cr-Mo cracks from street use (fatigue) it wasn't built correctly.

3)If you twist the small mild steel housing, isn't it also junk then? Same situation on the 31 vs. 35 spline axle.

3)If you LBF it (vulgar term for buying lower-cost parts), and it fails, you then have to go to the upgrade, so it costs more in the long run. Maverick's Pappy said it well: "Why is there never enough money to build it right, but enough money to do it over?"

4) If you are boderline with 850, what happens when you get it to 950...any you will, right?

My $.02
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Old 07-10-2003, 10:56 AM   #3
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Well, first, chro-mo parts don't crack because they're "too stiff", they crack because people get too radical with their weight reduction or got sloppy with their design. So, let's dispose with that myth up front. Now, it may be true to say that a particular vendor's part may crack when fabbed in chro-mo, but it's not really the fault of the material.

With that being said, I'd go with the stronger housing because that's what the vendor is recommending, and I'd go with the bigger axles for the same reason. Just because you've seen someone run a lighter setup doesn't mean that it's smart to do so, and half-assing stuff always causes me a lot of undue stress even if parts don't break. The piece of mind is worth the minimal increase in cost, and it seems like something as fundementally important as the axle is a bad place to look for a cost reduction.
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Old 07-10-2003, 12:41 PM   #4
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Well 950 is beyond capability of the rotating assembly, but speed is addictive, car aint even complete n im already looking at big blocks for the future The amount of **** I've broken you woulda thought I learnt my lesson, but seeing a $13,000+ list of parts that need to be paid for before Christmas - not to mention labor costs - well, it kinda makes u think "holy **** what am I doing!" lol. But yeah $200 bucks for piece of mind I suppose is the best way to go, I'll sell the nitrous kit outta my daily driver if I hafta I cant see this car running before 2004 though unless my loan comes through, so I guess I should go for the better parts and spread the cost out over a longer period of time.

With regards to the cracking Chrome Moly, I was told "in a storm, what will be more likely to break? A stiff tree or a flexible plant?" i.e. on the street the stiff tree (chrome moly) will see alot more banging than the mild steel that will flex - but isnt that the whole idea? To eliminate flexing in the rear end housing?

Thanks for the responses, you just cost me more money :P
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Old 07-10-2003, 01:58 PM   #5
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you are correct that chr-mo is more brittle, but if fabbed correctly it WILL NOT BREAK under its intended use.

I drive a 120+mph go kart that its WHOLE SUSPENSION is a chromoly frame. it flexes like crazy, and it the chassis never has broken.
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Old 07-10-2003, 04:02 PM   #6
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I take exception to calling (4130 or 4140) Cr-Mo "brittle". One of it's charms is that it is 25% or more stronger than mild steel, but very tough. Though means it deforms or flexes a lot before it fails, and unless it is heat treated to the strength/hardness of a grade 8 bolt, it doesn't "snap".

The stiff tree/flexible plant analogy is flawed, IMO. The Cr-Mo "tree" would bend about double or more before it failed. It would also take a Cat 5 Hurricane to fail it relative to a Cat 1 which ripped apart the flexible plant. You can't eliminate flex or "strain", but you can miximize it with stronger or bigger parts.

JordanM is correct. The suspension "spring" on a kart is frame flexure. Cr-Mo makes a good anti-roll bar at the correct hardness.

Cr-Mo parts that fail at weld joints, or crack are either fabbed improperly or have improper heat treating. Bad welding could cause both. Of course, severe overload would also fail them.

We praise 4340 cranks and rods for their great strength and durability. 4340 is just Ni-Cr-Mo. Nickel is added to the chrome and molybdenum to help the heat treating. It allows full hardness in deeper sections and in martempering, a great way to heat treat things like con rods, allows a 99% martensite formation as low as 300F.

My $.02
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Old 07-10-2003, 09:51 PM   #7
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You want to talk money spent! Wait till you need to have the chassis jigged and welded! WOW! My new car is just about done and it was built to SFI 25.1D spec. Chromoly tube chassis from firewall back with double frame rails,Chromoloy funny car cage from front of the car to the rear including chromoly 4-link,wishbone bar,anti-role ect. This car is built using a custome made chromoloy sheetmetal rearhousing, centered pig(not offset like normal 9") with back brace and so on. I know of two different guy's that have ran mild steel Fab 9's and both rear housing ended up bending. Both cars weighted around 2800lbs. And both cars where tube cars. Check this site and ask for information from screen name Hellbent, he bent his fab 9" and his car was somewhat light mild steel car. Its the blue Chevell that you can view in the pic's section.

www.gofastzone.com

I have heard the same stories about street driving chromo chassis cars on street and supposed crack in the welds or what not? I think that happening to a car is do to the poor construction of the car and the chassis builder. I have own one other chromoly car in the past and the car would see often street use in the summer and plenty drag use both on street and track. Not once did the car ever fail getting certified nor was there any chassis cracks or what not.

If you have a heavy car without a doubt go chromoly rearend. Plus, if you check the site I mentioned you will find a 66 GTO Mild steel tube car that runs 35 spline axles and the car get 780hp of nitrous and the motor is a 632ci that made 1211hp without bottle. He has been using the same axles for over 5 years and never a problem or broken axle. See the pics of Fasterthanmost. This car is also street driven and runs 7.67et in the 1/4 mile. I know this because these are all friends of mine that I race with in my town.

If you can do chromoly do it and if for any reason at all just look at the value of a chromoly car vs mild steel car,just about doubles
value.

I should have some pic's soon of my chromoly rear assembled as soon as it gets back from powder coating. If you would like to see some shots of this in a F-BODY give a shout! It is trick looking!
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Old 07-10-2003, 10:04 PM   #8
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Yep, I could go build a chrome moly tube chassis car - but thats not what I'm doing, don't need it to run 9's n thats as far as this particular car is going. I'll definately go for the chrome moly rear and 35 spline axles then, I doubt I will NEED that much but its nice to have peace of mind anyway.
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Old 07-10-2003, 10:20 PM   #9
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Here's what little I know about metal fatigue. "Fatigue" refers to the tendency of the metal to fracture under repeated cyclic stress. The magnitude of the stress may be considerably below the ultimate yield strength. The number of cycles required to cause fatigue failure at a particular stress magnitude is large, but decreases as the magnitude of the stress is increased. For some mild steels, cyclical stresses can be continued essentially indefinitely if they are of low magnitude. I suppose the same is true of CM as well.

Some of you guys know a lot more than me about materials, so forgive me if this seems ignorant. But I have also heard a lot of stories about CM parts breaking/cracking on the street. This doesn't surprise me and I don't necessarily think it means bad design or fabrication. If we are talking about a chassis or a part built for max-effort drag racing, you don't want it any stronger than it has to be. That's because if the design is optimized the only way to make it stronger will also make it heavier. And who wants to haul more weight down the track than needed? A properly designed drag racing part would not be expected to tolerate the same number of stress cycles as a street part. If you can drive your drag car for 100,000 miles on the street it must be too heavy, right?

So, I guess I am violating nettiquite with my ramblings, because Mr. Nitro only wanted to hear from people running CM on the street. Well, I have run a lot of CM bicycles on the street, if that counts. But I wouldn't hesitate to run a CM chassis on the street if it were designed for it. In fact, I have discussed this with Jim Salemi of G-Force race cars. Basically, he said that he would use a similar design but with larger/thicker tubes if he were building a car that would see any significant street use. Of course, it would be heavier than the same frame made for drag racing only. But there would be no problem using CM. It sounds to me like neither of the CM or mild steel housings are designed for the street. If the CM is stronger, it would be better, but not necessarily adequate.

I would sure get the 35 spline axles though.

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Old 07-10-2003, 10:29 PM   #10
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Well, my original post did state that I wanted to hear only from people who were running these things - but its become apparent that theres plenty people NOT running these things who ARE knowledgable - so I am open to hearing everyones comments on this - I like to have responses to my technical queries backed up with an explanation, which all of you seem to have done. I get tired of pure opinion posts that state "chrome moly sucks" - I want to know why it sucks, and vice versa

Before I go ahead and buy anything though, I will discuss it with Chris Alston and see what his views on its streetability are. My chassis fabricator highly reccomended using the Chrome Moly 3.250" Fab9 housing, but my brother who lives over in England for some weird reason, has a chassis guy building his tube chassis car who's telling him Chrome Moly is unsuitable for street use - but there are many, many things I do not agree with that the guy tells people - for example, his opinion that anything over a 2600 stall is not streetable. But anyway thats another story for another time.
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Old 07-11-2003, 08:18 AM   #11
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Quote:
Originally posted by JWINN
I know of two different guy's that have ran mild steel Fab 9's and both rear housing ended up bending. Both cars weighted around 2800lbs. And both cars where tube cars. Check this site and ask for information from screen name Hellbent, he bent his fab 9" and his car was somewhat light mild steel car. Its the blue Chevell that you can view in the pic's section.

www.gofastzone.com
Nice to see another West MI guy in here (Jason AKA Hellbent is a coworker of mine and has instructed me to come talk to you when it's time to stick a 6-point bar in the Impy).

Rich brought up CM bicycles, a topic that I'm, shall we say, somewhat familar with. The whole idea of using CM is to make a more-efficient structure; that is, owing to its material properties, CM allows you to build a frame that's lighter while maintaining the same stiffness and fatigue resistance. The only mild-steel bikes that you'll find are in the sub-$300 range; anything beyond your Wal-Mart Huffy or Murray uses CM and for good reasons. Now, you can take things to their extremes and buy a superlight race frame that's only good for a season or two of use, but that doesn't mean that CM's a bad material; it's just that weight reduction was taken to the extreme. It would seem to me that if I'm going to demand CM when buying a $500 bicycle to support my 165 lbs at 25 MPH, then I'd really like that in a racecar.

Nitro, if you want more info on this topic, digging into the Carroll Smith "... to Win" books would be a great idea. They ain't perfect, but I don't know of any better source for this info (they're certainly better than my usual somewhat-flawed analogies involving comparisions between drill bits and coat hangers)
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Old 07-11-2003, 01:00 PM   #12
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Eric,

That's funny you work and know Jason. He's a pretty good buddy of mine. You can tell him I said he's having a Jesse James identity crissis. LOL! He needs to get his Chevell out this year so he can have half a chance at the track or on the street against me. Tell him he needs a 3rd stage of nitrous and I promise to not let everyone know I beat his *** with my Haybusa off the bottle.LOL!

In response to other quotes:

It's always funny to hear the people that give expert advice on something but have absolutely no experiance with it. Very technical text book theories and or tests results quoted are indeed impressive to say the least and nice to know but don't always hold true!

I do have experience with both mild steel chassis and chromoly chassis and for a street or track car mild steel would be more than fine. But when it comes to the rearhousing I know for a fact the Fab 9" with a car above 2500lbs will deflect and or bend weather or not it has been braced and gusseted. I've seen it happen! Go with the chomoly rear housing. But like I said about the chassis either material will work weather its complete tube car or just backhalf. But every person I have ever talked to always has said they wished they went chromoly.
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Old 07-13-2003, 10:26 AM   #13
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I hope most of your questions have been answeredred, on my cars that I have owned and driven on the street 69 Camaro NA 10:90's- stock 9", N case 31 spline axles (factory), N20 (secret), 81 Camaro NA 8:60's Shortened 9" stock axle tubes w/brace 35 spline strange axles and spool This car 60' in the 1:28 -1:35 range. 3 seasons and no problems on rear. We do paint a white line down the axles themselves and check for twist about evry 6 months on maintence. Believe me sometimes on other rears we have done its scary to see the axle twist 180 and sill not breaking (we do take them out if we see this) I have to agree Chromemoly will not break if properly welded. What probably happens is that all the vibrations a car see's on the street will really stress a suspension. especially a race car that pretty much has all solid mounting points, not much give. thanks Andel
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Old 07-13-2003, 10:26 AM
 
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