Why does a chassis wear out?

[Terry Carraher]

I wonder about this too, it doesn't seem like the frame experiences that much flex. With car springs and torsion bars, there is much more motion. So I have to wonder if this is true or just part of kart "lore".

The purpose of stress relieving is that the act of fusion welding is essentially the same as when the metal is first poured, any steps that have been performed on the metal since it was made are, for the most part, undone in the region of the weld. If it is a mild steel, then this is likely not that big of a deal, but a higher alloy steel should be stress relieved, otherwise the weld is a potential weak spot. The type of fusion welding, be it TIG, MIG, etc., should have little effect on this, the act of melting the base metal and adding filler metal is why stress relieving is necessary. The heat input is enough to change the crystal structure in the steel. The other way around this is to braze the frame together since this doesn't heat the base metal enough to change its properties.

-Terry

[Scott Davis]

Good Explaination Marc!

[Darren Swisher]

What does this say for the used kart market?

Can a used chassis be competetive with a new chassis?

Darren

[John Clasen]

John G. It really isn't necessary to do a great deal of scientific testing to determine if the chassis is less effective after two seasons or even after four races. Just get out the stop watch and you will begin to see the tail of the tick tock!
On a road course this is not so critical, but for sprint shifters it certainly is at the top levels.
J.R.

[Bill Pedersen]

TIG welding is the preferred method of manually welding 4130 chrome moly thin wall tubing. The National Hot Rod Association (NHRA) requires that all 4130 chrome moly tube welding must be done by approved TIG heliarc process. On any F1 or Indy cars I have seen all of the 4130 are also TIG welded. With a skilled welder, TIG is the most accurate of all of the welding processes. You can weld very thin (.010-.020) material to really thick 4-inch material without burning thru the thin sheet and yet still have good penetration on the thick. When you are TIG welding, you can see what you are doing and where you are welding a lot better than with MIG. There is no smoke and if you use a WP20 torch, it is about half the size of a MIG gun.

TIG also allows careful control of the puddle and the puddle temperature, by using either a foot control or a finger control for temperature adjustment. This control allows you to accurately add just the right amount of filler rod at exactly the right place in the weld puddle and to increase the heat at the beginning and decrease the heat at the end of the weld as needed.

MIG allows the least control of the heat and the puddle. Temperature control is usually varied by stopping the weld and changing the setting on the welder. The filler rod/wire feed rate is constant and you vary you puddle size by varying your rate of movement along the weld. The puddle size varies the penetration. Moving too slowly will blow holes through the tubing. Moving too rapidly will lay a nice bead on top of the metal without any strength that results from the depth of penetration.

I’m not aware of an inexpensive MIG welder that has acceptable heat control for welding thin wall 4130 tubing safely. You can do it and I have MIG welded 4130 when I had no other choice, it’s just not as good as TIG unless you have a robot and computer controlling the process. Manual MIG welds tend to invariably start the very beginning of the weld too cold. This means that every time you start a MIG weld, the first fraction of an inch doesn’t get the same penetration as the rest of the weld. Also, the end of the weld tends to get overheated on thin wall tubing. Since most of the welds on karts are only an inch or so long, the cold start and the burning of the base metal at the end of each weld can cause problems. Using a CNC welder would take care of most of the problems. MIG welding is so much faster and less expensive (manual) than TIG and is why it is used in a lot of applications.

Metallurgists classify 4130 steel as “air hardening”. This means that after having been welded, it does in fact return pretty much to its original state. So much has been published about the supposed vital importance of "normalizing" and “stress relieving” that it comes as a shock to many to learn that a lot of small aircraft 4130 tube fuselages, landing gear, engine mounts and a variety of important parts for the planes have been built without this being performed, and have given long and safe service. Many books written for aircraft mechanics say nothing at all about normalizing or stress relieving 4130 after welding.

Lincoln Electric published a document earlier this year on welding 4130, which states: “Thin wall 4130 tubing normally does not require stress relief. For parts thicker than .120", stress relieving is recommended and 1,100ºF is the optimum temperature for tubing applications. Use a 900 F-temperature crayon and mark approximately 1” away from the weld area. Use an Oxy/Acetylene torch with neutral flame. It should be oscillated to avoid hot spots.”

The new “Performance Welding” book states: “One of the most repeated and most taught mistakes in aircraft welding is that each welded assembly must be stress relieved after welding by reheating the welded area to blood red by heating it with an oxyacetylene torch. The fact is that this quick reheating process actually does far more damage to the welded tubular structure than it ever does good.”

NHRA has very detailed specifications on diameters, wall thickness and locations for the driver’s areas on the Top Fuel and Funny Car chassis. They sonic test all of the tubing in this area, but they only do a visual check on the welds. There is no requirement or specification for stress reliving the weld area.
Back purging is not normally necessary on 4130 tubing. Many years ago I took the aircraft certification welding test. The weld cluster test was welded without a backup gas. The weld cluster is sectioned for inspection and requires 100% penetration to pass the test.

I recently made a wheel chair for my Father out of thin wall (.020”-.065” Titanium tubing and it did require back purging. It is a lot of work, a real pain and uses a lot of argon gas. While a backup gas may have a very slight cooling effect on the inside of the tube, it should not have an effect on whether or not the item would need to be stress relieved. The backup gas is used to purge the oxygen out of the weld area.

Another kart that is TIG welded that has not been mentioned is made by CTS in California. The welds are beautiful and the workmanship is the best I’ve seen on a kart.

[JClark]

TIG Welding is fine in most application's. Most 4130 CRMO thinwalled fuselage's were and still to this day Gas welded. The weld is only as good as the operator. Most people who fly planes, trust OA welding it's been proven since before WW2. Not many applications where MIG is acceptable on a A/C. Most chassis in Nascar are MIG. I've built Sprint Car Chassis, Rail Dragster's, Aircraft useing OA. I have a TIG welder, don't use it for Kart chassis that much because of the localized high heat and the problems which ensue. Gas welding is not as pretty but with proper pre and post procedures is just as good or better.

[John Clasen]

T-Man, perhaps you could share with us where you get the insider information on what grade of tubing is used by what manufactures? Since you just slammed a half a dozen or more major brands, it would be important to alot of shops and their customers to understand how this could be.

I seem to remember that in '99 and 2000 the speed boys and several others used and won consistently on the MBA chassis. Maybe the MBA factory got the tubing that was susposed to go to Tony Kart given the low win ratio for Tony Kart during that period. Ummm.

A Curious J.R.

[Chuck McCue]

"T man",
I believe the SKM built Italkart faired very well at the last Pro Moto, S1 and Super Pro. Where did you get your tubing knowledege?

CM

[Jeff Franz]

T man,

I am unsure if you received the message that the board moderators sent to you. If not, let me kindly remind you that you will need to update your user profile to display your real name and an active email address.

To make the appropriate changes, kindly use the "my profile" link in the top right area of these forums.

Thanks much,

Jeff Franz

[bird]

Well if YOU looked at the chassis info you would know that by your standards Golds are not inferior karts then wouldn't you? OK I'm done feeding the troll.

[Ray Knight]

With regard to naming certian brands as being built out of lesser quality materials. I have the opportunity to work on many brands of chassis and while there are differences in material in some many are made of different materials in different locations on the kart to create desired handling characteristics.Paroline Mfg. builds for 10 or more brands but you are dead wrong including KGB as a 2nd grade kart.I have had several chassis come un-painted from KGB and they are 100% Cro-moly.Please be careful on 2nd &3rd hand inside information.

[Henrik Thun Rasmussen]

I've found that when a chassis gets old the tubing starts to crack. about the good/bad chassis could it be that a chassis handling changes through its life when it is brand new it is alot more rigid than when it gets old. And if this is true would you have race it a couple times to find out what its like. to brake in ?

[Rodney Ebersole]

Boy, this has been a good read so far! do you think it did any good to my old Margay TNR when I Cryo treated it and baked it?

[Marc Miller]

Rodney,

I doubt it.... when you cryo'd it, it most likely became brittle.... then you baked at and it softened it.. going from one extreme to the other like that is like taking a piece of aluminum and bending it back and forth until it breaks.

So.. I was assuming you were kidding... but wasn't sure.

MM

[Mike Goebel]

quote:

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Originally posted by Rodney Ebersole:
Boy, this has been a good read so far! do you think it did any good to my old Margay TNR when I Cryo treated it and baked it?

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That's sort of like a frozen dinner . Remove from freezer and bake. Did the chassis taste any better?

Mike G.

[ August 12, 2001: Message edited by: Mike Goebel ]