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Author Topic:   Chassis Flex??
FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted September 14, 2005 04:47 PM  
On another post "Dirt Works" there was something mentioned about chassis flex. I know everyone has an opinion on this and thats what I want is your opinions as to why flex is good or bad and your experiences.

My thoughts are that maybe a little flex is good BUT anytime the chassis flexs its absorbing what the suspension should be handling. Therefore making it unconsistent. I can see like was stated in the other topic how it would make the car twitchy and basically unforgiving BUT I also think that with a good setup and driver its much faster with little to no flex. I had a mod that flexed a good bit and it was inconsistent although it did run good just not very consistent. My new car when it was built I brought up this topic to the chassis builder and his thoughts were basically the same as mine. When he finished the car he clamped the front of the car to the jig and let the rear hang off, he then attempted (with a big big bar) to twist the chassis. He said that was how you checked it??? He said the car had VERY little flex. And this car is consistent and pretty fast too. So lets hear your opinions!!

joeltjen
Dirt Maniac

Total posts: 182
posted September 14, 2005 07:52 PM  
my bosses son had a car that flexed really bad ( was a copy of a major brand latemodel with a stock clip put on) and you could scale this car, go run 2 nights and put it back on the scales and it was way off. re-scale it, go race, same thing.... way off. you could see the car "go away" in the heats. it was what it was and you couldn't get it any better with out a torch and welder.

IowaFuzzy1
Dirt Maniac

Total posts: 136
posted September 14, 2005 07:59 PM  
Flex=Fast I had a 05 Skyrocket on my frame machine this week,they have very little pipe ahead of the firewall and whats there is pretty light stuff and they do flex.This car is a consistant top 5 car at 3 tracks in this area.Flex actually lets the suspension work better aka.smoother,the trick is to figure out just how much to let it flex.Too much and you start having problems with cracking tubing at high stress points.Not enough flex and you have a car that is difficult to chassis tune,they work but they're just not quite as good as that guy ahead of you.

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 14, 2005 08:03 PM  
Here is my take on a "flex" car.
Flex must be kept to a certain amount, meaning not too much. How much is too much? I dont know to be honest kinda a trail method on that one. And how it flexes also should be taken into consideration.
if flex is about correct amount then
Pro
1)Easy to set up, meaning if you miss the set up your not going to be totally ruined.
2)driver forgiving, same reason behind a leaf spring car a leaf spring is very flexable but makes the car forgiving for driver error.
3)Lighter chassis overall, yeah a wieght limit but then you can put wieght where you want at to make the car handle better.

Cons
1) if flex is too much then car is inconsistant.
2) folds up easier in a crash
3) wont last as many races, will start to twist more than other cars
4) driver could learn bad driving habits.

now for a rigid car
pro
1)consistant
2) very fast when set up right
3)last long
con
1) brittle sometimes(chromoly cars) and break alot of welds
2) hard to set up
3) not driver forgiving
here is my take, i would want a car with some flex in it. I think some chassis have too much flex, others are too rigid.

Kromulous
Dirt Forum Champ
Total posts: 796
posted September 15, 2005 07:21 AM  
From an engineering standpoint, i cant understand how a builder can build a Mod out of DOM or ERW steel and pro-claim that there chassis flexes good.

Regular steel has a very low memory, so lets say the whole rear clip flexes up and twists 5*. Regular steel wont return to its natural condition, or its nuetral state, it will just flex them 5* and stay there. Actually it will come back a little but not much.

Chrome moly has a high memory, it will return almost all the way back to its nuetral state.

As well i dont see how builders can weld chrome moly and not heat stress releive it. I know that they claim you can weld chrome moly with mild steel filler wire and it wont harm the make up of the CM steel, but the problem is, if the heat of the welds pull and distort things it cant find a true nuetral point. Which is what heat stress reliveing will do, relaxes the metal at a given state, so when it flexes it will try to return to that point.


Anyway just some of my thoughts on chassis, and flex. I got some more expiernces i'll post later.

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 15, 2005 09:01 AM  
The cars not lasting as long would be due alot to the dom tubing.
but the chromoly tube cars have a large problem with breaking welds constantly. After a few races on one it seems to really grow on number of welds breaking. Rocket late models are the best built chromoly cars for dirt racing(imo) but they still have a shelf life before they start to get twisted somewhat.

IowaFuzzy1
Dirt Maniac

Total posts: 136
posted September 15, 2005 08:15 PM  
Chromemoly can be mig welded but it will crack next to the weld.The only way to properly weld chromemoly is Tig.With Tig the two pieces become one if properly done.Sprint cars are Tig welded as are Funny cars and Dragsters. This is easily demonstrated if you know someone who is a GOOD Tig welder:wire weld two pieces of chromemoly together,then Tig two pieces of it together.Take the two welded assys,put each one in a press and smash them both flat,the wire welded one will break, the Tig one{if properly done}will still be one piece.

IowaFuzzy1
Dirt Maniac

Total posts: 136
posted September 15, 2005 08:36 PM  
Chromemoly can be mig welded but it will crack next to the weld.The only way to properly weld chromemoly is Tig.With Tig the two pieces become one if properly done.Sprint cars are Tig welded as are Funny cars and Dragsters. This is easily demonstrated if you know someone who is a GOOD Tig welder:wire weld two pieces of chromemoly together,then Tig two pieces of it together.Take the two welded assys,put each one in a press and smash them both flat,the wire welded one will break, the Tig one{if properly done}will still be one piece.
The DOM&ERW cars that are built today distort too easily because they are built from tubing that is too thinwalled,thicker walled tubing retains its flexibility because of wall thickness.Flexibility is due to the resistence between the inside and outside surfaces of the tube;too thin=no resistence=bends,too thick=to much resistence=no flex.(IMO)firewall forward and rear structure behind the main cage should be minimum 85 wall.

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted September 15, 2005 08:53 PM  
When welding with chromemoly no matter if its TIG, MIG, or stick rods to get a PROPER weld you absolutly MUST pre-heat the work peice a minimum of 350 degrees! I wonder of the chassis manufacturers do this when building chromemoly cars????? Maybe thats why their cars break????? Oh, and TIG is not the only way to properly weld chrome. It can be done stick, MIG, or TIG if you do it right!

[This message has been edited by FlyNLoIMCA17 (edited September 15, 2005).]

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 15, 2005 10:42 PM  
I talked to my uncle about this. He used to build some pro stock drag bikes back along time ago. He said they went to chromoly frames. They built thier own frames. He said at first they MIG welded the frames, and broke them about in half sometimes. So TIG welded them, lots of cracks from heat.
He is a very intellegent person(meaning an upper level engineer). He said they brazed a few frames together, and when brazed correctly the frames lasted a long time. You could not do this to a race car frame because of saftey but it lasted for the drag bikes. He told me to properly weld the car together if I were to build it out of chromoly would be to tig weld it(very good welds) then send it thru an oven to help to normalize the metal. Meaning the whole frame would be brought up to a gradual temp then held there then cooled back down naturally(let it sit and cool). then i will not have as many problems with weld cracks and other problems with chromoly.
I am not sure what temp you need to take them too.

As far as preheat, the problem with that is how do you know how much actual heat is in the metal, and how do you do it uniform around the area? Every welding book i have seen says just eyeballing it with a torch causes alot more problems than it helps.
Sprint cars are TIG welded but it took alot of trial and error to get it right, meaning a few drivers lost thier life from welds breaking, or tubes breaking beside the weld.

I have been told that some funny car, top fuel frames are not even painted just greesed kinda so easier to check for broken welds becuase tehy break them constantly. I dont know how true that is, but we painted some rail cars for a few drag guys and they said that about the top fuel cars.

Also I agree that many cars today are too thinned walled.

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted September 15, 2005 11:05 PM  
Zero, you check the pre-heat temp. of the metal with a temp stick which melts at a given temperature. Heat it with a torch. I see your point about the uniform heat. There could be heat spots and such. You can heat it higher to insure that all the material is at least 350.

aggressive
Dirt Full Roller

Total posts: 33
posted September 16, 2005 06:42 AM  
http://www.avengerchassis.com/avenger_tech.htm
Its my belief that this is the best way to weld moly...its tig/purge and no i'm not a salesman for them...lol

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 16, 2005 08:08 AM  
I have seen some welding fixtures to fill the tube with shielding gas. This helps in the weld but waste alot of gas and adds to the price.

leapinlizard
Dirt Freak

Total posts: 402
posted September 16, 2005 09:52 AM  
Tig welding is the proper and only approved way of welding air space frames.

If it's good enough a DC9, it's good enough for me.

BTW, I don't have the cracking problems you guy's speak of. although I do stay on top of it. My car is all moly cept for the front and rear bumper. What I do have is a weight problem. My car weigh's about 200 pounds lighter than most cars.

Kromulous
Dirt Forum Champ
Total posts: 796
posted September 16, 2005 10:25 AM  
Zero, your Uncle confirmed what this building i work in, which houses about 300 Engineers of all disiplines say.

Interestingly enough i talked to one Engineer who actually races Drag cars, and he built his own frame last year (all tube). Its a 60's Ponitac Station Wagon that runs 7.20's. Anyway he told me that after doing some research that an acceptable way to weld moly was to Tig it but you use a mild steel filler wire. The filler wire is the key, because the moly tube will not change as bad using it.

The problem with welding Moly tube is the components that make up the moly slightly crystalize when you weld it. Which makes the metal brittle in that area. Welding it requires a welder that knows how to really focus the heat in a small area, while using the least amount of heat he can to get max pentration, more heat more crystalization, and more cracks.

Add that to the fact that when you weld something there is a certain degree of pull or push on the tube. So by the time you get it all together there are alot of tubes pushing and pulling in a blue million directions. Which will compound the cracking problem.

Heat stress releiving will relax all those push & pulling forces to a nuetral point. So once you remove a chassis from a oven its in a relaxed and nuetral state. Perfectly relaxed, problem is not many places have an oven that big and its not cheap to have done.

Interesting site, that Avenger chassis thing.

Krom.

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 16, 2005 10:49 AM  
Did the engineer tell you how hot the car must be heated up to do this? i wounder just how hot. and how long must it be kept up at that temp?

[This message has been edited by zeroracing (edited September 16, 2005).]

artie727
Dirt Full Roller

Total posts: 26
posted September 16, 2005 12:00 PM  
quote:
Originally posted by zeroracing:
Did the engineer tell you how hot the car must be heated up to do this? i wounder just how hot. and how long must it be kept up at that temp?

[This message has been edited by zeroracing (edited September 16, 2005).]


Zero, I'm not sure or the actual % of chromium in the tubing that the chassis builders use but,1% chrome, .5 moly needs only a preheat prior to welding and per ASME, requires no PWHT (POST WELD HEAT TREATMENT) The above listed material is manufactured with a minimum tensile strength of 50K. If you were to put the chassis in the oven it would be gradually heated to somewhere in the neighborhood of 1100 degrees for about an hour and the heat would be gradually decreased until cooled to 400 degrees. 1% chrome can be weleded with a 70 series electrode all day using any process you want, it just has to be done right. Purging the ID of the tubing is not required.
Wrapping a completed weld with insulation allowing it to slow cool is all that I would do.
If someone knows the actual chemical properties of the chrome tubing chassis builders are using I can tell you EXACTLY what needs to be done to weld it.
Oh, DO NOT put your car in an oven and heat it to 1100 degrees. It will not come out like it went in, it will move as the stresses are displaced.

------------------
Artie Perilloux
A & M Motorsports

Kromulous
Dirt Forum Champ
Total posts: 796
posted September 16, 2005 12:26 PM  
You sound like a real welder Artie, any other things you see off on what i was told or posted? I talked with a few Engineers, hadnt got a chance to talk with a real welder with some knowledge.

I was reading about that Metal-Lax deal (Aggressive's post), man that thing is neat. Wonder how much one is?


leapinlizard
Dirt Freak

Total posts: 402
posted September 16, 2005 02:56 PM  
No one is going to mention brazing as a acceptable way of joining moly?

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted September 16, 2005 05:31 PM  
zero mentioned it. Look above at his post.

rocket36
Dirt Freak

Total posts: 209
posted September 17, 2005 02:02 AM  
a commonly used method to assist in stress relieving chassis, both moly & mild steel, is abrasive blasting. works the same way as shot-peening conrods relieves stress. needs a competent blaster with patience to do this though as you need to work in a sequence to have the correct effect.
heat treating the entire frame when finished is ideal, but there would be few facilities actually capable of doing this correctly. our c/m rocket is entirely mig welded but i would be interested to know if every piece is c/m. all the tube would be but what about the battery cradle, steering mount, rack mount, dry sump tank mounts and all the little tab mounts???
the smart manufacturers will reduce the wall thickness of tube in some areas when they are recieving compression forces, and often they will use a large diametre tube with a thin wall rather than a small diametre tube with a thick wall in some areas. there is alot more to building a good chassis than just assembleng a big pile of steel tube together in the shape of a race car.

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 17, 2005 07:48 PM  
You would need to increase the tubing thickness or diameter for tubing under compression. steel is much stronger under tension than compression. When you build a building you put concrete for compression and steel for tension(rebar in the beam) because steel is very strong in compression compared to its tension strength. But concrete is strong in compression but week in tension. I know that is about building buildings and such but same rules apply.

The larger dia, but thinner wall vs smaller od and thick wall because the larger dia is harder to bend.

A-A mfg now makes alot of tabs out of c/m

[This message has been edited by zeroracing (edited September 17, 2005).]

DEEDDUDE
Dirt Maniac

Total posts: 157
posted September 18, 2005 09:03 AM  
If someone knows the actual chemical properties of the chrome tubing chassis builders are using I can tell you EXACTLY what needs to be done to weld it.

Artie 727,My understanding is the builders use 4130, which from memory is .75-1.25 Cr, .15-.25 Mo. nominal 0.30 C. Using k-wool wrapped around the weld after welding helps slow the rate of cooling which makes for a stronger weld and reduced cracking.

1 Cr 1/2 Mo was once used in the petrochemical field. Now the more common 1 1/4 Cr-1/2 Mo is used.

The reason the Cr-Mo cars crack or break is the use of lighter material and when the chassis flexes it work hardness the metal that results in cracking.

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