tube chassis design

[lightweightstickers]

I have started a tube chassis in Solidworks. I wanted to get a good conceptual design before I start to build one. I have taken most of the measurements from a rev chassis since it is what I have available to me at the moment. So far I have run analysis of weight from aluminum tubing and chromoly in 1/16th wall and 1/8th wall. The chromoly weighs 15 lbs total using 1/16th wall. The aluminum 6061 weighs in at 9 lbs using 1/8th wall. I think I can cut some triangulation out to bring some more weight off.

I am curious if anyone that has built a tube chassis has any input on certain aspects of a tube chassis design. I plan to balance the sled; however, not sure if a perfect balance is what one would want. I am thinking it might be better to have a weight proportion i.e 60/40? The bulkhead isn't quite right yet either. I need a un-assembled bulkhead to take measurements from.

So far this frame is going to require about 100 L.F. of 3/4" tubing. This doesn't include the 10% I would add for *&$% ups such as bad cuts and screwed up bends.

I have included a solidworks rendering of what I have so far.

Looking forward to some input.

 

[SNWMBL]

Stick with .035 4130. 6061 will weigh the same since you need heavy wall tubing (more like 1/4" instead of 1/8") and extra material to brace it. Not to mention anywhere you weld 6061 it will become annealed so heat treating would be neccessary. There's a reason everyone uses chromoly.

 

[LoudHandle]

Stick with .035 4130. 6061 will weigh the same since you need heavy wall tubing (more like 1/4" instead of 1/8") and extra material to brace it. Not to mention anywhere you weld 6061 it will become annealed so heat treating would be neccessary. There's a reason everyone uses chromoly.

I disagree with some of your advise but agree with most of it. The Chromoly will be far easier for most to pull off. Far more forgiving of those who have not perfected their skill sets. Your comment on needing heavy wall aluminum tubing is suspect; it is largely defendant on joint fitment and chosen weld process and the welders skill level. With proper design one could get away with 0.080" wall tube in some less stressed areas and likely 1/8" wall in the more heavily stressed areas. If you TIG weld it with 5356 even if you do heat treat it, the welds will take some movement without fatiguing prematurely, assuming proper joint design and respectable craftsmanship. Most engineering calculations discount the ends of the tubing where the weld typically is located when not triangulated, when triangulated correctly the load is dispersed enough to negate the need for heat treat for personal use. If I were building these to sell, I would cover my azz by heat treating either one post fabrication. If heat treating either one you can get by with much thinner wall tubing and not worry about failure.

My two cents on your Solid-works drawing is way too much tubing, but you have not really told us your intended use, power plant of choice, etc. to size it up properly. I would base it off of the PRO RMK platform rather than a has been sled, but that is just my opinion also (and I understand availability to measure is critical).

Also note 6061-T6 aluminum will not bend without breaking or annealing the bend area, use 6063-T6 aluminum for the tubes that need to be bent.
Hope that helps!

 

[kyle28]

Tunnel has way to much bracing and the front you need some more bracing around the a-arms. think when you hit a tree. its going to pull the front mount and push the rear in. .035 4130 is what I did mine out of. TW metals has good price on it btw. Also you got to consider lining the tunnel and under the motor into your weight. I used ABS plastic to line my tunnel as it will not dent like thin aluminum. But thin aluminum flashing works to. I based mine off a Rev, Revs are a great platform and were ahead of their time. If you strip a rev and a pro down their pretty similar!
its a big project! hope this helps.

 

[lightweightstickers]

response

Thanks for the input so far. I am currently a mechanical engineering student and started this as a class project. It has since evolved into a personal want.

My intended use will be personal. I haven't decided what power plant I want to use. I was leaning towards a turbo nytro motor, but not sure because I would really like a super light nimble sled. Probably going to run a 154 track. Not sure what skid; stocker, ez-ryde? Any suggestions?

I am a bit surprised by the .032 wall 4130. I didn't know anyone was using that thin of material. I will definitely run some calcs on it and do some modifications to the model in solid works to see how it performs. This was a mistake on my part for not really considering that.

I agree with Loudhandle on the aluminum. I have discussed this with my materials professor on numerous occasions. Most of the problem with aluminum is the welding. We have done break tests on this in the lab on properly welded aluminum and poorly welded aluminum confirming this problem. Your also correct that I should model after a newer sled, but like you have acknowledged, I don't have access to anything else at the moment. I am thinking if I were to pull out a tape measure on the mountain and start measuring sleds, it might not go over so well.

Any input on the current diameter of tube I have selected. I have looked at a few pics of sled online. It looks like the tube size varies. As far as lining the tunnel goes, I was thinking about doing what Ski-dooin it has done with the carbon fiber.

Thanks for the input everyone. Keep it comin.

 

[AKtoad]

Biggest problem with alum. is cyclic fatigue. In my experience aluminum is far easier to work with than 4130. The real trick is trying to have as few as welds as possible. Trying to compare steel vs aluminum with just stress/strain analysis is very difficult as most use static methods to compare them such as Mohr's circle. Dig a little deeper into analysis of materials and you will find an abundant amount of factors that go into choosing a material. Even using basic dynamic methods can pull you in the wrong path for an application. FEA feature in solidworks can get you close but be prepared to crunch some numbers. Not sure what tools and skills you have but consider what tools you have when you design your chassis.

 

[skidawg]

wall thickness

Very good info from everyone and it's funny I came across this topic, because I plan on doing the same thing this winter. I am torn between the three metals. Aluminum, 4130 steel and titanium. I have been welding for over 40 years and have the experience to weld any one of the three. I plan on building two tube chassis, one will have a turbo apex motor in it and the other will have a 996 n/a twin. The goal with the yami is obviously to make it lighter but also stronger, thus I will probably make that one out of 4130. On the big twin, my goal is to have a light weight tree banger. I do also plan on building a jig for those roll over events, that end up in the trees or rocks. Both sleds will be based off the 14 pro rmk. I guess the one question I have for everyone, is wall thickness? I do plan on using .75 tubing for the O.D. All things being equal, what would the comparable strengths be for the three metals listed? Ti, 4130, & Al.

 

[LoudHandle]

Very good info from everyone and it's funny I came across this topic, because I plan on doing the same thing this winter. I am torn between the three metals. Aluminum, 4130 steel and titanium. I have been welding for over 40 years and have the experience to weld any one of the three. I plan on building two tube chassis, one will have a turbo apex motor in it and the other will have a 996 n/a twin. The goal with the yami is obviously to make it lighter but also stronger, thus I will probably make that one out of 4130. On the big twin, my goal is to have a light weight tree banger. I do also plan on building a jig for those roll over events, that end up in the trees or rocks. Both sleds will be based off the 14 pro rmk. I guess the one question I have for everyone, is wall thickness? I do plan on using .75 tubing for the O.D. All things being equal, what would the comparable strengths be for the three metals listed? Ti, 4130, & Al.

For a rough estimate, you take the tensile and divide it by the cross sectional area (I.E. the ID area subtracted from the OD area). You then cam compare weights and the other beneficial and detrimental material attributes. Such as stiffness vs flexibility, etc. There are volumes of information on all these and other materials as a result of the bicycle industry. You also have the option of vari-wall tubing and double or triple butted tubing within the bicycle supplier system. I've researched the idea of a sub 400 pound wet RTR sled for nearly 30 years now and I keep coming back to the PRO RMK geometry in nearly all Carbon Fiber with some Kevlar thrown in to the really stressed areas for durability. All my tubers to date were purpose built full mod hill climb sleds, exclusively aluminum, no chromoly, and no TI other than fasteners. An aluminum / ceramic (5% Scandium) chassis will actually be stiffer than either a chromoly or a TI chassis just due to the properties of the metal, in a pound for pound comparison. I am not trying to tell you which to choose just what I have learned from my research. The possibilities are endless, just limited by the cubic dollars one has available to throw at it. Good luck on your builds whichever direction your research leads you!

 

[Wheel House Motorsports]

they are a blast to build... just time and $$$$$$

you would be blown away by how strong 3/4 .035 is... but nothing takes tree hits that great regardless.

they are a labor of love, and its really sad when you wad them up. much more so then a stocker.... especially since the whole chassis is 1 part and you cant replace portions of it.... easily.

 

[SNWMBL]

For 4130, .035 is what I've seen everyone use and its what I used. The hardest part is bending it without wrinkling the tube. The best bends I've seen to date on .035 were from a Ridgid ratcheting tube bender, no joke https://www.ridgid.com/us/en/geared-ratchet-tube-benders

If I could weld Ti, I would use Ti on my next chassis but I can't, so it'll be 4130 again.

 

[Kennyspec]

I have also been thinking of building a tube chassis to swap my Nytro engine into. I was thinking mainly 1" .065 4130. Seems like I am thinking way overkill lol.

 

[lightweightstickers]

update

Sorry, didn't mean to abandon this post, but finals took over my life for bit there. Anyways I am going to start working on this again. I am going to make some modifications to the chassis, to which I am then going to play with the FEA tools in Solidworks like crazy.

 

[Justin_Sane]

Im going to support the thin wall chromoly steel train of thought. (Im not an engineer but I am the production manager of a machine shop.)

titanium and aluminum, excellent materials, strong and light.

Chromoly steel, 4130, 4140, etc. very strong, not so light.

aluminum and titanium are much more expensive. They are also less tolerant of crappy welding, fatigue faster, etc. Although they dont rust the same way, which is kind of a bonus, because steel rust will weaken the metal, while aluminum and titaniums oxides sort of protect the metal underneath the oxide layer. therefore the steel should probably be powder coated which sort of negates the cost difference. IMO for a starter project, my advice is to stick with chromoly tubing. Once youve got some experience with it, then you shoot for something more adventurous. NASA didnt build the space shuttle first.

Another note which might just be my opinion, is that I would move some of the triangulation points so that you do not have four tubes joining one tube in the same place (im referring to the tunnel where the tubes join from the top and side). They way it is, some of the joints make it tougher to fit the tubes together cleanly for the welding. Since you are notching four tubes at an angle, they will interfere with each other at the joint, which makes it tougher to get good clean fitment and welds (important, but especially with aluminum and titanium.)

ps, there are also many tubing benders that you can pick up that are manual and bend steel without wrinkling or collapsing tube. They have been coming down in price for a few years because more people are getting on the bandwagon with making good, and affordable tools.

Another good method of testing the design, is to take solid copper wire and solder the connections into a miniature model of the design, and sort of do a miniature load test and you can find weak points that way (without all the math)

Ive been planning to do something like this as well, however my plan is to use a sheet metal tunnel and standard bulkhead (which I already have), and build the rest of the components myself

 

[SNWMBL]

ps, there are also many tubing benders that you can pick up that are manual and bend steel without wrinkling or collapsing tube. They have been coming down in price for a few years because more people are getting on the bandwagon with making good, and affordable tools.

Such as? I've yet to see any brand of manual bender (aside from the one I linked) bend .035 without wrinkling it. I'd like to give the Mittler Brothers 950 a try but I don't know anyone with one. http://www.trick-tools.com/Mittler_Bros_Model_950_Manual_Tubing_Bender_950_MB_2290#.UrJnVCcvySo It's only rated for .049 with a 3.5" CLR, but from my experience, dies are sometimes good for the next wall thickness down.

I'm being serious too. I wanted to use 1" or 1-1/8" .035 for my next bulkhead but couldn't find a bender with rated for anything thinner than .058 (rarely .049) with huge CLR's. So as of now I'm planning on using 7/8" .035 for most of it with the Ridgid bender.

 

[M5-]

When I bend Cro Mo I pack it tight as hell with sand which helps reduce the wrinkling but not eliminate it. I have a manual bender and it wasn't until I got custom tube size specific dies that I got rid of most of the wrinkles. The dies need to fit perfect and have a bit of a shoulder on them so the tube can't flatten as it bends because you are compressing and stretching the metal.

M5

 

[Modoutlawkart]

To make a long story short, DOM tube would be the best all around for you application. In 100' chromolly maybe lighter but not enough to justify the extra $250. The DOM is almost the same weight really easy to weld and costs way less and it is very rigid and will flex with no memory. My guess in comparing like tubing the DOM tube would be 3/4# heavier in 100'. The DOM is way more forgiving to weld and work with. Aluminum is way out of the question when its cold and exposed to shock it fractures pretty easy, if it was me I would say away unless you want to get hurt. Here is what I do all DOM chassis here. http://www.youtube.com/watch?v=mDP4O791QRo Good Luck and have fun!!!

 

[Justin_Sane]

Such as? I've yet to see any brand of manual bender (aside from the one I linked) bend .035 without wrinkling it. I'd like to give the Mittler Brothers 950 a try but I don't know anyone with one. http://www.trick-tools.com/Mittler_Bros_Model_950_Manual_Tubing_Bender_950_MB_2290#.UrJnVCcvySo It's only rated for .049 with a 3.5" CLR, but from my experience, dies are sometimes good for the next wall thickness down.

I'm being serious too. I wanted to use 1" or 1-1/8" .035 for my next bulkhead but couldn't find a bender with rated for anything thinner than .058 (rarely .049) with huge CLR's. So as of now I'm planning on using 7/8" .035 for most of it with the Ridgid bender.

JD squared model 32, for one example. Its meant for heavy walled tube for roll cages, but it can be done. Depending on the CLR and specs of the bends. I've also seen another small bender I'm going to have to dig up that's pretty good for small stuff. I'll see if I can find it. I saw it a few years ago and it blew my mind, of course now I can't find it. Iirc it was overpriced though. The ones you linked are very similar to the jd squared benders though.

Plus manufacturing techniques such as heat, lubrication, and packing always help when problems arise. There's many ways to skin a cat.

Edit here's one of the ones I was looking for.

http://shop.useful-tools.co.uk/mandrel-tube-bender-2-p.asp

 

[SNWMBL]

I had a model 32 for a couple years and liked it okay, but sold it hoping to pick up something a little better.

 

[m8magicandmystery]

ok..probably a real dumb question...

why is square tubing not considered in builds..??

can not the flat faces be advantages for gussets/plates etc..??

 

[LoudHandle]

ok..probably a real dumb question...

why is square tubing not considered in builds..??

can not the flat faces be advantages for gussets/plates etc..??

Numerous Pros and Cons

The first and likely the biggest is Builders EGO, I asked that question of a professional race car builder once. He looked at me like I just killed his favorite hunting dog and i was lower than dirt and replied "that no self respecting chassis builder would cheat and use square tube".

The joints are planer (2 dimensional) which means they will fail easier, but they are far easier to fit and weld too though, especially for a sled where most of it will need to be skinned. Whereas a round tube joint is non-planer (3 dimensional) and is harder to fatigue and make it fail.

Square is typically more expensive as are the bending dies, but with square other fancy tube notchers and stuff are unnecessary. The other issue with square tube bending is, without machining custom dies, you may need to make two or three planer bends to do what you could with round in a single bend.

Square actually is stiffer and resists torsion better than round in equivalent size and wall thickness.

Those are the most common reasons sighted, there are likely plenty of others but in a nut shell it is builders choice and ego at stake. There is no real challenge in working with square tube.