Need some a arm help

[99screaminshee]

i have a friend who is going to build me a arms for next years race season i just wondered if i am missing any thing or anything to look out for

as of now i have the stock bolt/bushing tubes from a set of stock arms 4 lonestar ball joints 15 feet of 1"x1" square tubing and other metal for gussets and shock mounts

 

i talked to the local quad shop guy he said i should go +2 +1

some people say to go +2 no forward changes

the reason i want to use square is that round tubing can kink the square has for main points of strength

also id like to use my stock shocks but they say that would make it ride shitty

id like to get a set of works triple rate shock with no rezzies because they shouldnt get that hot during ashort mx race so no need to spens the extra 2 bills

let me know what u guys think

[Holyman]

Sounds like you've got everything. Make sure you put pics up once they're done. And make sure the guy uses a good jig when he's welding it up so he can make more if they turn out good

[racer]

not to come across as an ass, but the round is much stronger. a circle is the strongest shape, that is the reason no one else uses square. the only square tube arms out there (to my knowledge) are factory raptor and 450 upppers, but they are alum. go with round. if you are worried about strength, though you shouldnt be, use chromoly.

 

also what gauge of steel are you using?

 

and be sure that its american steel, that chinese stuff sucks.

[J.J.]

the reason i want to use square is that round tubing can kink the square has for main points of strength

 

and

 

not to come across as an ass, but the round is much stronger. a circle is the strongest shape, that is the reason no one else uses square

 

 

The main factor resisting bending in a beam is it's second moment of area in the plane of bending.

The second moment of area is calculated from the cross-section of the component.

If weight and torsion is not an issue then an I-Beam has the most resistance to bending, period.

 

The properties that you should be looking for in deciding the material is the shape efficiency, in other words, what cross section is going to have the most resistance to bending with the least amount of weight.

In most cases, when considering bending, a round tube will be more efficient than a square tube.

That means that a piece of round tube with the same second moment of area as a square tube will weigh less for the same length.

 

 

Second moment calculator for square tube

 

Second moment calculator for round tube

[racer]

damn you and you mechanical jargon, im not that far into college yet. so whos correct, me or him? tell me if its me cause ill correct it and apologize.

[J.J.]

Can't really say who's correct or not, one needs to calculate the second moment of area of the steel stock that you're working with, if you're buying the stock from a reputable dealer they should have tables available, for weight per length and the second moment of area.

 

Both box sections and round sections are used extensively in making frames today. Compare the frame of a Cannondale or CR to that of a Banshee or old school YZ. Big difference in physical dimensions...

A box section can be made to be stronger than a round section, however, due to the extra weight involved in the box section, the engineer choosing a box section will have to choose a lightweight metal.

 

The thinner the wall thickness, the larger the outside dimensions required.

 

IMO the stock that 99screaminshee is working with, can be made strong enough, with the gussets in the correct place.

Do not try to bend the square bar.

 

Some tips:

Try to fit the ball joint for the upper and lower arm in the horisontal position with the shocks sagged to the rider and bike's weight. This will prevent the ball joints from binding at the extreme ends of the travel.

(Thanks for the pic Stu)

 

Do not try to bend the square bar.

 

At the shock mount on the lower arm, drill a series of holes along the shock mount, to enable different ride heights and use different shocks. drill the line of holes parallel with the lower arm.

Hope this has helped, try to post some pics of your project!

[99screaminshee]

thanx for the replys guys

i never thought about keeping the balljoints horizontal good idea

 

by the way the steel im using is an eighth inch thick

when i cut off the stock a arms i checked their thickness and they are thin as hell so i put a junk arm in a press and bent it it took 25 pumps after the press

made contact and then i put in a peice of the square tubing and it took 41 pumps to bend it.... so im gonna try it if it works good if not oh well ill build more

oh yea this is american steel too no chinese shit here

 

J.J. whatever it was u said went way over my head

[99screaminshee]

1 more thing the steel im using weighs in @ 1.4 lbs per foot

i dont think weight is an issue seein as i weigh 225 and race mx

[J.J.]

1 more thing the steel im using weighs in @ 1.4 lbs per foot

i dont think weight is an issue seein as i weigh 225 and race mx 

Build it strong bro

Using the 2nd moment calculator above,

The square tube you're using comes to 0.05694in4,

For comparison:

If you had to use 1" round tube, 1/8' thick, you'd get a 2nd moment of area of 0.0335in4.

 

Therefore you can safely say that if the steel properties are the same for both, the 1" square tubing will be more resitant to bending than the 1" round tubing.

 

Have you thought about the caster?

[azsandrat]

The reason that round tubing is used is that it is stronger,easier to bend and cope to fit the bosses that the ball joints thread into. Its also lighter per linear foot than Square tubing. If you do the math 1 inch round rolled flat it 3.14 and square is 4.0.

 

As for setting up the angles...33deg is the magic number for the anle of the ball joint. You want to be able to run a longershock if desired. When placing the shock mounts you need to run a set of arms so you can make sure that you don't slam the frame into the ground and break your wrists. You want to check this using a 20 inch front tire but most people use 21 or 22. What ever you do you want the shocks to stop the motion of the arm not the ball joints binding as it will snap them right off the bat.

Make a jig as well so they will be all the same. If ya wing it it'll be a bitch to set up the front end.

 

Most builders go +1 fowards for the banshee. The reason for this is that the Banshee is nose heavy to start with. If you MX you would run a shorter swing arm. This moves the center of gravity back 1 or so inches making it handle better. You can move the foot pegs back an inch and shorten the brake and shifter and it moves the rider back an inch and really makes them HOOK UP and handle alot better.

You have to remember we all love the Banshee but its 17 year old technology. It was made overnite to go after the 250R market. So not alot of thought was given to its suspension. Bad ass motor though. It came from a street bike.

 

I'd use 4130 chromoly. Its twice as strong and will flex. Mild steel won't flex much at all before it takes a set. You can get 4130 at www.wicksaircraft.com . They will sell it by the foot so you don't have to buy a min. length. If you need some help or some pieces to help your project give me a call at QUICKSAND MOTORSPORTS and I'll be glad to help you out.

 

Good luck with the project,

Gary

[99screaminshee]

wow this is gettin alot more complicated than i thought what do a set of those quicksand arms cost +2+1

[racer]

i have always wanted to build a set of my own, but using two shocks and only one coil. like they run on the sand rails and shit.

[azsandrat]

Powder coated a arms are 500.00 + shipping.

 

 

Two shocks sounds cool but too much weight up front.

 

 

I've been wanting to build a drag bike frame with inboard suspension like an INDY CAR only using a mountain bike shock for light weight. It would take some engineering to get the travel and leverage of the bellcranks correct. All it takes is time which is few and far between these days.

[racer]

Two shocks sounds cool but too much weight up front.

 

 

I've been wanting to build a drag bike frame with inboard suspension like an INDY CAR only using a mountain bike shock for light weight. It would take some engineering to get the travel and leverage of the bellcranks correct. All it takes is time which is few and far between these days.

true, but your idea sounds real cool. take a look at some of the Mtn bike rear suspension geometries. they are getting those things into like 10" of travel. i think that you could do it, but frame mods would be pretty substantial i would think.

 

 

I think im gonna try and make me something like this once i get her back running, its gonna look like THIS!

 

well maybe not quite like that .

Edited October 19, 2004 by therail