Braced Swingarm

[The Stranger:Me]

1)So ok why do "they" brace a swingarm?

2)Brace it for or against what? (vertical, horizontal, torsion??)

3)Why build it from Aluminium if you then need to brace it?
When I think of aluminium I think of soft, flexible, malleable. Why not use something like Chrome Molly.

4)And why use a box section, why not a lattice?
Cranes with a lattice boom generally are capable of lifting far greater weights than cranes with box section booms (over simplification I know, but) and have way way less weight in the boom.

Agh... so many questions in my head...

[James Deuce]

A lattice construction would require a huge amount of assembly and welding. Costs big money on a production scale. When I worked at Ford in the Wheel plant at Wiri we investigated lattice and honeycomb casting techniques and the tiniest flaw mean that a CNC lathe or drill unit would eat the wheel.

Yamaha uses a vacuum diecasting process that almost eliminates welding.

Aluminium is much stronger than chrome moly by mass. To get similar torsion resistance you'd have to build a very heavy chrome moly swingarm where you can just make an aluminium one big and use complex shapes to direct forces. Aluminium also flexes reliably.

[WINJA]

1)So ok why do "they" brace a swingarm?

2)Brace it for or against what? (vertical, horizontal, torsion??)

3)Why build it from Aluminium if you then need to brace it?
When I think of aluminium I think of soft, flexible, malleable. Why not use something like Chrome Molly.

4)And why use a box section, why not a lattice?
Cranes with a lattice boom generally are capable of lifting far greater weights than cranes with box section booms (over simplification I know, but) and have way way less weight in the boom.

Agh... so many questions in my head...

TORSIONAL IS A BIGGY , HAVE YOU RIDDEN A POWERFULL BIKE LIKE A ZX9 C1 THAT HAS NO BRACE , WHEN YOU PUSH IT HARD AND PUT ON THE POWER YOU CAN FEEL IT WALK AND WOBBLE ESPECIALLY HIGH SPEED SWEEPERS

[Brian d marge]

Have a look at the MV swing arm and the ones on the Gp bikes the shape on those are the most efficient. ( I havent seen a break down of the forces on a swing arm ,one thing I have been meaning to do )

The Shape of the thing ( or its Moment of Inertia , or the ability to resist bending torsional is a moment in the angular direction ..and a moment is a force mutiplied by a distance ) is the important thing , It can be done by lattice work

Sprung or unsprung wieght is inportant ( I always get this mixed up ) anyway the heavier it is the more the shock has to work as it has to control a larger mass moving ast speed... inertial loads ...( try stopping an oil tanker !!!)

Ease of construction a rolled shaped with 2 welds is easier to make than a lattice .( Didnt Macintosh use lattice swingarms ??)

Aluminuim the Fatigue limit of aluminium is 0.4 of its Uts and Steel is approx 1/2 of its uts though alloy doesnt show a distint endurance limit . so a gues is made , called proof stress

like most other materials the properties of alloy can be improved by the addition of impurities , to the point where its quite strong , but relativly brittle

Aluminium is about 300 mpa uts in it soft state elonation about 10 percent , but 7075 t6 is 570 Mpa uts and will elongated before facture about 11 % of its area

Mild steel is * 1030* is about 550 Mpa uts and will elongate 30 percent approx at break

So you select yuor material according to what you concider important in the design

and at the end of the day aluminium is sexier than steel ( though I personally like steel much more ,,,its quite an underestimated material is our steel,


Stephen ( rushing now as there is 2 min before beer time !!!)

ps those numbers are off the top of me head so ...they will be in the ball park thats all !!
Btw chrome moly or something like 4130 is a ( forgive me if i am wrong ) a bitch to weld or heat treat and is tougher 650 uts , but brittle 20 to 25 elongation and Horrible to machine , ( machinists step in here as I say its off the top so most likey in a ball park but in the wrong city )

[Motu]

A twinshock swingarm has less forces to contend with than a monoshock,which is why they look so spindlely compared to a monoshock,the monoshock has to support the weight of the bike aswell.They bend like the prongs of a fork - you can see the bracing needs to be put where it's most difficult to do.Torsional forces also come into play,and have always wondered why they use box section as it has no torsional strength at all.

[Fooman]

Aluminuim the Fatigue limit of aluminium is 0.4 of its Uts and Steel is approx 1/2 of its uts though alloy doesnt show a distint endurance limit . so a gues is made , called proof stress

Mild steel is * 1030* is about 550 Mpa uts and will elongate 30 percent approx at break

Btw chrome moly or something like 4130 is a ( forgive me if i am wrong ) a bitch to weld or heat treat and is tougher 650 uts , but brittle 20 to 25 elongation and Horrible to machine , ( machinists step in here as I say its off the top so most likey in a ball park but in the wrong city )

Eeehhhh, the engineer in me would say:

Aluminium doesn't have an endurance (Fatigue) limit, unlike steel - the fatigue limit for Aluminium (and other FCC metals), will be quoted at a number of cycles, e.g. 10 million. Proof stress is something not related to fatigue, it is just way of measuring the elastic limit.

Mild steel is 1008, or approximately 0.08% carbon - 1030 is 0.3% carbon, a medium carbon steel. With that amount of carbon, you would quench and temper to ~800-1000 MPa, otherwise it's a waste of money to specify the higher grade.

4130 is not a bitch to heat treat - it is designed for heat treatment (to 1500-2000 MPa). Otherwise, once again, it is a waste of the extra (expensive) alloy content. Machining, you need harder tools (e.g. tungsten carbide tips), but it's OK. It's a really common steel for making things like shafts, tension rods, gears etc.

As for the orginal question, as mentioned, the swingarms are braced to resist flexing. The swing arm flexs from vertical loads (although these are minimised by the rear shock), sideways loads and most of all, torsion from gyroscopic reaction of the rear wheel being forced to change direction. Hold a spinning bike wheel and turn it to see what I mean.

The amount of flex in any direction will be a function of the stiffness, which will be some equation with "EI" in it. "E" is the stiffness of the material (also known as Young's modulus, or the elastic co-efficient) and is essentially constant (decreases with temp). "I" is the 2nd moment of area about the neutral axis in the direction of the flex. "I" is a function of the cube of the distance away from the neutral axis. e.g. a bar with a section twice as deep (but the same width) will be 8 times stiffer in that direction for the same material.

Aluminium has a modulus of ~ 70 GPa, steel is ~210 GPa. But because of it's lower density, aluminium can have larger sections than steel, for the same weight. This makes the EI factor larger than the equivalent steel section.

Hope this makes sense.

Cheers,
FM

[Fooman]

A twinshock swingarm has less forces to contend with than a monoshock,which is why they look so spindlely compared to a monoshock,the monoshock has to support the weight of the bike aswell.They bend like the prongs of a fork - you can see the bracing needs to be put where it's most difficult to do.Torsional forces also come into play,and have always wondered why they use box section as it has no torsional strength at all.

It's torsional about the axis of the rear wheel - the swingarm will see this as a force generating a bending moment along the side of the swingarm (the axle will push up on one side, and push down on the other). The hoops of material that go up or over the swing arm are there to reduce this "flappy" behaviour. Box section does have torsional strength, although it is limited by the ability of the wall to resist localised buckling.

Cheers,
FM

[loosebruce]

I reckon you're all talking shit, it looks cool and thats why they do it

[Brian d marge]

K
lesson 1
never bang out a quick reply just before beer time !

thanks for tidying that up

couple of things now that I have had my coffee and have time.
4130 , heat treating is ok , as said , but welding , you have to be careful with as its an air hardening steel, and becomes marsenitic around the weld , relatively brittle around the weld area
Also the choice of filler rod is important, Also pre and post heating, while not so important as the higher grades , should be done ,
So yes I would say its a bitch to weld ( and with differing tools easy to machine )

As you can see Im not a great fan of the stuff , though if needed such as a gear ( which I dont do , as there are more experienced people than I ) I will use it,,,

Proof stress , yup sorry bout that, it was the ratio of ulitmate to fatigue stress I was thinking of ( Rushing, without thinking My Ludd!)

And as was said , E I while you cant change the youngs modulus , the shape can be change

Finally I didnt want to get involved with the explaination of moment of inertia, So tried to keep post simple ( and just ended up comfusing the reader !!!)

Lesson 2
Never bang out a reply with out making sure of the facts

( yeah right !!)

Stephen

Ps the fella who did the Norton F1 rotary racer is also not a fan of single shock systems, cant remember his name but I seem to remember it was twin shock for a while

[Motu]

Box section does have torsional strength, although it is limited by the ability of the wall to resist localised buckling.

Cheers,
FM

That's why they don't use it for driveshafts in rear wheel drive cars - an 80mm tube with 2mm wall thickness can cope with whatever a V8 can put out....you'd never fit a box section driveshaft under a car to handle a pokey 4 cyl 1300.That's how much tortional strength box section has....