Steel vs. Aluminum Frames & Swingarms

[The Red Scourge]

This may be a stupid question, but I've yet to find a satisfactory answer.  What are the actual benefits of an aluminum frame and/or swingarm?  People like to use aluminum swingarms on their RD builds, but why?  Is it a looks thing?  I get that steel will flex more than aluminum.  The Big 4 in Japan switched over to aluminum frames by at least the late 90's.  The only thing I really found looking online were people saying it was cheaper for manufacturers to produce aluminum ones. 

[m in sc]

aluminum can be stronger in a given space, and formed easier into shapes that are more rigid. that's why there aren't really any steel airplanes.   My 1st gen sv had an all aluminum frame and it was strong as hell. the early deltabox frames were also very strong. Not really a cost thing.  plenty of bikes come with steel arms & frames, but they typically look dated. remember, some of that swingarm is unspring weight. 

[The Red Scourge]

Thanks Mark!  I can always count on you swinging on through with some wisdom. Interesting that you mentioned the Deltabox frame.  This came to mind because I may be picking up a basket case FZR that's in an FZR400 Deltabox frame.

remember, some of that swingarm is unspring weight.

Ah!  Good point!  I hadn't thought of that.

[m in sc]

also the plasticity of aluminum is considerable less than steel, typically. (depending on the steel of course).

[sav0r]

All things even, you can make an aluminum frame like 20% lighter but it will still be the same strength. With the advent of large OD frames stiffness became a lot less of an issue though. For a long time, stiffer and stiffer chassis were the goal.

Box style large vertical section swingarms are a direct result of monoshock bikes and are used on twin shock bikes for basically style only. A thin wall cro-mo swingarm could be made to be lighter than something like a DG swingarm, but people like the DGs for nostalgias sake.

All of that said, it seems aluminum frames have some properties that seem to work well with motorcycles when they are leaned over and suspension gets all wonky. Even modern MotoGP bikes are aluminum frames, though I think Ducati won a season or two with a proper carbon frame. But manufacturers build in a bunch flex into these machines as to help cope with various forces when in the lateral position but try to keep them stiff vertically. That natural frequency of aluminum seems to just be around about where it should be to work well, even Ducati moved away from their trusty trellis. There are all sorts of trickery with flexible top triple clamps and carbon tube inserts in the frame, but is all a departure from design goals of the 80's, 90's where increased stiffness was basically the only goal. In large part because lean angles are so extreme these days.

[The Red Scourge]

All things even, you can make an aluminum frame like 20% lighter but it will still be the same strength. With the advent of large OD frames stiffness became a lot less of an issue though. For a long time, stiffer and stiffer chassis were the goal.

Box style large vertical section swingarms are a direct result of monoshock bikes and are used on twin shock bikes for basically style only. A thin wall cro-mo swingarm could be made to be lighter than something like a DG swingarm, but people like the DGs for nostalgias sake.

All of that said, it seems aluminum frames have some properties that seem to work well with motorcycles when they are leaned over and suspension gets all wonky. Even modern MotoGP bikes are aluminum frames, though I think Ducati won a season or two with a proper carbon frame. But manufacturers build in a bunch flex into these machines as to help cope with various forces when in the lateral position but try to keep them stiff vertically. That natural frequency of aluminum seems to just be around about where it should be to work well, even Ducati moved away from their trusty trellis. There are all sorts of trickery with flexible top triple clamps and carbon tube inserts in the frame, but is all a departure from design goals of the 80's, 90's where increased stiffness was basically the only goal. In large part because lean angles are so extreme these days.

Thanks sav0r!  That does make a bit of sense.

[bitzz]

A couple of things:
"They" changed how to make aluminum parts in the '80s. They used to fill the molds from the top,which allowed oxidization on the top skin as it was poured, meaning the pour was full of oxidization.
Someone figured out that if you injected molten aluminum from the bottom, with a vent at the top... keep filling till aluminum squirts out the vent, you can make stuff with no oxidization... which is MUCH stronger, but MORE importantly CHEAP.

Aluminum is stronger per pound that steel.
Aluminum is a lot less flexible than steel BUT is still flexible.
Steel, after it distorts, will return to it's original shape FASTER than aluminum.
Steel is cheaper and easier to work with. To build an aluminum frame you need specially designed castings. Steel? you need a welder.
Back in the "good ol' days" steel frames were brazed, now they're tig welded (A LOT easier)
We originally got round section swingarms because a "round" is the strongest section per pound. You can't beat a circle when applying forces... but a swingarm doesn't have to strong in 360 degrees, it has to be strong in the X and Y axis and a rectangular tube has more material in the X and Y axis than a round one. Research "TRUSS". Rectangular tube "handles" twist better than round.
... and it is cheaper and easier to make a swing arm out of round.
I was friends with Denis Curtis (RIP) of CMR Racing (the guy that built all the frames for "Team America") and he convinced me to replace the DG swinger on my RD racer. Immediately I could go into a corner harder and, more importantly, get back on it sooner coming out of the corner.
My next CMR bike   https://i.imgur.com/w1zQXSP.jpg
Sorta a XR69. Notice steel swinger and 6 points of connection frame to steering head.
Or a CMR swinger for a TZ-G    https://i.imgur.com/mWxBgV5.jpg

SO... our friends at Ducati had a "trellis" steel frame for years, it worked well... but they couldn't win races any more so they TOTALLY redesigned the frame as a RIGID aluminum thing THAT NO ONE COULD RIDE.OOPS! (There was NO WAY the trellis frame was going to support a 200HP motor. They HAD to do SOMETHING)... and it's taken YEARS to figure out how to make it work (if Ducati hired a frame guy that KNEW aluminum frames BEFORE they redesigned that frame, things might have been different). If Nickey Hayden or Rossi can't make it work, you've done something wrong... then came Casey Stoner (I don't know if they got it fixed or if Stoner was just that GOOD).

Here's another secret: The trellis frame Ducatis weren't particularly FAST, they were just EXCEPTIONALLY EASY to ride. The "ride" inspired confidence. It's easy to be a HERO on a 900SS.

You might notice: I'm into frames.
I grew up drag racing and learned early that ANYONE with half a brain can make horsepower (that's the easy part)... what is HARD is getting that horse power to the ground.
Tires and suspension (and by extension: Frames) are where it's at.

... and DG swing arms... NO... anything DG made was JUNK. Still is.

[m in sc]

I agree witht eh dg comments (and the rest). the aluminum quality was just garbage on the arms and the heads IMHO.

[bitzz]

The squish in the heads is all wrong. They're about 24cc... so you'll probably LOSE performance installing them.
The swing arm is made to imperial dimensions, with metric bearings.
... and the pipes are just trash... and HEAVY

Urban legend is that DG didn't weld anything. They had the muffler shop across the street do all their welding... and as I understand it, that welding shop is making DG swing arms again.

[m in sc]

I agree with the OG head profiles, they do suck, bad. I have heard from multiple trusted sources the stock rd heads when cut correctly actually shed heat better due to materials, and i certainly can see that/believe it.

Never heard the welding part. but its plausible, or at least in the beginning i could see that. early ones (pipes)  have really, really shoddy welds for sure. 

[1976RD400C]

My SV. The steering head is cast as one piece.

[tony27]

SO... our friends at Ducati had a "trellis" steel frame for years, it worked well... but they couldn't win races any more so they TOTALLY redesigned the frame as a RIGID aluminum thing THAT NO ONE COULD RIDE.OOPS! (There was NO WAY the trellis frame was going to support a 200HP motor. They HAD to do SOMETHING)... and it's taken YEARS to figure out how to make it work (if Ducati hired a frame guy that KNEW aluminum frames BEFORE they redesigned that frame, things might have been different). If Nickey Hayden or Rossi can't make it work, you've done something wrong... then came Casey Stoner (I don't know if they got it fixed or if Stoner was just that GOOD).

Here's another secret: The trellis frame Ducatis weren't particularly FAST, they were just EXCEPTIONALLY EASY to ride. The "ride" inspired confidence. It's easy to be a HERO on a 900SS.

You might notice: I'm into frames.
I grew up drag racing and learned early that ANYONE with half a brain can make horsepower (that's the easy part)... what is HARD is getting that horse power to the ground.
Tires and suspension (and by extension: Frames) are where it's at.

... and DG swing arms... NO... anything DG made was JUNK. Still is.

You got your timelines wrong there, Stoner raced the original steel trellis chassis then swapped to Honda in frustration at not being listened to by Ducati on what needed to change on the bike(seems to be a common theme eg Lorenzo waiting 1 1/2 seasons for seat changes), Rossi came next saying that him & Jeremy Burgess would have the bike sorted in 30 seconds, basically they wanted a Yamaha with power which meant the aluminium frame that Rossi was so used to. It's taken them a long time to get the complete change in design philosophy to work

[sav0r]

Stoner was on the carbon bike in '09 and '10. Ducati abandoned it for the aluminum twin spar in '11 which is when Stoner went to Honda. Stoner said he could have won the championship in '09 had he not missed those three races. He won 4 races that season but finished in 4th overall.

https://www.motorsportmagazine.com/articles/motorcycles/motogp/carbon-fibre-motogp-it-s-long-story

Of course, carbon-fibre frame add-ons aren't entirely new. This tech was especially a big deal in 2009, when Ducati equipped its Desmosedici with a full carbon-fibre chassis and HRC used bonded carbon sections to improve the behaviour of its troublesome RC212V. Suter later did the same in Moto2.

The carbon Desmosedici won seven races in 2009 and 2010, all with Casey Stoner on board. The chassis didn't last much longer, not because the material wasn't right, but most likely because Ducati's concept of using the engine as part of the chassis no longer makes sense when ultra-high lean angles demand chassis flex.

https://www.crash.net/motogp/news/226508/1/stoner-carbon-frame-ducati-best-bike-in-2009

"When we had the carbon frame in 2009, I believe we had the best bike on the grid, and unfortunately I wasn't able complete the season. But in my opinion, from the Catalunya GP on, the bike was incredible," said Stoner, during his impressive return test for Ducati at Sepang last week.

I wrote this post like four time but kept getting distracted and never submitted it.