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In driveshaft-as-a-suspension-link configuration I would be more worried about achieving the load carrying capacity and stiffness onboard - i.e. differential / spool axle bearings and carriers.
Beefier mountings and bearings quickly bring the weight up (and over?) the level of seperate driveshafts and suspension links for the same stiffness.
Ah ! But you need driveshafts anyway, so you gain a free zero weight suspension link for nothing.
Locating the diff properly, and providing some decent diff carrier bearings that will accept some thrust loading, should not present any huge obstacle.
An adequate driveshaft will very likely have more lateral strength and stiffness than many of the frail spidery suspension links we are used to seeing.
Dig deeper Tony.
I can weight and calculate you some exact numbers later but our suspension links weight around 100g per link. Beefier bearings, diff mounts and tripod -> UJ change could easily mean +250g per side...
Why should designing laterally stiff differential bearings have to weigh any more ?
It's just a matter of straight load paths to the frame.
And how much heavier are a pair tapered roller bearings over a pair of deep groove ball bearing for the diff carrier?
Your driveshafts still need to have articulated and sliding joints, why should a pair of Cardan joints be significantly heavier than CV or tripod joints ?
As Z has already pointed out, if the driveshafts are up to scratch anyway, they will be more than sufficient to locate the wheel without needing to be any heavier.
Agree with Tony.
And there is a little bit less chassis to build (ie. the part that the lateral link attached to).
And a little bit less upright (ie. don't need the top part anymore - see Jag).
Z
Random (possibly stupid) thought: building an "upside down" Jag-style IRS allows you to ditch lower Aarm and therefore cleaning up some space in between the rear wheels (think aero). Wonder if camber compliance could be treated somehow though...
Tony, Z, forgive me for posting the information bit by bit as I have a lot on my hands right now.
I did a quick scaling and our a-arms come at an average of 130 grams per a-arm. Modified upright's would remove some 30-50 grams per side so we're looking at around 350 grams of reduced weight.
Depending on the rulebook there is more or less benefits on the frame. Considering FSG rules (standard for European teams?) pretty much all that chassis is already there - with regular FSAE rules you can shave some tubes / 'coq to further reduce weight.
Our diff bearings weight a total of 140 grams with a hefty FoS. I didn't do any calculations but I'd estimate double row angular bearings good for dynamic axial loading could be found for around 250 grams total. Also single row angular contact bearings would be an option (wheel bearing style solution). Tapered roller bearings seemed to be out of the question or I just couldn't find any of reasonable dimensions.
In seperate driveshafts / suspension links configuration the differential is quite insensitive to lateral stiffness so the mountings can be fairly light. Then again lateral support can be taken into account in quite a reasonable manner so the added weight could stay around 50 grams per side.
I have no time to calculate differences between UJ's and tripods but my quick assumption was based on tripod being lateral stiffness insensitive and having one more contact per joint than UJ's.
So we have 25 grams per UJ to use to break even and any compliance in the UJ would mean suspension compliance... Hard to say where that would end up.
There's potential. And also for disaster.
Markus,
I like your analysis. The thing is I cannot see any advantage over a more traditional layout, except maybe from my "stupid" aero advantage as described above.IMO it is extra risk and complication (possibly).
I forgot to add I'm eager to see Delfts '13 car as they did something I'd consider more beneficial than using driveshafts as suspension links: they designed a car without driveshafts. http://fsae.com/groupee_common/emoticons/icon_smile.gif
Well that would be a fun design, but the intake sealing to get the 4 gas hub motors through one restrictor while still allowing for suspension travel seems like a nightmare...Quote:
Originally posted by Markus:
I forgot to add I'm eager to see Delfts '13 car as they did something I'd consider more beneficial than using driveshafts as suspension links: they designed a car without driveshafts. http://fsae.com/groupee_common/emoticons/icon_smile.gif