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I see...again IMO not applicable on the front. If you modify the design to allow steering, again you have not enough space, due to the fact that you occupy a significant space in height -the x-brace will have to be as high (approximately) as the upright... Unless you have a front axle forward of the bulkhead.
At the rear, the trailing arms could extend back past the axle centreline so that the X member cleared the diff and final drive. All fairly simple, just another form of twist axle.
It is all a lot more difficult to do at the front, because of a space conflict between anything that directly links the front wheels together, and the chassis.
With wide flat tires, and a smooth flat track, a light beam axle may be surprisingly effective.
Harry,
At last, hate mail! Thanks, I must be making progress. http://fsae.com/groupee_common/emoticons/icon_smile.gif
Regarding:
WHEEL PODS. The banning of UWA's wheel pods was a disgrace! Supposedly "against the spirit" of open wheelers? What crap. Look at the latest Indy cars. They are thoroughly "podded" at rear for reasons of safety and fuel efficiency. Welcome, FSAE, to the typically brain-dead officialdom of motorsport that immediately bans anything useful. (Mumble, mumble, "where in the rulebook???", mumble, mumble, "should be flogged!"... http://fsae.com/groupee_common/emoti...n_rolleyes.gif)
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8" TYRES. This is a bit off-topic, but as agreed there is an advantage to smaller wheels. I guess the first team that finds out how big this advantage is, will be the first team that tries it (hint: off-road quad racing tyres for mud/clay can be very soft?).
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STEERING. Production car R&Ps minimize backlash by using a spring loaded pad to push the "floating" end of the rack against the pinion. This increases friction (ie. between pad and rack), and is more difficult to do with a central pinion because of "rack-rattle" at its ends.
Backlash is easily minimized with bevel gears by allowing the "crown" gear and its shaft to slide vertically in its housing (ie. in cylindrical bronze bushes or needle bearings). A spring loaded pad pushes down on the centre of this shaft preloading the crown gear against the pinion with minimal frictional torque (because forces are at small radius).
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CG HEIGHT & AERO. The major masses are the driver and engine/drivetrain. These can be mounted as low as possible, whilst also using a live aero-undertray, by cutting a hole in the undertray roughly the size of the rear half of the fuselage. The low rear fuselage floor, with low mounted driver's bum and engine, pokes down through this hole. A flexible membrane seals the gap between low rear floor and undertray to prevent negative pressure loss.
So, CG as low as possible.
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CG HEIGHT & STRUCTURE. The Twin-Beam layout has, without a doubt, a lower CG than any current FSAE wishbone layout. Most of the structure (beams, BJs, P&Ss, chassis mounts for BJ/P&S) is as low as physically possible, ie. within 10cm of ground level! Only the wheel-hubs/bearings are at a "high" level (this is an advantage of smaller diameter wheels).
(Voice rising, face getting redder.) Not only do wishbone layouts have roughly half their structure significantly above axle height (ie. upper wishbone and upper half of upright), they also have a lot of heavy chassis structure up high (count number of chassis tubes going to upper wishbone mounts!). And then some genius decides to use pushrods leading to rockers and spring-dampers mounted as high as physically possible! On top of nose or engine!! Why???!!!
(Entering Full-Rant Mode.) What we have here is a lot of unimaginative drones who refuse to think rationally about their problem (eg. by considering CG height). They mindlessly copy pre-existing and bad designs, but are quite happy to pour their energies into long-winded but nonsensical justifications of their "optimisations"! "Oh, it gives us better access to the dampers..."
(Calming down, slowly.) The argument for wishbones over beams is similar to (actually somewhat worse than) the argument for bi-planes versus mono-planes in aeronautics. Sure bi-plane structures might, to a simpleton, be lighter and stiffer than mono-planes, but that doesn't make them better overall.
The major forces on an FSAE car go from ground-level wheelprints to low-as-possible chassis CG, so there are good structural reasons to position the members carrying these forces also low. A low CG is an added benefit.
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CG HEIGHT & SUSPENSION MODES. I posted extensively about "coupled" suspension back in 2005, for example here, but clearly this is a step too far for most FSAEers. (Incidentally, back in March 2000 Racecar Engineering I asked "How long will it be before racecar designers of the 21st century catch up to the steam tractor designers of the 19th century, and start designing racecars with a soft twist mode?" I note that 12 years later there is some talk about front-to-rear interconnected dampers in some high level race series. Geez, the Citroen 2CV, "the world's cheapest car", had better in 1939...)
Anyway, the relevant point here is that FSAE RACETRACKS HAVE NO BUMPS! Except for the rules, FSAE could be won with no suspension at all. Nevertheless, a bit of damped movement, say +/- 5mm, is useful in suppressing "bouncing on the tyres". And the right sort of soft Twist (=warp) mode makes it easier to have predictable LLTD for good handling, especially over slightly undulating ground.
The bottom line is that suspension Bounce (=heave) and Pitch modes only need very short travel. This is easily done with beams by having short travel (+/- 5mm) bump rubbers fitted at the mid-beam position. With these the chassis can be run very low with no scraping, so low CG, but there can still be plenty of wheel travel in Roll and Twist modes to keep the scrutineers happy. The soft Roll mode is, of course, no problem because no camber change, and the undertray doesn't scrape because it is beam mounted.
Z
Hehehe, I can just imagine the evil FASE organisers reading that "there are no bumps"....
And planning the next event at a BMX track.
Harry,
The HyperProRacer in your link has a live spool axle at rear, and lateral swing arms at front. It is an attempt to make a safer superkart by providing much better driver protection (roll cage, belts, etc.). I talked to the builders at a recent autoshow - nice guys and I hope they have success with it.
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Tony,
The type of twist-beam you describe is similar to that used on 1930's Mercedes GP cars. The beam must be torsionally flexible and have some means of lateral constraint (P&S on the M-B cars).
If, as you described, and on the M-B cars, the rear beam is behind the diff, then it has greater than 100% camber compensation. That is, during outward body roll the wheels lean into the corner.
Other variations have appeared over the years, such as an interconnected trailing-arm rear suspension on Volvo station wagons of about 20 years ago.
Z
PS. Back in 2005 I was suggesting that the organisers should deliberately put bumps on the tracks. They could then delete the mandatory +/- 1" suspension rule, and leave the decision up to the teams (good suspension would win). It will never happen.
Quote Z 'The banning of UWA's wheel pods was a disgrace'!
Erik, the UWA pods were never banned!
Teams know that turbulence created by a rotating wheel creats drag and every year the rules committee get several enquiries about wheel pods, mudguards or similar.
The response is consistant. It has been determined that an 'open wheel' car is one where the entire wheel is visible in plan view.
The UWA team were asked to trim the pods a little so the wheels were not covered. They decided to remove the pods instead.
Pat
Go tell that to the FSG organisers... There are two points at the track where some cars catch some air for split seconds, making us feel a bot like WRC drivers (OK, exaggerating a bit but you get the point). Agreed that in a perfectly flat surface, double wishbones are no match for beam axles though... IMO beams could be lighter that double wishbones if polished up a bit, maybe a little heavier as actual structures but leading to a decreased overall weight (fewer mounting points -> "fewer" chassis needed to facilitate load transfers etc)Quote:
Originally posted by Z:
Anyway, the relevant point here is that FSAE RACETRACKS HAVE NO BUMPS!
Z
A Little off topic but in reply to Pat, We were asked to modify or remove the wheel pods, we followed the rule to the letter it was written and determined that there are many teams who also break this rule, we did get a rule clarification (and upon checking, we were the WA team to speak to the rules committee about this), We as a team decided to design to every other definition of an open wheeler as we were under the impression that if we had to remove our conflicting part, other teams would also (ie rear wings pods etc) and it would not be done. We didn't jump up and down about it because it wasn't worth it, we didn't cut them up as they were part of a students ongoing work/thesis.
I personally don't see the problem in allowing/disallowing them, but I think if a rule is enforced it needs to be followed by all.
back on topic here;
@mech5496, we use a custom gearbox with a neat little aluminium casting after a few years of refinement it has been quite good to us, although at 2011 comp we had a few issues they can be prevented by finishing a job properly...
@Z the low mounting of the beam I imagine would be similar to the flexures we currently use, helps bring the CoM down, we already have the drivers "bumsert" lower than the flexure pickups (I think it may be the lowest part of the sprung..but don't quite me on that)
Bumps on track would be a fun and interesting experiment, similar to the wet skidpan...
(and we're still trying to find where you are hiding in our tech meetings Z.)
Is there a slick ATV race tire out there though? And will that be any lighter than a 10" or even a 13" FSAE tire?Quote:
8" TYRES. This is a bit off-topic, but as agreed there is an advantage to smaller wheels. I guess the first team that finds out how big this advantage is, will be the first team that tries it (hint: off-road quad racing tyres for mud/clay can be very soft?).
All of the examples I've seen have been either very aggressive knobby patterns or at best something that looks like a traditional sprint car tire. In either case the loss of net footprint area and tread stiffness would be a disadvantage.
On top of that you're looking at heavier tread-caps and sidewalls (typical sports car tires are mono or dual-ply, an ATV tire is likely to have a 6 ply-rating) in general and are running a higher aspect ratio than what is currently seen on FSAE tires. I doubt you save any weight or inertia over a 10" or even a 13".
I'm not saying that a proper 8" tire couldn't be developed, just that I haven't seen anything that's close.
Is there a slick ATV race tire out there though? And will that be any lighter than a 10" or even a 13" FSAE tire?Quote:
Originally posted by Zac:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">8" TYRES. This is a bit off-topic, but as agreed there is an advantage to smaller wheels. I guess the first team that finds out how big this advantage is, will be the first team that tries it (hint: off-road quad racing tyres for mud/clay can be very soft?).
All of the examples I've seen have been either very aggressive knobby patterns or at best something that looks like a traditional sprint car tire. In either case the loss of net footprint area and tread stiffness would be a disadvantage.
On top of that you're looking at heavier tread-caps and sidewalls (typical sports car tires are mono or dual-ply, an ATV tire is likely to have a 6 ply-rating) in general and are running a higher aspect ratio than what is currently seen on FSAE tires. I doubt you save any weight or inertia over a 10" or even a 13".
I'm not saying that a proper 8" tire couldn't be developed, just that I haven't seen anything that's close. </div></BLOCKQUOTE>
I was looking around a while ago just to see if anyone had them. American Racing and Hoosier both make tires in 8" size for MiniCup cars. However, it looks like these tires are supposed to last a few races, or possibly even a season on those cars so the compounds are at the exact opposite end of the spectrum as FSAE tires (very hard instead of very soft).