rear frame

[Ax]

hey all,
im working on this design concept concerning the rear end of the car. i have gone through the designs of other teams out there and i have noticed many of em using a detachable rear end made outta aluminium (im guessing) which is the 'driveline mounted on the engine' setup. i have gone through some of the pros and cons of such a setup in this forum itself. but my main concerns hadnt bin listed there.. they would be,

-is such a rear end hanging onto the engine with no tube frame for support stiff enough?
-would it take control arm loads as well as damping loads?
-how much could it contribute towards weight reduction?
-could it handle the vibrations of a 4 cyl engine?

[Ax]

hey all,
im working on this design concept concerning the rear end of the car. i have gone through the designs of other teams out there and i have noticed many of em using a detachable rear end made outta aluminium (im guessing) which is the 'driveline mounted on the engine' setup. i have gone through some of the pros and cons of such a setup in this forum itself. but my main concerns hadnt bin listed there.. they would be,

-is such a rear end hanging onto the engine with no tube frame for support stiff enough?
-would it take control arm loads as well as damping loads?
-how much could it contribute towards weight reduction?
-could it handle the vibrations of a 4 cyl engine?

[Chris Allbee]

Did the cars you noticed usin 'em fall apart? If not, then that should answer many of those questions.

The detailed answers to those questions are part of what makes this an engineering competition.

Good luck with your design.

[Dsenechal]

The "detachable" aluminum rear ends are the diff carriers. The carriers are not meant to hold any load from the suspension, meaning you have to use some form of swept rear control arms.

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chris Allbee:
Did the cars you noticed usin 'em fall apart? If not, then that should answer many of those questions.

The detailed answers to those questions are part of what makes this an engineering competition.

Good luck with your design. </div></BLOCKQUOTE>

True, "not falling apart" is HALF the battle at least. I have heard multiple design judges bring up this point: with a fully stressed engine, how "round" will the cylinders stay, and how "straight" will the main journals stay? If you really want to do a stressed engine, you should be expecting those questions...

[Ax]

@ Dsenechal
i know wat u mean by the diff mounts. but i aint referring to those. i am talking bout rear ends in which the control arms are connected as well as the dampers and the diff mount.
And the question has bin specifically directed to those teams which already run these setups. and jus a general answer to my questions based on their experience would help, not looking for a detailed rep.
regards.

[Yellow Ranger]

depends on the engine too. A horizontally split, big 4 cyl can take a lot more torsional loading than an aprilia whose case is split vertically...

Also, just because a team made it through competition with a particular setup doesn't mean it stayed together for them the rest of the year. Hell, some teams only get these cars running just before competition, and if you plan is to have it last for a week of testing and the comp, you can get away with a lot more.

Here's a thought: model what you want to do. Figure out how much loading will go into the motor with this setup. Create a solid motor FEA and look at stress/deflection when the motor is properly constrained and loaded. It isn't exact, but if you see any red flags at this stage, you probably shouldn't run that setup.

[Mike Cook]

I would be slightly uncomfortable with completely stressing an engine (f4i) but our team has run the type of box you are referring to for a couple years now. Its nice because its completely modular and you can build it separately but, I can't say that it is the lightest setup and it is pretty complicated to machine.

I think with the rules concerning the main hoop supports, you have to have some frame structure extend to the rear of the car. With this being the case its probably better to not do a rear diff box.

[Brett Neale]

Adelaide's 2007 and 2008 cars had a setup like this with two different engines - the F4i in 2007 and the RXV550 in 2008.

We stressed the engine quite a bit in 2007 but used a tube frame to connect to the bulkhead in 2008 since we were unsure about how the Aprilia would take it. Let's just say the 2007 bulkhead itself was stiff enough, but the way it attached to the engine and the limited tubes we had going back there wasn't as stiff as we'd like. It mounted directly to the rear mounts of the F4i with the larger swingarm mount machined off.

The 2008 car on the other hand had a much stiffer bulkhead connection, it used tubes both top and bottom and had a much nicer structure.

Anyways, both designs had fixed diff carriers, mounting points for rear A-arm pickups and pivot points for the rear damper bellcranks. The dampers on the 2007 car mounted into the engine mounts below the intake on the F4i, in 2008 they mounted in a similar point but on the frame itself.

In terms of longevity, the 2007 car ran for 3 months prior to competition for some pretty intense testing, then after comp it ran for another 7 or 8 months doing hillclimbs, driver training and some other test days for the 2008 team. Never missed a beat except, ironically, at comp when we had a leaky fuel tank. The 2008 car had a similar test schedule prior to comp, but AFAIK the 2009 team hasn't run it at all this year for some reason.

I'm at FSG this weekend so no pics until Monday!

[Osth]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The "detachable" aluminum rear ends are the diff carriers. The carriers are not meant to hold any load from the suspension, meaning you have to use some form of swept rear control arms.
</div></BLOCKQUOTE>

Not quite, we used this rear frame solution at Chalmers for three years, 2005-2007, with a milled Al rear fram either bolted together (06-07) or welded (-05).

At least the -07 car had a fully stressed engine without any problems that I know of (associated to the stressed engine http://fsae.com/groupee_common/emoticons/icon_smile.gif), and that car finished the endurance both at Silverstone and FSAE West as well as some extensive test driving (7-8 endurances pre Silverstone I think).

The engine on the -07 that was fully stressed was a 4 Cyl Yamaha Fazer (I think they are called?!?), so the solution most definiatly works.

So I think based on our experience you can say that it works without breaking the engine etc, but there are also some problems with it:

It is somewhat expensive for the cost report(We used CNC machined Al parts).
We where critizied for bad triangulation of it (Could of course be avoided with a good triangulation http://fsae.com/groupee_common/emoticons/icon_smile.gif)...
It seems to be too weak during actual driving and flex too much under suspension loads.

On the positive side:
It looks great, especially with some fancy anodizing of the Al.
If performed correctly, it should have the potential to save weight (In the -07 case I don't think a real benchmark was done so don't know how much though).

I found no good picture of on the actual car, but there is a CAD image to show the concept on:
Chalmers -07 Frame (http://web.student.chalmers.se/groups/formula/joomla/index.php?option=com_content&task=view&id=53&Itemid=80)

I know that our last two teams dropped the Al rear frame and stressed engine, and I think it was due to new rules prohibiting fully stressed engines so you should probably look into that as well.

[Ax]

hmmm. any more teams that have used this kind of setup? we use an f4i too and would like to know the outcome after stressing it as discussed above..

[James-H]

We used a plate rear end at Manchester in 2008. It consisted of two 10mm thick laser cut aluminium plates spaced by steel tubes with x bracing welded between these tubes. It carried the diff and took all the suspension loads as well as bolting directly to the engine (R6). The car ran fine without falling apart, but we (along with the design judges) had concerns about the stiffness of it.

So for 2009 we had a spaceframe that ran the complete length of the car. However we used the plate idea to mount the diff still but with these 'diff plates' mounted inside the spaceframe and still mounted off the engine. The advantage that this gives is that it is easy to locate the driven sprocket in line with the engine output accurately. Again we had no reliability issues with these diff plates falling off the engine due to vibration.

[Wesley]

I always thought it would be interesting to build a test rig and do some stress testing of an engine. Just tweak one side of it and determine the maximum bore and main journal deflection/deformation. I'm pretty sure with the F4i it would be ok, given decent load paths.