Rear differential setup

[URracing]

I've noticed a good number of teams mounting their rear diff off the rear bulkhead of the primary structure, right behind the motor. Some are even bringing their rear a-arm link back towards the cockpit on a FORWARD angle since there are no frame rails to mount it to past the differential. I noticed one car at the Michigan competition last year that even ran their driveline on a forward angle from the diff to the upright. Correct me if I'm wrong but I thought I read somewhere that the driveline had to be in a perpendicular plane, both horizontal and vertical, with the planes crossing eachother at the diff.

I'm just curious how teams made out with bringing that rear link back on a FORWARD angle. To me that's putting exponential forces on your linkage sort of like an over-center locking mechanism of such. I know it's a good idea since it eliminates overhanging components and weight on the back end.

Im currently designing our rear end and just curious as to how much chassis flex has an effect on the chain. I'd like to mount the diff as solid as possible, even using aluminum plates off the back of the motor to encapsulate the diff if we have to.

Just curious to see any input on this. It's a pretty cool setup except it scares me to run your rear link like that. If I understand it right the forces that link sees are mainly when the car is turning and braking.

[URracing]

I've noticed a good number of teams mounting their rear diff off the rear bulkhead of the primary structure, right behind the motor. Some are even bringing their rear a-arm link back towards the cockpit on a FORWARD angle since there are no frame rails to mount it to past the differential. I noticed one car at the Michigan competition last year that even ran their driveline on a forward angle from the diff to the upright. Correct me if I'm wrong but I thought I read somewhere that the driveline had to be in a perpendicular plane, both horizontal and vertical, with the planes crossing eachother at the diff.

I'm just curious how teams made out with bringing that rear link back on a FORWARD angle. To me that's putting exponential forces on your linkage sort of like an over-center locking mechanism of such. I know it's a good idea since it eliminates overhanging components and weight on the back end.

Im currently designing our rear end and just curious as to how much chassis flex has an effect on the chain. I'd like to mount the diff as solid as possible, even using aluminum plates off the back of the motor to encapsulate the diff if we have to.

Just curious to see any input on this. It's a pretty cool setup except it scares me to run your rear link like that. If I understand it right the forces that link sees are mainly when the car is turning and braking.

[exFSAE]

It's definitely a challenge, getting it rigid and packaging everything well while getting the suspension kinematics you want. Big challenge.

You're talking about having the half shafts swept forward or up? With suspension travel those things are going to move a bit anyway. Manufacturers like Taylor Race spec maximum splay angles. Closer to "all squared up" and perpendicular is better I believe, but you have to make compromises.

[Mike Sadie]

Having both control arm mounts forward of the wheel as you describe significantly increases the loads in the control arm tubes. Be sure (as you should anyways) you design the control arms for acceptable compliance, not just failure.

If I remember correctly, the rule of thumb regarding splay angle is 1 wheel hp loss per degree.

[blister]

Well it`s definitely a big challenge. UWA did a really beautiful rear packaging in 2006.
Also i think Graz had forward pointing driveshafts in 2007 due to packaging and wheelbase.

One big problem is the rearmost upper control arm mounting point. It can interference heavily with your drivechain and cause rigidity issues.

control arm angles: it could be advantageous to seperate the lateral and longitudinal loads in your arms to the two control arm beams.

I think its possible to save 10kg with a not-so-standard rear packaging but it`s a looong road.

[URracing]

Yeah it looks like a pretty compact setup with a lot of variables to consider. I believe we have decided on building a real basic car that is manageable during the competition since we are just gettin back into Formula.

Just one question though, what kind of results can we expect from running the stock motor/gearbox with a 14 tooth - 56 tooth 520 chain drive? I know the 56 tooth sprocket is one nasty thing to package back there but the reason I have asked is that our original design included it, but keep in mind it was also designed by students a while ago that are now graduated. We plan to get an AEM management system on it for fuel mapping (since we already have it) once we get it running with the stock ECU, but may also look into the piggy-back Power Commander too.

thanks

[Biggy72]

Get a smaller front sprocket. Also just make a spreadsheet to calculate how fast you'll be going in every gear with your stock transmission and tire sizes. This will give you an idea of what to run sprocket size. 520 chain will work fine.

[VFR750R]

i don't think the loss is linear.
For shallow angles almost nothing is lost but from 15-20deg? it will start to go exponential. Tripods (like universals) last longer if they are working anyways, they don't like to be at zero degrees. Tripods can also handle higher angles then universals.

I also don't think you will subject the frame or uprights to additional loads even with high half shaft angles.

The control arms however are subject to higher loads (cornering, not from half-shafts)but it can be calculated and the appropriate tube size determined.

[Brett Neale]

Our 2007 car had a forward driveshaft angle of around 12deg static. We calculated negligible power loss and a very small vibrational moment (less than 0.5Nm peak) when run at this angle. I think at full bump/droop the max angle was still under 15deg, which was well inside what Taylor specs for their tripods.

The diff is a Torsen mounted solidly to the bulkhead with a linear idler sprocket tensioner. This proved to be the stiffest option and is highly recommended over a pivot-type arrangement, and it also will never change your driveshaft geometry.

As for the wishbones, we made sure to make the rearmost pickup inline with the upright ball joint where possible to reduce loadings. The bottom wishbones were on more of an angle so we upsized them to suit. We also mounted spherical bearings on the chassis side to prevent any problems with rodends.

We found that the gains made in rearward weight bias outweighed the complexity, cost and drivetrain losses. The improvements over the previous car were huge in terms of getting power down and progressiveness of rear-end breakaway - it was much more predictable with the extra weight and stiffness in the rear.

http://formula-sae.netfox.com/images/galleries/0000/0111/8.JPG

[vreihen]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Brett Neale:

We found that the gains made in rearward weight bias outweighed the complexity, cost and drivetrain losses. The improvements over the previous car were huge in terms of getting power down and progressiveness of rear-end breakaway - it was much more predictable with the extra weight and stiffness in the rear. </div></BLOCKQUOTE>

Since I don't see any specs on your team's web site, would you mind sharing the static F/R weight bias percentages of your present car and the previous car? Just wondering how much weight in the rear made a noticeable difference in getting the power down.....

[Christopher Catto]

to take the extra loads on the control arms, make sure that the rear leg (which is "unstable", so to speak because the triangle has one internal angle greater than 90deg)) is of strong material.

By nature of the setup, this rear leg will be quite short anyway (like on old fashioned Lotus F1 and F2 cars). The bolt and the hole in the bulkhead need to be strongly and stiffly installed.

I think the gains in terms of compactness (see the Ferrari F1 from 2003 onwards?) are worth it because ideally you do not want the rear to overhang much past the engine and gearbox to avoid the rear of the car swimming around.

[Dan Lentsch]

We are going to attemt this type of "Hanging Diff" / foreward swept contorol arm setup this year. As a second year team it has been a HUGE challenge to do this. We are still unsure of how it will work out, but at this point I think we probably would have not done it since we now have an extended wheel base (67in), goofy looking control arms. and a crazy ammount of time invested into it. On the good side we will only have our half-shafts are swept foreward a couple degrees at most. We will be @ VIR and West if you want to see..

[Brett Neale]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by vreihen:
Since I don't see any specs on your team's web site, would you mind sharing the static F/R weight bias percentages of your present car and the previous car? Just wondering how much weight in the rear made a noticeable difference in getting the power down..... </div></BLOCKQUOTE>

We gained 5% extra rearward weight bias on the previous year's car.

[Chris Lane]

We're also using this sort of arrangement for our first year car.

I've sunk many many hours into getting my head around the packaging issues....

I'm nearly there though... :P