Semi-trailing redesign

[TT510Guy]

I am working on a new project (I call it Redux, long story)that involves trying to put down a fair amount to power (between 450-550ft-lbs at the flyhweel), to the ground in a fairly light (2500lb) car. The car is an early Datsun 510 and thus currently has semi-trailing arm suspension. I am open to a complete redesign of the suspension to get the power down. Currently a 510 with over 300ft-lbs of torque has significant wheelspin even in 2nd gear (1.9:1 plus 3.36:1 differnetial and 215/40-16 street tires) and even in 3rd (1.3) when it's below 70 degrees outside. I feel a fair amount of this is due to the semi-traling design adding negative camber (and changing toe) as the suspension compresses. That 510 already has relocated pivot points to compensate for lowering (1in higher, the max possible with given crossmember) and higher spring rate (875# which gives approx 225# wheel rate) and minimal static camber (.5deg).

Redux's intended use is daily driver / commuter with semi-frequent trips to autocross, dragstrip, and road course/track days. Racing slicks will be used, but very infrequently. Primary goal is significant improvement in straight-line acceleration with minimal loss in ride quality and low gain of overall weight. I have read through a fair amount of RCVD and have a fair understanding of general suspenssion designs, but am no engineer!

I am considering everything from multilink IRS to De Dion to live axle. I'm not a fan of live axles, but if I could get a reasonable design (and perhaps an alumunum 3rd member to cut the unspung weight) that seems like it will meet my needs I'm game. The design also needs to accomodate wheels with a fair amount of positive offset as I will run wider wheels but not fender flares.

A De Dion with Watts seems appealing, but from what I read De Dion seems generally poo-poo'd. I'm sure I'm overlooking other necessary information, and I apologize for the lenth of my first post...

Dave Lum

http://www.datsuns.com

 

[MikeHalloran]

A long, long time ago, I worked as a manufacturing engineer in Ford's axle plant. One thing that surprised me about the job was that we were always doing cost studies, on _everything_. Every week we'd get a pile of prints of something, and we'd have to figure out how to produce it, and how fast we could produce it, on tools that we had, or could get. One of the prints was next year's model, but we didn't know which one. We got reverse- engineered prints of competitors' parts, and of stuff they were thinking about maybe building someday, and prints that had subtle deviations from current parts. Somebody, somewhere, aggregated our output, and should have been able to estimate the incremental cost of, say, a 1mm change in bearing size or spacing, or a flange with one more bolt, that kind of stuff, in addition to exotic designs, and the exact cost of producing any other manufacturer's parts on our tooling in our factory.

One of the designs I remember well was a DeDion axle, with of course a Ford pumpkin. The whole department got enthused about it, because it was clearly producible with just a few new tools, it allowed a lower floor pan/ bigger trunk, and it looked really cool.

One thing that struck me about it was that the cross-tube connecting the wheels was pretty sturdy, because the lateral loads that a regular axle housing carried in compression, it had to carry in bending. I.e., the cross tube was only a little lighter than a regular axle housing, and there was of course a short axle housing nested inside it, so the car would necessarily get a little heavier overall.

For a live axle setup that handles well and can deal with lots of torque, look under the last generation of Camaros: lightweight Panhard bar, two trailing arms, and a long, deep torque arm from the pumpkin to the transmission crossmember. On a Camaro, the torque arm is right next to the driveshaft, so it will fit in the tunnel. On a racecar, you could offset the arm a bit farther to the right, so the crownwheel torque reaction would better help hold the r/r wheel down.



Mike Halloran
Pembroke Pines, FL, USA

 

[TT510Guy]

I see nothing bad against the De Dion there....a little more overall weight but significantly less unspring weight. Funny you mention Ford because I discovered Lincoln MK VIII LSC has an aluminmum 8.8 (223mm ring gear) differential that weighs within a couple of poinds of my Nissan 180mm (R180 in Nissan-speak) differential and would be using the Lincoln part in a De Dion design.

I will, however, check out the Camaro design, thanks.

Dave Lum

http://www.datsuns.com

 

[GregLocock]

I think the De Dion tends not to be used because it has some packaging problems and costs as much as an IRS but lacks the marketing cachet. Adding a Watt's link solves the compliance oversteer tendency, but make sthe packaging even worse.

From a suspension designer's perspective your proposal is a fairly nice solution, but I don't see that it has any special power handling abilities.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Peter7307]

De Dion seems to be something of a strange item from the production point of view.

It is flavour of the month for a model or perhaps two then it is discarded and all traces removed as though it never existed.

Rover used it for a while as did Aston Martin. Even Honda had ago at using it for one model.

Maybe the promise and reality never quite managed to meet?

Pete.

 

[MikeHalloran]

The cross-beam is a tough part to machine, because it's big, and because there's no clearance for a real boring head to machine the inner bearing race seats at the ends. The alternatives are to machine the ends before assembly to the crosstube, which is impractical for several reasons, or to back-bore and back-face those inner bearing bores from outside, working through the stub axle clearance bore, using a long cantilevered single- point tool. The flexibility of that tool and its small cutting edge means that it takes a relatively long time to machine the bearing seat. By contrast, the outer bearing seat can be rough- machined with a single- point tool and finished with a reamer, in much less time, even if the tool change is slow.

I.e., the machining cycle time is too long for a high volume vehicle, so the only way to support increasing production volumes is to add machine tools, and they're expensive.

Also by contrast, the inner and outer bearing bores of an IRS hub carrier can be machined at once, or at least in the same setup.

The modern unitized hub/flange assembly potentially makes the DeDion a little more attractive, because the crosstube assembly simplifies to a tube with a couple of bends and a couple of cantilevered bolted flanges that don't require extreme precision.

The DeDion still needs sliding splines in the halfshafts, and they are typically expensive or unreliable or both, especially at axle- style torque levels.





Mike Halloran
Pembroke Pines, FL, USA

 

[BillyShope]

If you've been reading RCVD, you know how difficult it is to completely eliminate IRS squat (which, with your planned dragracing, I would assume you'd want to do) without introducing a significant amount of roll oversteer. Still, that is the first thing I'd investigate, since ROLL oversteer does not introduce the instability problems inherent with oversteer caused by rearward weight bias, etc. If, however, you think you would be uncomfortable with the steering corrections that would then be necessary in the autocross application, you might be forced to consider a live axle.

For a live axle, I'd follow the KISS principle and adapt the design so successfully used by Jaguar on the early C-Types. This is simply 3 trailing links with Panhard with one of the links offset for dynamic cancellation of the driveshaft torque. This is briefly mentioned in RCVD, but is discussed more thoroughly in the student workbook for RCVD, with coverage and equations supplied by myself and a former Jaguar engineer. Or, you could simply go to my site, where I have a spreadsheet for the necessary setup for complete driveshaft torque cancellation. If this is done properly, the rear tires remain equally loaded...regardless of driveshaft torque...and the rear of the car neither squats nor rises. Here's the URL:

http://home.earthlink.net/~whshope

 

[GregLocock]

I wouldn't be quite so blase about roll oversteer.


It happens very late in the cornering process and is responsible for a disconcerting 'two bites' feeling when entering a corner.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bikesrfast]

I guess for your multipurpose use that has to include drag racing why not get an 80's Alfa Romeo Alfetta DiDion and even include the whole gearbox under there?

If go live axle then follow the linkage pattern/setup of the worlds most successful multipurpose car thats also similar in size and weight to your project, the Ford Escort RS1800.

 

[TT510Guy]

Intersting replies all, thank you. Seeing as the 510 already has IRS, the adjustable length halfshafts aren't an issue. Anti-squat I'm not so concerned about as I am about camber change under acceleration, which De Dion and live axle cure nicely. However, if I am driving on some bumpy roads I don't want to be tossed sideways like I have been in a '95 Mustang GT - I had a FWD Taurus SHO that was immensely superior to the Mustang over imperfect roads.

RS1800 has leaf springs, which per RCVD is pretty far from optimal. A lot of suspenion ills can be hidden by having low overall vehicle weight.

In other circles I have heard C4 Corvette suggested, of course I would need to somehow section the center to get it to about 52in hub to hub, anyone have experience with C4 rear geometry?

Dave Lum

http://www.datsuns.com

 

[bikesrfast]

TT510Guy - "RS1800 has leaf springs, which per RCVD is pretty far from optimal. A lot of suspenion ills can be hidden by having low overall vehicle weight".

Indeed the RS1800 has leaf springs but a simple statement that not exactly describes it correctly.

The axle location is controlled fully by the 4 links (well 5 with the panhard of course) and the leafs merely spring and with good effect too as there is a natural dampening effect with the friction between the springs and individual leaf frequencys aiding control as well as progressive rates that can be built in depending on the individual leafs characteristics (taper etc). Weight is a downside though. Before shocks as we know them now, leafs were a good thing because of these reasons.

 

[NormPeterson]

Wouldn't it be better to crib from the C5 arrangement and relieve the halfshafts (and the associated clearances and wear) of wheel location duty?

Would "Redux" happen to be a 215 CID aluminum-engined 510 that I have heard was a bit, ummm, "twitchy"?


Norm

 

[TT510Guy]

Thanks for the clarification on the RS1800, it's appreciated. Cribbing from the various years of Corvette's is also under consideration.

My "Redux" is not a 215 CID aluminum engined 510.

http://www.datsuns.com/projects/redux/

My current 510 (also a twin turbo V-6 car) has served as a proof-of-concept / development mule and "Redux" can be considered "version 2.0" which will address all the shortcomings of the current car. Straight-line traction is one of the shortcomings I am looking to address.

Dave Lum

http://www.datsuns.com