Vancouver Special

[Vancouver Special]

It's alive!

The engine ran for the first time this afternoon. Sounded like a chain saw just like the Phantom. Will start bolting on the drive train and the rear suspension next.

Joe

[Vancouver Special]

Uploaded one photo of the anodized rear suspension uprights. Weight just under 1 kg each.

Joe

[Vancouver Special]

If all goes well, the new car will be at the KUMHO Super Challenge.

Joe

[autobacs]

[DM 00]

Can't wait....

[Murr]

Re: Vancouver Special

Sorry to bring up an old thread.
I have been playing with stock chassis vehicles for a bit know. Getting tired of fighting with everything to work well within the limits of a mass produced car.

I am looking into building a streetable / hillclimbable / pylon eating Openi wheeled car. I am currently just laying out ideas, parts needed, geometery etc... And the only part I haven't been able to "dream" up is the rear diff. The engine would be chain driven to a central rear diff, I was thing of using a factory "FWD" diff And making my own bearing keepers and sprocket holder. something along the lines of a Honda civic so I can source a Quaiffe fairly easily and have parts easily obtainable. But from what I read, the diff has to have oil for gears to work properly (fairly obvious). That eliminates using a production type open to air diff.
What kind of rear diff is used in the Phantom and Vancouver special? I have looked, although not extensively for detailed pics or descriptions of the diff unit.

Thanks
Liam <getting Bored of my slow stock bodied suzuki
http://groups.msn.com/holyshift/shoebox.msnw

[Vancouver Special]

Re: Vancouver Special

Both cars use Quaife LSD (I bought them from Taylor Engineering). The model is the one used in a Hewland Mk.9 racing gearbox. Hewland stub axles (also from Taylor) are used at both ends of the LSD. VW CV joints and boots connected to VW axles, which then go thru VW wheel hubs. This is a pretty standard set up for most smaller formula cars. Basically a Hewland racing transaxle set up without the "wet" case. The advantage is that with the Hewland stub axles, you can sandwich a brake rotor between the stub axle and the inner CV to give you an in-board brake design (which the Phantom has). The original plan was to run in-board brakes on the Vancouver Special, hence the similar set up. However, at the end, out-board brakes were used due to packaging problems. If you use a Civic diff (the Quaife version is not much cheaper than a Hewland application and it's the same price as a VW application, check Taylor's website to confirm) you will be committed to use an outboard brake design since a Honda axle has an intergral inner CV. You also need to check the length of axle. If they are not the right length, you will be spending money modifying them. Then you need to take a close look at the Civic wheel hubs to see if they are workable for your design.

To convert a "wet" diff to a "dry" diff, you basically have to build a two-part can around the diff. The final drive flange is sandwiched in between these two cans. The typical Quaife has two end caps, For the new car, I had to throw away one of the end caps and design/machine my own. A custom sleeve is fitted over the main body of the diff with rubber O-rings. This sleeve has two thread holes (and plugs) 180 degrees apart for filling and draining (as well as for rotational balancing). The one original end cap retained had oil splash access holes tapped and plugged. A similar set up was used on the Phantom but instead of a metal sleeve around the main body, it had a large electrical heat-shrink tube shrunk on (pretty smart idea hey?). A medical syringe is then used to inject diff oil into the unit and a dap of epoxy is used to seal the tiny hole. Well, it never worked that good because it always weeped a little bit. In fact, at one of the first events, oil actually leaked onto the in-board brake rotors and I almost couldn't stop while charging into the stop box. Found out the design actually had no provision for venting and thermal expansion had pressurized the diff internal volume forcing the oil out. Both cars now have a vent hole drill into one of the bearing carriers.

By the time I paid for the Quaife, stub axles, bearings, oil seals, custom machined sleeve, end cap and bearing carriers, I could have bought one of these, http://www.taylor-race.com/items.cfm...=Chain%20Drive
This unit was not available when I build the Phantom. When it came on the market, I already bought the diff and the stub axles for the new car and was well into my design phase. If I have to do it all over again, I would just get one of these. Taylor has a shop drawing of the unit's cross section showing how it's done. Make sure you check it out.

Joe

[Murr]

Re: Vancouver Special

Hello

Thanks for the explanation and the link. Now that I have an understanding of what to look for, google comes up with many more clues.
I was thinking in my mind to built a case around a honda diff, machine a mount into the case,then run a single brake disc instead of 1 per axle. Have machined factory slip-in inner honda CV so it would be a flat face, therefor be able to bolt on porsche or vw CV's joints with custom length axles.

But TRE has built it all into a very nice tight, and light package.

Thanks for the info, and now for another question, do you have a picture site of the now "old" phatom special? I was never able to get in close enough at knox to ever take pictures of the car. Maybe that was on purpose, as probably 99% of things now a days were copied from someone else.

Thanks again
Liam Burr
http://groups.msn.com/holyshift/shoebox.msnw

[Vancouver Special]

Re: Vancouver Special

Be careful with running one brake rotor on the diff. Since it is an LSD, braking torque will behave exactly like driving torque except in reverse. A lot of the chain drive cars use single rotor. However, many of them also run a solid drive (no diff at all) like in the infamous LeGrand D Sports Racing car. To save me time in explaining why it is bad to run one rotor, I will reprint what Mark Ortiz wrote a while back when asked by a FSAE student. His reply and excellent explanation is at the end of this message.

As to photos on the Phantom. I am never big on keeping photo record on things that I do. As a result, I don't have many pictures of my cars (or anything else for that matter). I told myself to take a few more photos during the construction of the Vancouver Special. I did take some but again, it's not a routine thing for me. I do however have a close up shot of the diff with the oil can as well as the mounting side plates. I don't have a place to host them for posting with this message. If you want to see it, I can email it to you. If everyone is interested in see it here, I need someone to host it.

Joe


The following is by Mark Ortiz (consulting motorsport engineer and vehicle dynamics guru):

SINGLE OR DUAL REAR BRAKES FOR FSAE?

I am a Formula SAE team member. As you know, some teams use one differential-mounted rear brake disc and some use two discs mounted on the driveshafts or outboard on the wheels. I am having a bit of a time rationalizing the idea of using one rear brake. I spoke to the Wollongong team president at the ’03 Detroit event, and he explained that a single rear brake causes corner entry understeer (bad for small-radius SAE courses!) by effectively “locking” the diff. You said basically the same thing in the FSAE message boards. We are using the common Torsen 1 type diff. Does this diff lock in the same way when applying a braking torque as if you were applying an acceleration torque? Looks as if it would. I realize that a clutch diff could act differently under brakingdepending on ramp angles.

Situation:
Maximum torque bias ratio 80:20
Diff mounted disc
Car approaches corner. Brakes are applied. Enter left hand turn.
Weight is transferred to front and to right side. Left rear wheel loses most of its normal load and traction.

Will 80% of the braking force be sent to wheel with traction, while the wheel with low normal load receives 20%? Does the unbalanced braking or the locked diff create the understeer, or is it both?

Now with 2 outboard mounted discs it is obvious that I will always have 50% available rear braking force at each wheel (but different normal loads).

Is this correct? What equations can I use to calculate my braking force, or acceleration force distribution through a locking diff?

P.S. Are you a design judge, and where can I get info on FSAE-sized cam and pawl diffs?




Taking the last items first, a few people have talked with me about the possibility of my doing design judging, but this has all been purely tentative. My understanding is that SAE is no longer even paying expenses for judges, so the judges are actually taking a loss on the activity. I try to get paid if I’m going to have to work.

The only ready-made diffs for FSAE that I know of come from either Quaife or Gleason. These are both Torsen style. The UNC Charlotte team made their own in 2003, using Gleason gears. This was done purely to reduce weight.

You could probably make a ZF-style clutch diff, with the ramps on the pinion shafts, but I think you’d need to do the clutches and carrier yourself, and maybe use spider and side gears from a small car. You could probably also make a Detroit locker style diff yourself, but you’d probably have to make everything.

All things considered, I think it’s easier to make or find two small brakes instead.

Somebody may have some equations that describe the behavior of limited-slip diffs, but I don’t. Even modeling the friction in these units at known speeds and forces is a bit tricky, because the friction forces are a combination of Coulomb and viscous friction, in proportions that vary with load, speed, temperature, and lubricant properties.

Not only that, the forces are history-sensitive! That is, to model or understand the unit’s behavior at a specific instant, we need to not only know the speeds and input forces at that particular instant, we need to know the events immediately preceding that instant. For example, suppose we have either a clutch or a Torsen diff, with no preload. If there is no force on the diff, and we jack one wheel off the ground, and apply rotation to the carrier, we just spin the airborne wheel, get no locking, and transmit no torque to either wheel. But if the car is in motion, and the diff is transmitting torque, and then one wheel gets airborne, there is torque on the diff, and therefore loading on the clutches or worm gears, and therefore locking or friction in the unit, at the time the wheel goes airborne. That means we will continue to transmit torque to the wheel that’s on the ground, as long as there is no interruption of input torque.

Now, examining the situation you’ve posited, where we apply the brakes before a turn and continue braking while initiating a left turn, with a single brake acting through a Torsen: First of all, yes the diff does act the same when transmitting reverse (braking) torque. If the diff locking or transfer torque is less than half of the brake torque, both rear wheels are retarding the car (exerting a rearward force), but the torque on the outside wheel is half of the brake torque plus the transfer torque, and the torque on the inside wheel is half of the brake torque minus the transfer torque. If the transfer torque is large enough, it may exceed half of the braking torque. In that case we may have a forward force at the inside wheel. In either case, in a left turn we have a rightward yaw moment due to the diff locking effect, and this moment adds understeer.



It is more common for the inside front tire to reach the limit of grip during trailbraking than for the inside rear to do so. But if the inside rear locks, the outside rear is seeing whatever torque it took to lock the inside rear, plus the transfer torque. In an FSAE car with a single rear brake and a Torsen, there will be more transfer torque if there is continuous braking up to this point than if the brake is applied with the inside rear already unloaded due to cornering.

With two rear brakes, the rearward force is equal on both rear wheels up to the point where the inside one locks. If there is a limited-slip diff, there will be transfer torque. This may be very small, or if there is substantial preload and engine braking, it may be considerable. Even with an open diff, when one wheel locks, the braking forces are no longer necessarily equal on both sides of the car. We can say with certainty that the force at the unlocked wheel is at least as great as on the locked wheel, and possibly greater.

This also applies on the front wheels of a rear drive car, and the above remarks also apply to the front end of a front-wheel-drive or four-wheel-drive car, and to the front end of a rear-drive car with frictional device connecting the front wheels to prevent lockup and flat-spotting of the inside front tire. In all of these cases, the presence of limited-slip or anti-lock transfer torque creates a yaw moment that adds understeer.

[Murr]

Re: Vancouver Special

Great, thanks for the info once again. I have alot of design and stuff to keep me busy for a long while now.

"If you want to see it, I can email it to you."

If you wouldn't mind, I would like to take a look at your rear diff set up.
Pretty much unlimited size limit so what ever you have I would appreiate.
liam_burr at hotmail.com

Thanks

[Vancouver Special]

Re: Vancouver Special

[QUOTE=Murr]
liam_burr at hotmail.com
QUOTE]

PM sent.

Joe