Salisbury or Quaife ATB?

[kcapitano]

One last thing, wouldn't more lock on corner exit make oversteer worse? (just a question I have personally...Never really looked at diff tuning)


My understanding of it is that when both wheels are accelerating at the same speed (locked), they make the car want to go straight, thus reducing oversteer at corner exit.

Its dependent on weight transfer. If the weight on the rear tires is evenly distributed between them then both tires will resist changing speeds an equal amount and the car will want to go straight. If, however, there is a large percentage of weight on just one rear tire then that tire will react more longitudinal force then the opposing rear tire and a yaw moment will be introduced. This affect can lead to power on oversteer during hard cornering with a spool setup.

In regards to your original question, learning to maintain and tune your current diff would, in my mind, be a much more effective use of your time than redesigning your powertrain to accommodate the MK2. Without experience tuning your suspension and differential setup you won't be able to validate the effect of changing your differential. Redesigning everything for the new diff will also require more resources during the design phase, which won't help you finish the car sooner and without validation you will be right back where you are currently in a few years.

That's my two cents. To tell you a little of my experience, in 2012 I redesigned my team's drivetrain to work with a Drexler after having used a Torsen for over 10 years. My motivation was purely for reliability and finishing the car sooner and we never really looked at how the diff affected the car. Our drivers haven't noticed a significant difference, but this summer we are looking at doing some diff testing to validate its effects. Unfortunately its difficult to change the setup and because we don't now very much about it the diff testing is very low on our priorities list.

[AxelRipper]

$500 Torsen? Too expensive, heavy, and unadjustable.

I have to say, if you have the money and want something that is lightweight and can bolt up to your car and work spectacularly, the Taylor is hard to beat. When you can justify why you chose it in design for other reasons than it was easy and everyone says its great I imagine it works even better.

There is a differential out there that is cheap, light, and tunable, and requires about as much work if not less than a standard Torsen. (This paragraph sounds like something that Claude or Z would write, but I'll be kind and go a step further and give you the answer)

The front differential from an ATV. In our case it was the Honda Rubicon. Take it, machine off the ring gear (if you're using the newer style one) and build your bearing/sprocket adapters. Our setups have been about the same weight as the newest Taylor. And the diff costs under $300 online, brand new.

Now the adjustability is where it gets fun. Its a cam and pawl differential. There's a shim in there that you can change thickness of to go from nearly open to full spool. The internals will wear a bit, but it works quite well.

And if your suspension is tuned wrong (lifting inside tires, it opens up) then you can just pop it apart, grab the mig and spool it.

[dmacke]

Originally posted by AxelRipper:
$500 Torsen? Too expensive, heavy, and unadjustable.

After a couple years of design iteration our housing, gears, and bearings weighs less than 6 pounds. It's always a work in progress but I think it gives us more design flexibility.

The front differential from an ATV. In our case it was the Honda Rubicon. Take it, machine off the ring gear (if you're using the newer style one) and build your bearing/sprocket adapters. Our setups have been about the same weight as the newest Taylor. And the diff costs under $300 online, brand new.

This is definitely intriguing though. Will look into it.

[Z]

Brian,

The type of differential you choose is a major top-level design decision if you want a really competitive car. The whole car concept is affected greatly by the type of diff (eg. High or low powered engine? Middle or more rearward weight? Aero or not? Etc.). On the other hand, if you are only aiming for mid-field, then almost any old diff will do (providing it doesn't break!), with a spool being the cheapest and most reliable.

So very briefly, regarding your "whole car concept" decision making;

If Torsen, then you don't want the inner-rear-wheel lifting when exiting corners. So with the Torsen (and likewise with open-diff) you want more front roll-stiffness than rear, or more rear weight, or lots of aero, or a low powered engine, or just don't be in a rush...

If LSD, then you can easily set it up to be an "open" diff (minimum preload and ~90 degree ramps), or a "spool" (max preload and more angled ramps). This covers most of the range of the other car parameters. Only when you want something inbetween (ie. because you are already at the front of the field) do you need to know how to "tune the diff". Here ~90 degree braking-ramp, 45 degree accel-ramp, and some preload gets you most of the way.

Z

(PS. Kcapitano has it right regarding corner exit with a spool. That is a good example of how the same diff can give you severe oversteer or understeer, depending on the rest of the car setup.)

[Jay Lawrence]

Just a comment on the cam & pawl: we used one on a couple of cars and did not like it (this is prior to me being on the team). For us it gave a kind of ratcheting feeling, particularly on corner entry. This is probably tunable to a certain extent, but from what I was told the drivers were not big fans of it.

[Brian S.]

Thank you for all of the input guys, I really appreciate it.

I am trying to get a proposal together for the direction I would like to take this year’s car, and this diff choice is what I am stuck on. We just got back from Lincoln a few weeks ago with a car that has yet to move under its own power, as it struggled to pass sound and then the scavenge pump for the dry sump failed. This was the first time in four years that we did not complete every event. It seems that the few members we have left want to scrap this design completely and start over from scratch. They also want to do a 2 year build. I am trying to propose that we only slightly tweak last year’s design and spend a few months before competition testing and tuning. While we test and tune the 2014 car, I would like to start working on a fresh redesign for the 2015 season. We do not have many knowledgeable designers, but we have an adequate amount of good manufacturers, so I feel that this plan could work well for us.

Here are the specs of our 2013 car:
Weight: 491 lbs wet, half tank of fuel, 52% rear
Power: Most recent dyno run was 76 whp (CBR600F4i)
Aero: None, but we have an undertray that was developed, just needs to be made

Z,
From what you are saying, it sounds like our current Salisbury style diff would be great for what I want from our 2014 car. I feel that the Taylor Race MK2 will work well if we decide to make a small, single-cylinder car with aero for 2015, which is what I would like to do.

Not having to adapt the Taylor Race MK2 to our current design will save us a lot of time that we can use to develop other areas of the car or just test longer. I also feel that the design judges will appreciate an attempt to tune our diff as that was one of the things they called us out on.

For those of you suggesting cheaper diff alternatives, thank you, but we already have both of the diffs I am considering. However, I will look into some of these suggestions for the 2015 car.

Brian

[jd74914]

Originally posted by Jay Lawrence:
Just a comment on the cam & pawl: we used one on a couple of cars and did not like it (this is prior to me being on the team). For us it gave a kind of ratcheting feeling, particularly on corner entry. This is probably tunable to a certain extent, but from what I was told the drivers were not big fans of it.

Sorry for the hijack Brian, I just wanted to comment on this for anyone reading who hasn't played with a cam and pawl. We experienced the same ratcheting feeling as well as a very high level of wear with our cam and pawl setup. For reference, our car is about 100 whp (75 kW) and would wear pawls in maybe 4 hours of driving.

Last summer we designed a Salisbury loosely-based on a BMW e30 differential and so far have been very pleased. All drivers have reported that it is much easier to drive than the previous cam and pawls. Like everyone else here, we also haven't had the time to test it, but with our clutch/preload combinations it has been very driveable (our main goal).

I think you're going the right way reusing the old Salisbury design. If you had enough problems competing in all events, you want to fix the things that stopped you, not redesign "successful" parts.

[Z]

Originally posted by Brian S.:
While we test and tune the [modified 2013 =] 2014 car, I would like to start working on a fresh redesign for the 2015 season. We do not have many knowledgeable designers, but we have an adequate amount of good manufacturers, so I feel that this plan could work well for us.

Brian,

That sounds like a good plan, especially if you can keep the same team together over the two years. If your "good manufacturers" spend a season modifying->testing->re-modifying->re-testing your current car, then they should become reasonably "knowledgeable designers" by next year.

With regard to your current car, I would suggest that the modification program involves as much "simplification" as is feasible. As in, "Do we really need this part? It keeps flexing, and sort of binding-up, and, err... Why don't we just toss it, and mount that bit direct to the chassis?" I know there is a lot of resistance to this sort of thinking, but I have found that real testing often proves that you don't really need that part after all.

Some suggestions for parts that can be "tossed" (yes, I do keep harping on about it, but anyway... ).

* Pushrods and rockers. Just mount the spring-dampers directly between chassis and wishbones. See pics of U of Cincinatti at FSAE-Lincoln-2013 for clues.

* Steering-R&P AND a Bevel-Gear-Box. You only need one or the other (eg. a 3:1 BGB with a Pitman-Arm at the bottom eliminates the R&P, and is smoother, stiffer, cheaper, less free-play, etc.)

* Anything more than a push-pull-rod for gear changing. And while you are at it, just a hand-clutch so you can toss that extra foot pedal...

Doing above will make your designers a lot more knowledgeable as to what is really needed on the 2015 car.
~o0o~

Back to diffs.

I am sure that your current car, being relatively high-powered and with only ~half-weight on rear wheels, can smoke both its rear tyres coming out of almost any FSAE corner (ie. they are all fairly low speed, so high torque is available). So the Salisbury-LSD is probably the better option. If in worst case you are getting far too much inner-wheel spin, then you increase pre-load and ramp-angle to make it more like a spool.

The proposed 2015 car, being lighter, with lower power engine and aero, would be better matched to the TR2-Torsen. In fact, with more rear weight (say, about 60%), then a plain open-diff could work well enough.

Note that with, say, ~2/3 rear-weight and equal sized tyres all round, it is natural to carry most roll-loads (= LLTD or ERMD) at the front. This has the outer-front-tyre carrying ~1/3 of the cornering forces (because inner-front is almost unloaded), and both rear-tyres sharing the other 2/3 of the cornering forces. So equal loading on all (3) tyres, and good balance. This also has both rears planted at mid-corner, so good acceleration out of the corner even with an open-diff.

Z

[Brian S.]

Z,

I am familiar with a lot of your ideas for the simplification of fsae cars from reading the suspension thread and others, and I have to say that I agree with most of what you say. Our paddock was actually right next to Cincinnati’s at Lincoln and I really liked their suspension setup. I brought a few of my teammates over to it and told them that we should do direct-actuation if we do a complete re-design, and I just got a bunch of blank stares. It’s tough to propose simplification ideas especially to our young members who want to make a mini-F1 car. It’s even worse if you propose to simplify a system that someone else is working on because they think you’re insulting their intellect by “dumbing it down”. I had a nice “I told you so” moment when the overall results were posted and I saw Cincinnati placed 9th overall.

At Drexel we have a five year program because we have 3 6-month co-ops. This helps us in that we can keep members for an extra year and there are always people at school in the summer. However, it hurts us because people will disappear for 6 months. I have two more years, so I will be overseeing the next two cars. Hopefully we can all pull together to make a very good car in 2015.

Back to diffs,

I appreciate everyone’s help in making this decision. I am going to order some new ramps and any parts I need for servicing the diff from ICP. We are currently running 80x80 ramps, so I was thinking of getting 45x80, 60x80, and 60x70. Do these sound like good choices?

Brian

[SNasello]

Brian,

those ramp angles sound like pretty good choices to me to start with. If you end up using the diff over the next few years you can slowly build up a range of spare parts for it that you can use to tune it on the new car.

You should also look into getting shims of various thicknesses to adjust the preload, if possible. Also, think of a method to measure the preload in your diff, so you can check if it is in spec when you do find a setup that you like.