1-Way vs 1.5-Way Rear Differentials
 

I wanted to separate out the idea to get more traction (haha) among the racers on board.

I think the traditional, no-thought idea is to use a 1.5way rear diff in the back for our cars, especially the non-AYC ones. It provides lock-up on acceleration as needed and moderate lock up on decel, braking, any time you're off the gas, for stability. I'm starting to question whether or not that stability is "needed" and how much hindrance it is, or at least mine is, on turn-in.

This *might* be specific to me/my set up. Speaking with OS Giken, they use springs to "negative preload" the diff - they counteract additional locking from the pressure ring, while the Cusco using springs like a shim to add preload. Theoretically, how the OSG is setup should help with turn-in, despite still also being a 1.5way.

However, *if* there's enough stability in braking/high speed, which I think there may be, particularly with our EBD, center diff, etc., than being open on braking/decel can greatly help turn-in, while maintaining lock-up on powering out of corners. You can use suspension to tune out corner entry understeer, sure, at more or less the expense of making the car looser in every scenario, to a degree. AWD cars in general struggle with corner entry, not corner exit, and I'm wondering if a 1-way could really do the trick here.

Are there any other factors I'm not considering about 1-way diffs? The stability argument makes it sound like any car with a 1-way diff, rwd, cars, open diff cars, would completely lose it in high speed stability/braking, which is obviously not the case. Any other reason NOT to use a 1-way diff, and anyone here with experience in both, in both autocross & road racing?

A 1.0 would remove the ability to lock the rear on entry all together and therefore limit the driver's tools to rotate the car.
I'd rather have the 1.5 and a way to tune how much of a throttle lift is required to lock the rear (albeit not an easy adjustment). The sprung pre-load or sprung anti-pre-load is to tune how aggressively the driver wants the rear to lock, that has got to be a personal preference or based on driving style.

rear locking binds the rear wheels together and inhibits corner entry/rotation, locking the rear is not optimal for corner entry ease, they have to travel different distances. the partial lock of a 1.5-way fights that

I disagree, it does not inhibit entry/rotation, it allows for more easy rotation on entry. The lockup is a tool and must be used as such, it is momentarily removing some rear grip (if tires are moving at different radii read: car turning) which will likely result in oversteer. For our cars, being front heavy and prone to understeer, that is a pretty helpful tool to getting around those extra tight turns.

If you are locking up the rear and getting too much oversteer or losing stability on entry, consider adjusting the 1.5 to be less aggressive or modify your driving style. More straight line braking and less trail braking should keep the rear in line when you want it to be. Just my .02

Another note, for example, during my last AutoX event, there was a big sweeper (by autox standards) which I found making a double apex to be my fastest option because I couldn't get a lift-off oversteer. If I could have, I am confident that would have been the faster option; brake once turn once, on gas earlier. But I am also open front diff and was spinning inside front, probably the more prominent reason for my having to box the turn.

It does not "remove" grip, unless the wheels are completely locked, i.e., welded together or aggressive decel lock, which means that literally one wheel is going to get dragged. limited slip diffs resist differences between the two wheels. similar argument as between the front and rear wheels - locked center will not result in the car being loose upon turn-in. you want the car as open as possible upon turn in

I would put money on the fact I couldn't be faster with a 1way vs a 1.5way. Just two very slight differences in setup and bias and probably slightly different driving style.

Since Evos nearly always have a propensity to lift an inside rear at heavy decel (some do with with only moderate depending on setup), if you have a 1way wouldn't you lose a big chunk of your engine braking torque once that wheel lifts?

On paper though, there seems to be merits to use that partial lockup on decel in the rear. Unlike the front where its purely a hindrance.

hadn't thought about engine braking torque, though by the time you're in corner entry how much would that really account for if most of your braking happens down the straight?

in theory with the wheel lifted, with a 1.5 the wheel on the ground takes the burden of that, vs with a 1-way that is open?

i think there's not too much response here as it's not something that's explored very frequently, but i'm thinking more and more for awd open everything on entry/braking is the way to go. i dunno if it'll have much difference in terms of lap times, though i should technically be able to add grip back to the rear via softening the RSB with a 1-way, but i think there will be a huge difference in terms of the car's willingness to turn in.

There are many little tweak Ive felt through different setups which were like "well, I guess one way felt better here but maybe not there" and the clock really doesn't show any different. Definitely something a pro team would try both ways and make a choice. For our level, seems no reason not to stick with a 1.5way rear, 1way front. Shocks, spring/bar rate balance, roll and CG center/axis, tire pressure balance, etc all seem to be enough variable that have cross talk into the effects of which diff configuration may work best.

I know my answer sounds (it feels like it does at least) wishy-washy, but Id say just go with your gut here and stick with it. Considering the variables, I think you'd find improvements in some areas but maybe not others.

Locking and unlocking the center diff will have a much bigger impact on turn in, because front grip is impacted by it.

i think i'm stuck with modifying 1 diff, which will be the rear, so the front is the stock helical, though the acd is tuned. my full setup is: 8/10k ohlins, a few WL bushings to correct geometry that everyone does on 8/9, -3.3 camber front -1.8 rear, caster at 5.5 degrees, tanabe hollow FSB w/ cusco brackets reversed and on soft (which made virtually no difference), WL 24mm RSB on full stiff, ER ACD tune, and the Cusco 1.5 Type RS set up by jon@tre. this setup actually used to be pretty darn loose on 17x8s - on 18x9.5s, it seems to push more. Whatever the case, obviously not your level of build but, this should not be a "pushy" setup, i think my rear diff is hyper aggressive.

You're right that we don't have the resources like big teams to test setups, but I'm willing to bet a 1-way would give me back some corner entry rotation/reduce the resistance of turn in

Yea, that's what I posited a while ago in the ACD tuning thread, and opening it up has made a sizable difference in turn in, but I just think my rear diff is binding on turn-in, IDK. I'm more or less set on doing this at SOME point, I'm not sure when, probably next year, and I'll report back what I find.

I agree with you, for max grip going about the turn, you want diffs open and slipping, not limiting the slip, to allow all tires to do their independent speeds.

BUT our cars are more particular and for some turns, especially turns where the car wants to push, having a way to get the rear loose on command is the best tool to get the car pointed in the right direction (rotation via lift-oversteer). If you do not want to use this tool, ever, then go for a 1.0. There are certainly other ways to get the rear loose, typically more compromising options tho. See if Jon can tune out some of the aggression of your Cusco RS 1.5 since it does have that ability.

In the meantime, with all that entry oversteer, how about posting some diy-gymkhana videos, heh?

i have entry understeer due to, in part, the cusco 1.5way being too aggressive and binding the wheels together. rotation via lift-oversteer is more suspension dependent, not directly related to diffs, especially on our cars where the inside rear is going to have nearly no grip/be off the ground

I would actually suspect your setup may have something else effecting turn-in if thats your issue. My car is a razor on entry. Its pushy mid corner but super sharp at entry and rotates on exit. The sharp turn-in is what makes my car feel so great is fast transitions, you can just crank the wheel and it goes.

I've also found some things Id like to update in the ER tune on the ACD. I've found I like the least aggressive setting for the first half of the turn and med to high for exit and putting power down. So I need to spend some time this winter figuring out how to flash it myself.

i've tried with a variety of settings, i dunno. i listed my suspension setup & specs above, nothing seems particularly blatant to me... if anything, i'd think it'd be on the looser side of the spectrum

I would check to make sure nothing in your suspension is binding which can definitely happen over time. Thats more along the lines of thinking you have "something" causing the push rather than the setup working.

I cant find any specific info on the tanabe bar but it seems like itll be 30-50% stiffer than stock which would possibly lead me to think you still need more rear bar. How much do you hand an inside rear mid corner? Just a tiny bit, none at all, or its hiked way up there. How low are you overall? Is your Ground to top of fender >26" (assuming 25.3" tire)