- First Name
- Austin
- Joined
- Sep 27, 2020
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- Houston, Texas
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- 2019 Subaru Crosstrek
- Your Bronco Model
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It's difficult to understand how it will feel to steer an equipped vehicles and what it feels like when it kicks in as I've not driven one before, only a Ford t-lok. When you say the steering is heavy do you know why that may be? If it isn't being engaged I don't understand why it would feel any different than an open. While driving with normal amount of traction the worm wheels and gears on each side do their own thing as I understand it, just like an open which is obviously useful for turning. Further, does heavy steering imply a less comfortable daily driving experience?The short answer for those that don't want to read everything: in the front, up to about 2.5:1 TBR. The rear, anywhere from maybe 2:1 to 4.5:1, depending on how the vehicle will really be used. More detail:
For background, on the most basic level, the term "torque bias ratio" is what we use to characterize a differential's performance level, ie "locking effect". Torque bias ratio relates how much friction the differential generates in order to provide resistance to wheel spin, and transmit torque between outputs. So, the higher the TBR, the "stiffer" or more aggressive the differential is tuned to be. For reference of scale, an open diff is roughly 1:1 TBR; a locker is infinite:1. So while we're definitely playing on the low end of the scale when dealing with even very aggressive LSDs, that scale is somewhat logarithmic.
That's important, because it has direct bearing what is most appropriate for a given application, based on how the vehicle is to be used. The right choice is, as all things in engineering, whatever compromise works best for your needs. For starters, its important to understand that the more aggressive the differential is, in terms of either TBR or preload, the more it will also resist allowing the vehicle rotate, change direction, turn, etc.
This is especially important if the driven axle in question is also a steering axle. The more aggressive the differential in a steering axle, not only will the vehicle be less inclined to change direction, but it will also be more difficult to steer - the steering will be heavier, awkward, possibly having a gyroscopic feeling. It can also accentuate torque steer. Anyone that has tried to steer a 4x4 with a locked front diff will attest to impact it can have. So, for that reason, I would set a practical cap of about 2.5:1 TBR on a front axle.
There are varying schools of thought, of course, and the design of the suspension and steering systems have bearing on this too. But in my experience, something around 2.5:1 is still benign enough to be liveable in a typical SLA front suspension. It will still have a slightly heavier steer feeling than an open, but you get used to that quickly. I ran one like that in my Mountaineer for years. But if the suspension is torque steer prone, something more like 1.7 or 1.8:1 would possibly be a better choice.
In the rear axle, TBR can be more flexible (bearing in mind the comments of the first paragraph). It really does depend on what you're doing. The higher the TBR is, the less you'll spin a tire. The more torque you'll be able to send to the high traction side. The more readily and effectively it will respond to brake modulation (or traction control intervention), should it come to that. But also the more quickly you'll be able to saturate the traction limits of both tires in quick succession. This later point is more applicable driving in bad weather, snow, etc. A high TBR diff will "snap" on the driver fast, with less warning, possibly leading lateral instability / tail slides / oversteer skids.
So for that reason, and this may seem counter-intuitive, but high TBR levels are more beneficial with higher traction surfaces. You can take better advantage of higher TBR when the overall traction limits are raised. For a daily driver in all weather conditions, the 2-2.5:1 is still a good pick. For more aggressive driving in sportier cars, closer to 3:1 seems to be a good starting point. For all-out track cars, 4:1 might be desirable, but it depends on the chassis set up and track you're driving. Higher TBR can work off-road as well, where it can be beneficial when there are wide splits in traction on varied surfaces (grass/dirt/mud/gravel/etc). I think in general off-road (non rock crawling) situations, 4ish:1 is probably the practical limit if combining with a daily driving duties. I'm talking on fire roads, trails across open fields, going to the beach, prerunning, or even rallying, etc. But you need to be aware if the TBR is high, it will come with the aforementioned challenges in winter use.
Of course, I say all that, but the V8 Mountaineer I mentioned above also had a 5:1 TBR, high preload diff in the rear end. Combined with the vLSD transfer case, the 2.5:1 front diff and studded snow tires, it was extremely capable in places I had no business being. But that also had direct bearing on the skill level needed to stay on top of what the vehicle is doing. I often come back to the question of "what would I want my mother to drive?". And that would be back to something like 2.5:1 TBR...
Without having driven a helical equipped car I'm also not sure how it would feel when it kicks in. I don't want a violent over correction while driving on the highway in the rain, but I'm pretty sure they wouldn't be popular if they weren't safe for a daily driver.
If one drove an "aggressive" TBR helical, could you describe what that may feel like / what happens to the vehicle? Like I said, I had a t-lok on my Mustang so that's about all I know.
I'm guessing a mellow TBR would just feel like it isn't working much as an LSD but please correct me if I'm wrong.
For a fair number of us on the site I'd say that an LSD instead of lockers would be more beneficial the more I learn about all of the differences. Less expensive, practical, a bit safer if designed appropriately. I'm surprised and disappointed Ford didn't offer an option for it. And yes I did read your post or maybe it was someone else's who talked about why they're being phased out and how lockers are more marketable. Still unfortunate. I am feeling more comfortable with going with a base and regear and install LSDs.
Is the reason why a helical won't lock when one tire is hanging up in the air is because zero times whatever the TBR is is still zero force applied to the other wheel? Pretty sure that's why but wanted to check.
For anyone interested, here are the helpful videos I've watched and articles I've read about helicals. EDIT: I can't find the articles in my history right now but when I'm at my computer I'll try to find them. Some are dry, well most are, but one has video of an SUV with differential comparisons on the same terrain. Things are clicking better for me so maybe it'll help y'all.
Rocketeer Rick, does this video seem accurate to you? Seeing it in action is selling me pretty well on the idea.
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