Overdrive/Underdrive turning radius measurement

[CRAWLER75]

Hey guys. I got OD/UD gears today and was able to install and do a simple test in the backyard. I haven't done any extensive testing yet. This was my observation while driving around with measuring tape. And I'd like to share the data with those who might be interested. The main reason I got this gear set was to improve turning radius. It's already good, but I wanted to make it even better. I was very curious about how it would work conjunction with unlockable differentials. So this is what I've found so far.
I made full circle on flat, grippy concrete and measured the diameter.

Test truck is mostly stock + savox 1210SG servo


W/ C1 Hyrax tire turning radius in inches (left turn/right turn)
------------------Stock gears--------OD*UD--------Gain
Unlocked----------53"/52"----------50"/49"------(-5.7%)
Locked------------66"/61"----------62"/59"-------(-4.7%)

W/ C2 Hyrax
------------Stock gears---OD*Stock ---OD*UD------Gain
Unlocked----56"/53"-------55"/53"-----52"/50"-----(-6.4%)
Locked------75"/67"-------71"/65"-----64"/60"-----(-12.7%)


Couple of notes
*C1 Hyrax was mounted on Gmade SR02
*C2 Hyrax was mounted on ebay rim with some negative offset and the truck was standing about an inch wider than C1 setup
*for whatever reason, the test truck turned sharper to the right
*Most noticeable improvement was when running the class 2 tires with diffs locked.
*[Front Diff lock only] resulted halfway between locked and unlocked..no surprise there
*This data may not be meaningful on the trail

So far, I find that unlocking diff is more useful than installing OD/UD. However there are other benefits from OD/UD and I will be doing more field testing in the coming weeks.

[Rockatanski]

Thanks for sharing your experience!

[wrightcs77]

Thanks for testing. I will be getting a set of these soon.

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[meatmonkey]

I'd been wondering this for months! Thanks for taking the time to actually measure it. It would figure the larger the tire the larger the difference due to the change in rollout. I'm kind of surprised that you had to run both the od and ud to see a meaningful benefit. I woulda thought that even a mere 5-7% of ratio difference would have done the trick.

How was the install of said HD underdrive and overdrive gears? Did you feel the need for any shimming?

[CRAWLER75]

Originally Posted by meatmonkey

I'd been wondering this for months! Thanks for taking the time to actually measure it. It would figure the larger the tire the larger the difference due to the change in rollout. I'm kind of surprised that you had to run both the od and ud to see a meaningful benefit. I woulda thought that even a mere 5-7% of ratio difference would have done the trick.

How was the install of said HD underdrive and overdrive gears? Did you feel the need for any shimming?

Since I had no previous od/ud experience, I was very excited to test it out. As it turned out, it wasn't as dramatic difference as I hoped it would be. As small as it was, there was tangible difference. I am going to comp tomorrow so it will be tested.

As far as installation goes, it was pretty straight work. I replaced axle housing with metal one while I was at it, so cable needed little adjustment. I haven't checked if shims were needed. But there wasn't any binding or excessive play. It took about an hour for the front and 30 min for the rear.

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[CRAWLER75]

Update:
So I’ve been running this setup for a few months in multiple events now. Steering advantage with open diff and od/ud drive has been plenty evident. It impressed many judges with sharp turning. You need a strong servo to go with obviously.
I couldn’t tell if climbing was improved as it was already pretty competitive to begin with.
It does drag slightly driving straight line on high traction surfaces. No noticeable tire wear yet.
Basically it makes the truck more optimized for turning than going straight.
If you are interested in comp and you want to have an edge over competition, I highly recommend doing this mod. If you like trailing, you won’t need this.


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[o0wiz0o]

Great information. Many thanks.

[NKlamerus]

I think this is the info I was looking for, someone running an open front differential AND overdrive up front.

[Inspector86]

That is in line with what I would expect. I suspect if you had rear dig it would turn even tighter.

[MOguy]

add a little caster, Rotate the bottom of the front axle out. Even a little will help.

[MOguy]

Originally Posted by CRAWLER75

Update:
So I’ve been running this setup for a few months in multiple events now. Steering advantage with open diff and od/ud drive has been plenty evident. It impressed many judges with sharp turning. You need a strong servo to go with obviously.
I couldn’t tell if climbing was improved as it was already pretty competitive to begin with.
It does drag slightly driving straight line on high traction surfaces. No noticeable tire wear yet.
Basically it makes the truck more optimized for turning than going straight.
If you are interested in comp and you want to have an edge over competition, I highly recommend doing this mod. If you like trailing, you won’t need this.


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You can open the rear and keep the front locked and if you can get in a position where one rear tire has less traction you can almost dig.

If you get a controller setup where you can also unlock you front end only your sidle hill will improve by allot.