Overdrive/underdrive question

[triafgg]

Hi, I'm new at this so have a question.
I've read posts saying how good OD/UD is but £90 seems a lot so was thinking about tyre sizes. I have a few sets of 1.9 tyres and some are up to 6mm taller. One rotation is 12mm difference so would putting the smaller pair on the back effectively overdrive the front giving the same effect as the OD gears? As the axles remain constant but tyre circumference is different it must have some effect but I can't quite work out what is, if anything.

[Voodoobrew]

It will look silly but yes smaller rear tires Wil give you overdrive. Overdrive is usualy calculated by percentage though. So front diameter/rear diameter*100=% typically you want around 115-130% which people say as the front is 15-30% overdriven

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[triafgg]

Yes, the two I have only work out at 108% so to get the right percentage would need a much larger difference and as you say would look odd. Thanks for answering the question. One thing to take away I guess is if you were forced to mix different sizes on axles best to put the smaller ones on the back so that you weren't under driving the front at all which I guess is the worst scenario.

[ferp420]

i usaly dont like the over / underdrive setups on my scalers but my more comp style type crawler it seems to help one has 6" tires up front and 5.5"tires in the rear and it works great
on large flat smooth climbs like glass or large wet slabs of granit in the creek the od/ud set up has a tendancy to break traction on ether the front or rear sooner than if the front and rear run the same gearing

[Inspector86]

I sometimes use OD/UD gearing on my comp crawler, but in my experience OD/UD is not necessarily a good thing for a trail rig.

[Panther6834]

I know this is an old(er) thread...but, I'm still hopeful someone will be able to answer my question. I have a Cross RC HC4, and I have an EXTREMELY "unusual" idea for it. It involves running a rear tire that's larger than the front. In 'stock' form, the T-case doesn't provide any OD to the front, which would lead to the front tire "dragging". As such, I (obviously) need to replace the T-case with something different...something providing OD...so as to 'compensate'. I've also created a list of 19 possible rear tires, and 46 possible front tires, both lists of which contain tires with a variety of different diameters. However, in order to properly determine what percentage of front OD is needed, and which front diameter tires will work best with which rear diameter tires, I am in need of a minor piece of information.

From my research, I've only found three aftermarket T-cases that will work (without mods) in the HC4 chassis - the TGH Creeper (30% OD), TGH O.G. EBR Edition (15% OD), and RC4WD Over/Under (with selectable 27%, 33%, 42%, 57%, & 67% ODs). Obviously, depending on the front/rear tire combo, almost any of these ODs could work...but, at the same time, I do not want to use any OD higher than the 33% option (preferably, no higher than 30%). However, as I already mentioned, I'm missing "piece of the puzzle"...or, to be more specific, how to calculate the difference between front & rear tires. Although I will probably have to calculate using several OD percentages (to determine the best possible front/rear tire combos), for my example, I will use the 12% option. Obviously, it's not the tire diameters that matter, but their circumference. In order to calculate the front/rear variance, I need to know which 'direction' to calculate.

Using the 'example' 12%, would I need a front tire with a circumference that's 12% smaller/shorter than the rear tire's circumference? Or, do I need a rear tire with a circumference that's 12% larger/longer than the front tire's circumference? To some, these two sound as if they're the same thing, but the difference is like night & day. For example, a 25% reduction of 100 would be 75, but a 25% increase of 75 would NOT be 100 (it's 93.75, for those who might be wondering). Moving back to tires...and, still using that 12% example...let's say a rear tire with a 17.28in circumference (5.5in diameter) was used. If the "correct" way of calculating is that of a front tire with a circumference that's 12% smaller than the circumference of the rear tire, then a tire with a 15.2in circumference (4.84in diameter) would be needed. However, if the calculation needed to be the other way (ie. rear tire with a circumference that's 12% larger/longer than the front tire's circumference), and the rear tire was the same tire, then that would mean a front tire with a 15.43in circumference (4.91in diameter) would be needed.

My question is, *which* way of calculating is correct? Still using the 12% OD example, would the front tire need to have a circumference that's 12% larger/longer than the rear tire's circumference? Or, would the rear tire need to have a circumference that's 12% smaller/shorter than the front tire's circumference? Obviously, if the T-case's OD is different than 12%, the same difference would need to be used in calculating the correct circumfrences, and, thus, the correct diameters. Assistance in solving this will be GREATLY appreciated.

[mash914]

I think I remember reading somewhere that those numbers are sometimes an average of the two different ways of calculating that you describe. I think if it were me, I'd say you can calculate it either direction, just be consistent.
I'll use my enduro as an example. I'd get the actual gear ratios in those transmissions and determine the overall gear ratio to each axle, all the way from motor to 'wheel' (spur / pinion, transmission, transfer case, axle ratios).

In my Enduro's case (no separate transfer case) the:
Spur / pinion ratio is 4.83:1
Transmission ratio to the front axle is 2.6:1
Transmission Ratio to the rear axle is 2.9:1
Axle ratio is 3.75:1

Overall gear ratios work out to:
Front overall ratio is (4.833 X 2.6 X 3.75) = 47.1:1
Rear overall ratio is (4.833 X 2.9 X 3.75) = 52.6:1

As you said front is 10.4% faster than the rear
and the rear is 11.7% slower than the front

To pick the correct tires, you just need to make sure your ratios are starting from the same perspective. In my case to have a neutral drive, I'd need either:
Rear tires 11.7% larger than the front
-or-
Front tires 10.4% smaller than the rear

So if you choose let's say 106mm (class 1) front tires, you'd need rear tires that are (1.117 X 106mm) = ~118 mm rear tires
or if you pick rear tires first, those 118 mm rear tires would need 10.4% smaller tires....(0.896 X 120) = 106 mm front tires.


Sorry if my explanation is a bit confusing - when you're dealing with gear reductions, sometimes faster / slower, larger / less seem a bit counterintuitive

[Panther6834]

I get what you're saying...but, whereas you're vehicle already has some OD (through the axle gearing), mine has none...front-to-rear is 1:1 out-of-the-box. In order to do the smaller-front/larger-rear tires, I'll be adding OD (via an aftermarket T-case)...but, at the same time, 'what' percentage that OD will be is yet to be decided.

Between the 19 possible rear tires, 46 possible front tires, and 4 possible ODs (15%, 27%, 30%, & 33%), in order to determine the best possible front/rear tire combos, I have a LOT of calculations to do. Unfortunately, I can't just "go with the average of the two"...it's not even close to that simple. With the 15% OD possibility, the variances are already large enough that the wrong tire diameter combo could cause the front, or rear, to "drag". And, as the possible OD percentage increases, the variance increases, making the right combo even more important.

Yes, the vehicle will be built using a crawler chassis...and, yes, it's going to have solid axles & leaf spring suspension...but, this vehicle will not be going typical 'slow' crawler speeds (think "Mad Max"). That's why I NEED the rotational distances of the front & rear tires to be as close to equal as possible. I know the wording I just used is 'wrong ', so I'll try explaining another way

Let's say, work the vehicle going forward, traveling 3ft, the rear tire rotates two times (ie. a tire with an 18in circumference). Now, let's say the front had a 25% OD (yes, I know that's not one of the possibilities, but I'm just using that percentage for this example), which would mean that, in the 2 rotations of the rear tire, the front tire would have rotated 2.5 times to travel that same 3ft. For whatever diameter the front & rear tires are, the 2.5 rotations of the front tires need cover, as closely as possible, the same 3ft as the 2 rotations of the rear tires...but NOT less then 3ft.

The thing is, change to a different OD percentage, and the front and/or rear tire diameters need to be different. This is why accurately calculating the circumference differences between possible front & rear tires is so detrimental...if the calculation is off by more than a few percent...and, even worse, if it's off in the wrong direction (ie. it causes the rear tires to "cover more distance than the front")...it could cause damage to the drivetrain...especially in a vehicle that'll primarily be doing 10-20mph.

~ More peace, love, laughter, & kindness would make the world a MUCH better place

[Panther6834]

Good news...the answer came to me as I was trying to go to sleep. Because the front & rear axles are geared the same, the (short) answer is this:

Whatever the OD is, the same percentage increase is applied to the circumference of the rear tire.

Technically, this applies no matter where the OD is coming from, be it the T-case, front axle, portal gears (if portals are used), or any combination of these. Whatever the total-final OD is to the front, the circumference of the smaller front tires needs to have that circumference increased by the same amount for the rear tire.

Quick example: If the OD is 33%, and the front tire has a circumference of 12in, then, to maintain equal distance travel at the rear, the rear tire needs to have a circumference of 16in (12 times 1-1/3 equals 16). No complicated formulas, nothing involving 'rpms'...turns out, the "solution" was basic math.

~ More peace, love, laughter, & kindness would make the world a MUCH better place

[MOguy]

Originally Posted by triafgg

Hi, I'm new at this so have a question.
I've read posts saying how good OD/UD is but £90 seems a lot so was thinking about tyre sizes. I have a few sets of 1.9 tyres and some are up to 6mm taller. One rotation is 12mm difference so would putting the smaller pair on the back effectively overdrive the front giving the same effect as the OD gears? As the axles remain constant but tyre circumference is different it must have some effect but I can't quite work out what is, if anything.

Your over driven tires should make more rotations then your underdriven covering the same distance. Smaller tires will have to spin more to cover the same distance compared to larger ones. I would think the smaller tires spinning faster would be what is overdriven.

[MOguy]

Originally Posted by Panther6834

Good news...the answer came to me as I was trying to go to sleep. Because the front & rear axles are geared the same, the (short) answer is this:

Whatever the OD is, the same percentage increase is applied to the circumference of the rear tire.

Technically, this applies no matter where the OD is coming from, be it the T-case, front axle, portal gears (if portals are used), or any combination of these. Whatever the total-final OD is to the front, the circumference of the smaller front tires needs to have that circumference increased by the same amount for the rear tire.

Quick example: If the OD is 33%, and the front tire has a circumference of 12in, then, to maintain equal distance travel at the rear, the rear tire needs to have a circumference of 16in (12 times 1-1/3 equals 16). No complicated formulas, nothing involving 'rpms'...turns out, the "solution" was basic math.

~ More peace, love, laughter, & kindness would make the world a MUCH better place

that is how I see it.

[Panther6834]

Originally Posted by MOguy

that is how I see it.

One thing I should have...but, forgot to...mention is that the foams used have to also be taken into consideration. Some foams, such as 'oversized' foams can increase the diameter by as much as 0.5", thus increasing the circumference by an even greater amount. On the other hand, some foams are as much as 0.5" (or, in a few cases, up to 0.75") smaller in diameter, and, depending on the running weight of the vehicle, can decrease the diameter (and circumference) by more then a reasonable amount.

For vehicles that will be going faster (or much faster) than 'crawling', this shouldn't pose too much of a problem, as those aware of this will buy foams that are as close as possible to the ID of the tire, or (if purchasing larger diameter foams) they will cut the foams down to match the ID. However, those doing actual crawling would have to carefully consider the "stuffed" diameters & circumferences of the front & rear, as THAT'S the circumference that would truly matter.

~ More peace, love, laughter, & kindness would make the world a MUCH better place