Tire size & OD 'quandary'

[Panther6834]

Calling all RC gearing & tire "math gurus"...you assistance is requested/needed.

PROBLEM: "Percentage smaller of front tire compared to rear tire"...or, "Percentage larger of rear tire compared to front tire" ???

I know that some reading the above statement are going to want to say, "It's the same thing"...but, you'd be incorrect. Yes, they *sound* like they're the same, just worded differently...but, in truth, they are COMPLETELY different. For example, you could say that 25% less then 100 is 75, and you'd be correct...but, 25% more than 75 is NOT 100 (it's 93.75, for those who might be wondering). So...what does this have to do with anything?

I have an 'interesting' project in mind, to be built using a crawler chassis, and solid axles (already purchased)...but, the 'plan' is to run larger tires in the rear. For example, the Arrma Felony (a vehicle I'm sure many reading this are familiar with) uses 100mm diameter tires in the front, and 107mm diameter tires in the rear...however, because of the ring & pinion OD to the front (Front 42T/15T, Rear 45T/14T), neither tire is "dragging" during vehicle movement. I don't know that the front OD percentages is...but, to put it another way, as OD is, the rotational speed on the front is just slightly faster than the rear, so as to 'compensate' for distance traveled during vehicle movement, due to the difference in front vs rear tire diameter.

I want to do something similar...except that I'll be using a crawler chassis & solid axles. The 'dilemma' I have is how to "refer" to the percentage difference, so as to help me determine what diameter tires to get. In other words, I need to know which "phrasing" is correct. As mentioned near the beginning, "percent smaller" is not the same as "percent larger"...but, I don't know *which* I need to be using in order to determine the proper tire sizes.

This is where I really need help from those more intelligent than I am in this area. I completely understand that, for this calculation, it's not the tires diameters that matter, but tire circumferences, as THAT'S where the percentage difference is important.

Here's what this all boils down to. Let's say, for example, the gearing I used gave the front a 12% OD. In regards to tire circumferences, would I need a front tire that has a circumference that's 12% shorter then the rear tire's circumference? Or, would I need a rear tire that has a circumference that's 12% longer then the front tire's circumference? One of these two is correct...but, I don't know *which* one.

To all the RC "math gurus", which is 'correct'?

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

 

[JDM74]

The right answer is do not use percentage. Do the math and calculate the right answer then see what is avaiable.

Calculate the front axle rpm for a given input rpm and then do the same for the rear. Pick one tire size and solve for its speed over ground at whichever axle you want to mount it on, then solve for tire diameter on the other axle based on its rpm and the same ground speed as the first tire creates.

Then go look for tires close to that diameter.

 

[Panther6834]

Re: Tire size & OD 'quandary'

Thank you for your info...but, in all honesty, none of it makes any sense to me. What does make sense (to me...and, I'm sure, to at least some others) is how I originally phrased my question(s).

Truth be told, I'd say there's a very high probably that the information I'm asking for, and the end-results of what you described, would equate to the same thing...just "described" in two completely different ways...except for one 'problem' in what you said - it had nothing to do with "speed over ground" (what you stated), but is about "distance over ground".

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

 

[OSRC]

Re: Tire size & OD 'quandary'

Thank you for your info...but, in all honesty, none of it makes any sense to me. What does make sense (to me...and, I'm sure, to at least some others) is how I originally phrased my question(s).

Truth be told, I'd say there's a very high probably that the information I'm asking for, and the end-results of what you described, would equate to the same thing...just "described" in two completely different ways...except for one 'problem' in what you said - it had nothing to do with "speed over ground" (what you stated), but is about "distance over ground".

Correct, speed and distance are completely different....though I've read your post several times and I'm not 100% what you are asking... with all due respect, all the unneeded "", '', (), etc. in your posts make them difficult to read. At least for me they do.

Speed of the tire is irrelevant, that's a variable based on current motor RPM. You're talking strictly about circumference of the tire which will be the distance covered by the tire for a given revolution of the motor. The circumference of the tire and the gearing is all that matters. JDM74 is spot on with doing the math (except for conflating speed in there.) I'm not sure what else you need.

Simple example: for every one revolution of the motor, if you have a 100mm tire, 2.5:1 front gear and a 10:1 trans you get 24.56mm of distance. (PI*100/2.5/10) - plug in your numbers for the rear gear and calculate what you need. I assume that part is understood.

Whether its shorter front or taller rear - both are right. Refer to it however you like. Just like with overdrive - front overdrive or rear underdrive are the same from a math standpoint - it's just in how you calculate it and what your base point is....which brings us back to the straight math. I would not use percentages... "if my gearing is 12% shorter, I need a tire 12% larger" - That's a shortcut that doesn't always translate perfectly. Raw math is your best bet.

 

[JDM74]

Re: Tire size & OD 'quandary'

Thanks for pointing out the speed error, I was thinking revolution but my mind went right to RPM for some reason. I guess I am so used to doing speed error calcs for different tire sizes and gears in 1:1 trucks. lol.

I'll offer this info, % overdrive or under drive is USUALLY in reference to whatever "stock" gearing is.

 

[DRED805]

I use this open in two tabs all the time to figure out front and rear final drive ratio separately. You gotta know your transmission and all other gearing values, but it lets you put in your tire size, too.

https://scriptasylum.com/rc_speed/top_speed.html

EDIT: For actual overdrive, use the two theoretical top speeds as the values to put in this, so tire size is taken into account...
https://www.calculatorsoup.com/calculators/algebra/percent-difference-calculator.php

 

[Panther6834]

Re: Tire size & OD 'quandary'

Whether its shorter front or taller rear - both are right. Refer to it however you like. Just like with overdrive - front overdrive or rear underdrive are the same from a math standpoint - it's just in how you calculate it and what your base point is....which brings us back to the straight math. I would not use percentages... "if my gearing is 12% shorter, I need a tire 12% larger" - That's a shortcut that doesn't always translate perfectly. Raw math is your best bet.

First, my apologies for how long this post is. Second, I'm hoping, in rewording a few things, other's "confusion" will be lessened...or, better yet, completely removed.

Technically, yes, both are 'right'...but, the formulas used would need to be different, because (as already pointed out) the "front to rear difference" is different than the "rear to front difference". Also, I agree that "raw math" is my "best bet"...in actuality, it's my 'only'. However, there are different formulas that can be used to achieve the answer. I completely understand that a formula based on rpm will work...and, for those who understand the workings of that type of formula, go for it. I understand distance-based formulas, so that's what I prefer to use.

The only "piece of the puzzle" I'm missing is whether the formula needs to be based on how much smaller the front tire needs to be, compared to the rear...or how much larger the rear tire needs to be, compared to the front. Thus is the reason why I asked what I asked - whether going by "percentage smaller of the front tire, compared to the rear" was the correct way to calculate, or whether "percentage larger of the rear tire, compared to the front" was correct. To some, they may appear to be the same...but, they are NOT the same thing.

As for the 12% I originally stated, they was just used as an an "example". The truth is, I don't yet know what OD percentage I'll be using, or need. What I DO know is that the front & rear axle's have equal (1:1) gearing. Unfortunately, I cannot just replace the R&P gears in the front or rear axle, as the gears are 'proprietary' (Cross RC uses a 'unique' design). As such, the OD will have to be done through an aftermarket T-case (I won't be using the kit T-case, as it has a 1:1 front/rear ratio). Based on what's available (TGH & RC4WD), I have OD options of 12%, 27%, & 30%. Which I decide to go with will need to be based on which front/rear tire combos work best...and, to determine that, I need the answer to my original question(s).

Briefly getting back to the tires, I've narrowed down the list of possibilities to 19 rear (2.2) tires, and 46 front (1.9) tires. In order to calculate which are the 'best' possibilities...and, more accurately, which combinations will work...I'll need to factor the different circumferences, along with the different OD possibilities. But, in order to do that, that's where I need to know which - front or rear - is the "governing factor" (my choice of words).

In the hopes of listening your confusion, I'm going to reword my original question(s)...and, in doing so, it doesn't matter 'what', specifically, the OD percentage is to be. Which of these two is the 'correct' way to determine needed tire diameters, when based on a known front OD:

1) The circumference percent smaller of the front tire, compared to the rear tire, or
2) The circumference percent larger of the rear tire, compared to the front tire.

Let's, for example, say I went with the 12% OD T-case...what I'm asking is this. Does the tread (ie. circumference) of the front tire need to cover a distance 12% greater than the rear tire? Or, does the tread (ie. circumference) of the rear tire need to cover a distance 12% less than the front tire? (These two are NOT the same). If the first question is the correct way of calculating, then that's the same as saying that the rear tire needs to have a circumference 12% greater than the front tire. However, if the second question is the correct way of calculating, then that's the same as saying that the front tire needs to have a circumference 12% less than the rear tire.

Hopefully we rewording things this way will make more sense to both of you (and others who might be able to provide the information). For almost all 'problems', there are multiple ways of coming up with a/the 'solution'. Each individual will attempt to solve the problem they're own way...a way (ie. formula) that they understand. As long as the 'answer' is found, there's no "wrong" way of finding the 'answer'. I'm just trying to get the "key" to finding the 'answer' is a way that I'm understand. I'm not saying that my way is better than yours, anymore then tires is better than mine. Both will achieve the same results in the end. The only 'difference' is that you understand your way better than I do, and I understand my way better than you do...but, again, it ends, both would produce the same 'answer'.

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

 

[Panther6834]

Re: Tire size & OD 'quandary'

Thanks for pointing out the speed error, I was thinking revolution but my mind went right to RPM for some reason. I guess I am so used to doing speed error calcs for different tire sizes and gears in 1:1 trucks. lol.



I'll offer this info, % overdrive or under drive is USUALLY in reference to whatever "stock" gearing is.

If you ready my most recent reply to DRED805, you'll see that you ARE correct (with your rpm-based formula). But, at the same time, the formula I intend on using is ALSO correct. Put another way, two completely different formulas that, in the end, would achieve the same result. It's just that you understand your method, which I don't...and, I understand my method, which (I'm guessing) you don't.

No big deal...just "different paths to the same destination" (you take the high road, and I'll take the low road...you know the words...lol).

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

 

[Panther6834]

Re: Tire size & OD 'quandary'

I use this open in two tabs all the time to figure out front and rear final drive ratio separately. You gotta know your transmission and all other gearing values, but it lets you put in your tire size, too.

https://scriptasylum.com/rc_speed/top_speed.html

EDIT: For actual overdrive, use the two theoretical top speeds as the values to put in this, so tire size is taken into account...
https://www.calculatorsoup.com/calculators/algebra/percent-difference-calculator.php

Thank you...I'm checking this/these out right now. Will let you know whether, or not, it solves my 'problem'.

EXIT: I checked out the first link, but it's of no help to me. The "missing link"...which isn't listed, is anything related to possible front OD. Plus, this calculator is based on the same tire size front & rear. The second link just left me 'confused', as I didn't see anything related.

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

 

[DRED805]

Re: Tire size & OD 'quandary'

Thank you...I'm checking this/these out right now. Will let you know whether, or not, it solves my 'problem'.

EXIT: I checked out the first link, but it's of no help to me. The "missing link"...which isn't listed, is anything related to possible front OD. Plus, this calculator is based on the same tire size front & rear. The second link just left me 'confused', as I didn't see anything related.

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

I said open it in two tabs. One for front, one for rear. You can put any gearing or tire size differences there may be. Just make sure you're putting the same motor kv and voltage in for both tabs. To find what you're looking for you need to use the top speed values from each tab in the equation I linked... that way the tire sizes are taken into account.

 

[Panther6834]

Re: Tire size & OD 'quandary'

I said open it in two tabs. One for front, one for rear. You can put any gearing or tire size differences there may be. Just make sure you're putting the same motor kv and voltage in for both tabs. To find what you're looking for you need to use the top speed values from each tab in the equation I linked... that way the tire sizes are taken into account.

That would be great...if I knew the gear ratios of the T-cases. But, I don't. The front & rear axle's have identical gearing. While I know the possible ODs (15%, 27%, 30%, 33%), depending on which T-case I get, I have no idea what any of their front/rear ratios are.

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

 

[Panther6834]

Re: Tire size & OD 'quandary'

Here’s some additional info, which might help. And, in doing so, I’ll be including ‘sample’ calculated tire sizes using both formulas.

SIDE NOTE - 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 33% (preferably, no higher than 30%).

To continue where I previously left off, I will stick with the 12% example I originally mentioned:

Let's say a 5.5in diameter (17.28in circumference) is used for the rear. If the "correct" way of calculating front tire diameter 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 is the other way around (ie. rear tire with a circumference that's 12% larger than the front tire's circumference), then that would mean a front tire with a 15.43in circumference (4.91in diameter) would be needed. The question is, which would be the ‘correct’ front tire? The tire with the 4.84in diameter? Or, the tire with the 4.91in diameter?

Does my question make more sense now? And, assuming it does, that brings us back to which of these two ways of calculating it is ‘correct’?

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

 

[ScaleLifeNewbie]

Why not get the tires you want to use, put them on the truck and figure out the gearing after instead of doing all this theoretical math? Wouldn’t it be easier to figure out a solution to a problem right in front of you instead of a problem you don’t have in person yet? Then with it in front of you, you can play around with gearing, tire sizes, etc. To me that makes more sense. I think there’s just too many “what ifs” going on.

Just a thought I had, feel free to bash me on how wrong I am lol.

 

[Panther6834]

Why not get the tires you want to use, put them on the truck and figure out the gearing after instead of doing all this theoretical math? Wouldn’t it be easier to figure out a solution to a problem right in front of you instead of a problem you don’t have in person yet? Then with it in front of you, you can play around with gearing, tire sizes, etc. To me that makes more sense. I think there’s just too many “what ifs” going on.

Just a thought I had, feel free to bash me on how wrong I am lol.

Not going to tell you, "you're wrong"...no bashing. And, yes, I know you & I have "butted heads" a few times in the past. But, this time, it's not a matter of one of us being 'right', and the other being 'wrong'...in this case, in a matter of speaking, we're both right, but in 'different' ways If the axles being used could have their gearing changed (like most RC axles), then you'd, most definitely, be 100% correct. [emoji1360]

Unfortunately...and, as confirmed by one of my LHS's "crawler gurus", the gearing of many Cross RC vehicles is specific to Cross RC vehicles, and the axles can't use ring & pinions from other companies. Part of this has to do with the steep angle of the pinions, but there are also other 'proprietary' differences (such as the position of the pinion to the ring). As I have a very chaotic work schedule the next few days, I don't be able to do this until at least Friday...but, I will post a photo showing a 'typical' axle, and a Cross RC axle, do that those reading this thread will finally have a visual comparison, and will start to understand how "different" the Cross RC axles can be.

Adding "insult to injury", Cross RC doesn't make optional gear sets with different tooth counts...users are, essentially, 'stuck' with the axle gearing, and the only way to add OD is to replace the T-case (be it a stand-alone T-case, or a 3rd-party transmission w/ built-in OD T-case) with one having front OD.

--------------------

On the 'plus' side, the solution to my problem came to me as I was trying to get to sleep last night...and, interestingly enough, turns out, no complicated mathematical formula is needed. No calculations involving RPMs. Just plain, simple, basic math.

"Short version" of the solution involved these numbers: 12, 16, & 33. Confused? I'll explain...but, first, a 'word' from our 'sponsor' - the number 33. As most are aware, the number 33 it's used synonymously with 1/3. Technically, 33% & 1/3 aren't truly 'equal'...but, they are "closer enough for government work", just as 32-pitch & Mod .6 (32-pitch & Mod .6 are not the same, but they work well-enough together that there aren't really any problems).

With that said we now the "long version". I'm one of those people whose brain can't easily let things go. If I'm trying to find a solution to a problem, I tend to "keep going", until I either find a 'solution', or until I am 100% certain that a solution is beyond my ability. Last night, as I lay there, trying to go to sleep, my brain refused to shut down - one way, or another, it was going to have its answer...lol

One thing I don't understand is "why" I was 'stuck' on using 12% OD for my previous examples, when 33% turned out to be so much easier. As mentioned above, 33% (essentially) equates to 1/3. As such, with a 33% OD, for every revolution of a rear tire, the front tire rotates 1-1/3 turns. So, where does 12 & 16 play into all of this? I'm glad somebody asked.

Starting with the front tire, let's say it has a circumference of 12in. With 33% OD, front will rotate 1-1/3 times in for every rotation of the rear tire. As such, if the rear tire has a circumference of 16in, it would cover the same distance as the front tire's 1-1/3 rotations.

This answers my original question of 'which'...thus, the 'correct' answer is, "the circumference of the rear tire, compared to the front tire, needs to be equal in percentage to the front OD. If the calculation were used in the opposite direction (based on a rear tire with a 16in circumference), then you'd end up with a front tire with a 10-2/3in circumference, which works cause a bit of (unwanted) be OD in the front.

Next comes the easy...albeit, time-consuming...step - for all 46 possible front tires, calculating the "matching" rear tire circumferences for 15%, 27%, to30%, & 33% OD possibilities (obviously, I won't be doing calculations for each of the 46 possible front tires, as many of them have the same diameter as others). Once that chart is done, looking over all possible fronts & be rear tires, I will eliminate all tires that don't have "matching possibilities", and see what remains. Then, and only then, will I be able to start narrowing down choices based on tires that I like the 'looks' of

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

 

[OSRC]

EDIT: Forget it - basic math is a waste of time. Use whatever formula you think will work.

 

[Nova's Ark]

Another measurement to consider is how tall the finished height of the tire will be, once the RTR weight of your vehicle is applied. Depending on your intended usage, you may have soft dual stage tire foam inserts that are for rock crawling and are much softer, resulting in a large tire contact patch that will shorten the height and roll out of the tire. Or if you install a firm single stage foam that supports the entire tire, now the tire will be at full height at all times with a smaller contact patch.

 

[Panther6834]

Just plug in numbers and go.

Gotta love math...lol

Seriously, tho...sounds right, but I have to sorta-kinda work it from the other direction. This is partly because I don't yet know what OD I'll use, and partly because I'm working from a list of already-selected tires (19 rear possibilities, and 46 front possibilities). I realize I have a LOT of 'number crunching' to do, none of which I'll probably be able to get to for at least another week (my work schedule, as of right now, has me working every day through 7/29)...and, in doing all those calculations, I hope to determine every "workable" front/rear tire combination.

Another measurement to consider is how tall the finished height of the tire will be, once the RTR weight of your vehicle is applied. Depending on your intended usage, you may have soft dual stage tire foam inserts that are for rock crawling and are much softer, resulting in a large tire contact patch that will shorten the height and roll out of the tire. Or if you install a firm single stage foam that supports the entire tire, now the tire will be at full height at all times with a smaller contact patch.

I'm not going to say the foams are "not important", because I'm know they are...but, at the same time, they are the least important part of the equation right now. Whether soft or hard, whether single or dual stage, they can always be trimmed down (if need be) to achieve what I want/need. The most important part of the "what tire sizes" aspect is determining what OD to go with, and which tire diameters will achieve what I'm wanting to do.

I've never before attempted a vehicle running different diameter front-rear tires...and, maybe I'm a lil 'crazy' for even considering the idea...but, it is what it is, and (one way, or another) I will complete this crazy project. In the end, hopefully, Mel (Gibson, not Brooks) will be 'proud'...not that he'll ever know about it.

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

 

[svt923]

The most important part of the "what tire sizes" aspect is determining what OD to go with, and which tire diameters will achieve what I'm wanting to do.

What I believe Mr. CI is trying to say is the foam can have a significant impact on the actual diameter of the tire once mounted thus throwing any calculations off. Something like a Rock Beast XL is listed as a 4.75 inch tire but actually ends up at 5 inch when mounted with the stock foam. Likewise if you are running a soft foam, the diameter is going to end up smaller due to compression under the weight of the vehicle.

Example: These tires have the same dimensional specs

 

[Panther6834]

What I believe Mr. CI is trying to say is the foam can have a significant impact on the actual diameter of the tire once mounted thus throwing any calculations off.

I knew that. In most (albeit, not all), the tire diameter increases by approx 0.5". Smaller diameter tires, obviously, increase less (probably around 0.25-0.35"...right?). Also, the diameter increases can vary, depending on how narrow/wide the wheel is - wider wheel, tire increases diameter less; narrower wheel, tire diameter increases more. - but, on a truck this heavy (the parts, alone, are almost 15lbs), once electronics are in place, any "you're diameter increase" would probably be nullified.

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

 

[Panther6834]

I have to admit, this is quite the "journey". Or I continue, I don't want anyone telling me, "You're doing it wrong", You're going about it the wrong way", "There's an easier/faster way", or anything of the sorts. I understand, and realize, that there are different 'ways' of going about this. Some 'ways' work better for some, while other 'ways' work better or others...and, in the end, the results (at least, mathematically...actual/final tires choices are a "personal choice") would be the same. For example, some people love driving, while others do not. For those (like myself) who love driving, working as a Professional Chauffeur is perfect...and, for those who hate driving, maybe an office job is more suited to your liking.

Anyway, after spending a few hours calculating the circumferences of the 46 front, and 19 rear, tire 'candidates', followed by even more hours calculating rear tire circumferences (based on the 4 possible ODs), followed by eliminating possible front & rear 'candidates' that didn't have any corresponding "matches", I ended up with a total of 295 possible front/rear tire combinations, consisting of 243 combos that would almost certainly work, and 52 combos that might (depending of the foams used) work. It's kind of 'interesting', in a 'funny' way, that, of the 19 possible rear tires, several (those with a 5.5" diameter) have as many as 19 possible front tire "companions", while a couple of other tires (the two work the larger diameters) each had but a single match.

The next step will be to stay eliminating tires from 'consideration', primarily based on my own personal preferences. Obviously, I selected the original 65 tires based on those I liked...but, as I'm sure others would agree, a person might not like the visual aspect of version front/ready combos. Meowing down the choices could take weeks...and, as I don't plan to stay this build for at least another 1-2 months, I've got plenty of time to make your decisions.

To those who shared input (in this thread, was well as a very similar thread on a different forum)...'positive', as well as 'negative'...thank you. As with every custom project, it'll be an "interesting" journey. Once I've started the build, I will create a build thread...I've even already come up with a title: The Black Cat goes 'Mad' with a Cross RC HC4. Unless anyone else had any additional input, I don't think there's any real need for me to post anything further in this thread. Again, "thank you" to everyone who's commented and/or sent me PMs on this matter.

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