Mechanics of a half track with driven front axle

[Rudukai13]

Alright, so this is another project idea that's a little further down on my list, but I've been thinking over the mechanics of what I want to end up with, and originally I wanted to keep it under wraps but I'd like to get some thoughts and opinions from people who have a little more experience with the components I want to use. Here's my basic idea;

- Start with an Axial-based rig (Wraith, SCX, Yeti, etc.)

- Utilize Axial's overdrive and underdrive gear sets, installing the overdrive gears in the REAR axle and the underdrive gears in the FRONT axle

- Install RC4WD's Predator EVO track conversion on the rear axle

In my head, the use of the overdrive gears in the rear axle SHOULD offset the effective gearing-down of the axle when the track conversion is installed. Additionally using the underdrive gears in the front axle will slow down the large front tire to the point of not just dragging the rear track along quite as much. There would still be an effective slight underdrive to the rear axle, which should help with turning and climbing - and when the front tires lose traction on a hill, the large tracks in the rear can push the truck over whatever obstacle is being climbed. I'm also thinking using specifically the EVO track conversion would be better suited for this since the drive wheel is slightly larger than the standard Predator tracks, so there won't be as much of a difference to overcome using the axle gearing.

My question is - Will the combination of the over/underdrive gears be enough to offset the smaller drive wheel of the track conversion? Or would the track still just drag behind the larger front tires too much?

Any thoughts/opinions/questions are welcomed, just please try to be constructive. Thank you in advance!

[Rudukai13]

By the way I'd like to build this rig to compete in a TTC event, if I can find a TTC series that would allow a half track vehicle to be entered.

If I can't get the gearing between the front and rear axle to work properly, then there's always the option of removing the front driveshaft and making it a strictly rear-track-driven vehicle.

[hotwheels000]

I think you need to look at the roll out of the tracks vs tires to determine weather you need over or unders in either end, not the driven cog size. I might be wrong but in my head if it was the driven cog size you would have to compare that to the hex size driving the tire....

[Rudukai13]

You're mistaking the track as a sort of deformed tire rather than what it actually is - looped, moving ground. Remove the track and what do you see? The drive cog on the track is the component that spins and actually creates forward movement. You can think of that drive wheel as the actual wheel that's in contact with the "ground". Therefore attaching the tracks to an otherwise wheel-driven truck is effectively like attaching smaller tires to it. The issue is that when you leave the large crawling tire on the front, you've created an offset in the size ratio between the front tire and the rear "wheel". That offset means either the front tire will be force to drag the smaller wheel around, or the smaller wheel will dig in and just make the front tire spin and claw at the ground in attempts to move faster. Utilizing the over/underdrive gears should in theory mitigate some of the difference in that ratio between the front and rear, and would hopefully close the ratio enough that it wouldn't just cause problems and would actually be a benefit to the vehicle.

[nharkey85]

^^ this. No matter the length of the track, it will always progress the same amount when utilizing the same drive cog.

BUT....you will want to figure the rollout of the front tire AND how much of the track is moved by one rotation of the drive cog. That would technically be the tracks rollout.

I would imagine underdrive front overdrive rear and hope it enough unless there are different drive cogs available.


All that being said, IMO i domt think such a small track system will look right. "Halftrack" i see upwards of half the vehicle length tracked with a typical steer axle upfront

[Rudukai13]

Hey thanks for your input! I actually talked with Turtle about this a bit and he gave me a great idea for how to make this simpler, more scale and even more capable. It doesn't utilize the Predator tracks, and it negates the issue of gearing.

He suggested starting with a 6x6 then fabricating some over-the-tire tracks for the rear axles, similar to tracks for skid steer construction vehicles. Seems like a good way to go about it to me, and it'll have a better scale appearance and more performance than the Predator tracks!

[nharkey85]

That would look the part much better

[jebster]

just saw this thread and wanted to join in. a future project i'm planning to take on is a half track unimog. my plan was to use scx10 axles, 1.9 wheels on the front axle and predator tracks on the rear axle.

I haven't done the math yet but i was going to address the difference in effective wheel size (the 1.9 tire in the front and the drive cog of the predator tracks) using a transfer case. i was going to build a transfer case (I plan to have a front mounted motor/tranny) that i can use different pinions for the front and rear axles to get a an exact match. i already have a predator tracked truggy and know that if the speed between the two axles is not exactly the same either the front or rear tracks will get drug around (that is out i found out one of my axles had overdrive!), which is not good. if i'm going to match a regular tire to the tracks it will need very precise wheel speed matching.

the transfer case i have in mind would have a moveable plate so that i can adjust the pinion ratios. that will allow for fine tuning and eventually a way to adjust if i change tire sizes.


for the smart gearing/rpm/wheel speed people out there is the ratio of the pinion sizes i would need in the transfer case simply the ratio of the tire/drive cog sizes? for example, if the tire is 4.1" and the drive cog is 2", that is a ratio of 4.1/2 = 2.05. Using a 35 and 17 pinion in the transfer case would get me 35/17 = 2.06, which would be pretty close.

p.s. to get the pinions closer in size (i would like to get the pinions as close to a 1/1 ratio as possible) i could run under/over in the front/rear axles. not sure how to work that into the math yet myself as i'm just started to study up on this subject.

[jebster]

a quick followup on the over/under in the axles.

if the front as underdrive (43/13) and a 4.1" tire and the rear has overdrive (36/14) with the 2" drive cog does the math work like the following:

tire to drive cog ratio = 2.05
over to under drive ratio = (36/14) / (43/13) = 2.57 / 3.31 = 0.776

total delta = (tire to drive cog ratio) x (over to under drive ratio)
= 2.05 x 0.776
= 1.591

if that is correct I could get something like a 24 / 15 pinion ratio (1.6) or 19 / 12 (1.583).