GRC12 Pro Build

[OGmicromonster]

Well I decided to build a new truck for myself using PTI axles. I wanted to build a rig with a lower CG and a three-gear transmission. And I also decided to post my build here so I can get some input and suggestions on how I can make it a serious competition worthy rig. I'm convinced the PTI axles can hold their own so that's why I'm leaving them as is... for now. This isn't to say the current PTI truck isn't good – it is. I just think it could be better. Anyway... on with the build and the photos.

The first thing I did was select my center transmission. I decided to go with a Team Associated "Stealth" transmission I had left over from another project (Nitro T4 conversion). I also picked out my powerplant... a Holmes Hobbies Crawlmaster brushless. I shaved the part of the transmission case that could rub on the can for clearance (not shown). I also have a Team Losi 90-tooth spur (LOSA3927) and slipper pads coming so I can get a lower ratio if needed. The stock transmission comes with an 87-tooth spur, the largest available from AE or Kimbrough.





I also knew I wanted to put the standard slider drive train. So I need to make up some adapters. My father in law drew up these adapters and had them made for his truck, so I got a set for mine as well.





I assembled the axles like normal using some of the new GRC 12" stretch kit lower link/shock mounts. I'm going to shave the bottoms of these axles down like I did on my other rig.




Now that my drive train is pretty much complete, I'll move on to the upper link mounts. Stay tuned!

[tomik]

Nice! That should bring the CG down a lot!
How about the chassis and how are you going to attach the tranny to the chassis?

Will definetly follow this thread

[jetboat]

what did you use to adapt your axle side drive shafts???? They look nice and clean.

[OGmicromonster]

Originally Posted by jetboat

what did you use to adapt your axle side drive shafts???? They look nice and clean.

They are custom made input shafts. They are exactly the same as the stock ones (on the inside), but instead of having the ball end for the CVD on the outside, they have a 5mm end with flat spots.

[K_B]

Have you thought about flipping the axles rather than shaving them? The thing I like about flippin' 'em is that it gives you a nice flat spot across the top for mounting a servo without the diff case interfering. It reverses the rotation, but all you gotta do then is swap a couple wires on the motor.

Also, do you know if PTI will be selling those lower shock mounts individually or will they only be availbe in the conversion kit?

[drc323]

It looking good og .What chassis are you going to use ?

[OGmicromonster]

Originally Posted by K_B

Have you thought about flipping the axles rather than shaving them? The thing I like about flippin' 'em is that it gives you a nice flat spot across the top for mounting a servo without the diff case interfering. It reverses the rotation, but all you gotta do then is swap a couple wires on the motor.

Also, do you know if PTI will be selling those lower shock mounts individually or will they only be availbe in the conversion kit?

I would rather shave them. It gives the bottom of the axle a much smoother area. Plus no matter which side you mount the servo on it's going to be about the same height. I'm going to mount the servo the same way I did on my first rig (see photo below). Which doesn't interfere with the diff case. That's my thought anyway. Plus I would rather have the majority of the screw heads on the top so they don't get ground down from rubbing the rocks.



The lower shock mounts will be sold separately as replacement parts I'm sure. I can't imagine that they wouldn't.

Originally Posted by drc323

It looking good og .What chassis are you going to use ?

I'm going to design my own chassis. I have a few ideas. The one I would really like to do would be too expensive for a one-off thing and would be pretty tough to pull off to. So I'll just design a simple chassis.

[tomik]

Yeah, I'd also be interested of those shock mounts.

[aggressivellc]

Looks good keep us posted!

[OGmicromonster]

::::UPDATE::::

Well I had a little bit of time today and did a minor mod. No big deal but I thought I'd share anyway.

I wanted to be able to reduce the final drive ratio further and at the same time move the motor further away from the transmission case. The stock Team Associated T4 transmission comes with an 87-tooth spur gear, the largest one available from Associated or Kimbrough. With this spur the lowest I can get my final ratio is 43.48:1 without the motor rubbing (using a primary ratio of 6.69:1 - 87 spur and 13 pinion).



So I decided to swap out the Associated spur gear and slipper pads for Team Losi ones. They are virtually identical in design and are direct bolt on parts.




Now with the 90-tooth spur gear installed I have a primary ratio of 6.92:1 (using the 13-tooth pinion still) and a final ratio of 44.98:1. BUT since I have the 90-tooth spur on there now I have more room to put a smaller pinion gear. So I can drop down a tooth on the pinion and can have a final ratio of 48.75:1. I might be able to go down one more tooth if needed but I don't have an 11-tooth pinion to try out.

I still have the 13-tooth pinion installed in the photo.


Well that's it for today. I drew up my upper link mounts and will hopefully be cutting them out sometime during the week. I'll post those updates soon.

[drc323]

Looking good so far manCant wait to see more on this build

[OGmicromonster]

:::UPDATE:::

After a little bit of contemplating, I decided to go ahead and build a dig setup. Since one of the PTI's weak points is it's turning radius, especially with a 12-inch wheelbase, a dig seemed like a obvious choice. Here's what I did...

First thing was to cut a pair of drive shafts to make my new dig drive shaft. As you can see I'm using Jato shafts.



To join the two male parts I used a short length of graphite rod I had laying around (I used to use this stuff to make drive shafts for Xray M18's). I figured if it's good enough to make golf clubs and fishing poles, it should be good enough for a drive shaft.



It fit perfectly in the hole already in the shafts so all I needed to do was polish the graphite with some fine sandpaper. Once smooth I glued it into the short end and left the other end dry to rotate inside the corresponding male end.




Next I took the female section and prepped it. I just beveled in the inside a bit to make sliding onto the male ends smoother.



Now I needed to make my slider guide. I took a 1/8-scale truggy aluminum wheel widener, bored out the center to just fit over the female section and machined off the treads and the hex portion.
(Stock piece on the left and new machined piece on the right)
Side Note: My father-in-law did the actual machine operating. I just told him what I wanted.



I wanted to make it as smooth as possible so it won't get caught up on the rocks. So I also beveled the leading edge.



There were already holes the perfect size to thread for a 3mm ball stud. So those got tapped and a 3mm ball stud was inserted.



I took some PTI MMT shock pre-load spacers, cut off the tab and glued them on the female section with the slider guide in between.



Here is the whole assembly installed on the rear section. I still need to figure out how I'm going to lock the rear axle when the dig is used. BUT, I have to decided if I'm going to lock the rear or not. Anyone have suggestions?



To actuate the dig, I found a micro servo that I had installed in my Xray M18. Since I don't drive my micro anymore, this servo found a new home on this build. It's basically a Hitec HS-81MG rebadged in Xray branding.



Here it is next to my steering servo (a Hitec HS-5955TG) for size comparison.



Speaking of steering, I had an idea to gain more steering and wanted to try it out.

[OGmicromonster]

:::UPDATE Part 2:::

To gain more steering I made up a couple of extenders for the steering knuckles.



The stock steering knuckles were designed with the original MMT chassis in mind. Since that chassis' wheelbase is much shorter (about 8") a lot of steering isn't necessary. As a matter of fact they were designed to minimize the steering so the truck was controllable at speed.

Anyway, moving the link mounting hole forward would provide more steering by giving the link more clearance on the front diff case and also having more throw.
Here's how it mounts on the steering knuckle. The screw holds the carbon fiber piece in place through the original link mounting hole. This takes the majority of the load. The second phillips screw prevents the carbon fiber piece from pivoting around the first screw. It's small so it doesn't weaken the knuckle much. The new mounting hole is about 1/4-inch further forward and allows for more steering angle.





There is still plenty of clearance behind the wheel.



Full lock with the new extenders.



As you've probably noticed in the photos I have my shocks mounted (Tamiya 501x rears) so I can start designed the chassis. I've already drawn up the upper link mounts that will go on the axles. And as K_B suggested, I decided to flip the axles. After thinking about it, there were many advantages to doing this. Thanks K_B! I'll still be shaving the underside (which was originally the top) to allow more clearance and to get rid of sharp edges.



Hopefully I'll have my upper link mounts soon. They are being cut as I type this update.
Until next time.... SEE YA!

[kawika]

Well. Why didn't I think about that as a solution to my steering problems. Looks great! That's much easier than the TLT knuckle conversion I'm in the middle of.

[Skeeno]

I'm digging the dig set up. I've been contemplating adding that to mine.

[OGmicromonster]

I figured if I'm going to put a dig on my rig (hey that rhymed! :-P) I wanted one that was slim and clean. I didn't want unnecessary parts hanging off of it. I still need to figure out the braking mechanism. As it sits now it's basically just a rear disconnect.

[Skeeno]

How do others do it? Do they lock the rear upon disconnect?

[OGmicromonster]

Originally Posted by Skeeno

How do others do it? Do they lock the rear upon disconnect?

Yeah, most people lock the rear as the drive shaft is disconnected. They use different methods, but that's what I need to figure out. I want to use a reliable and sturdy locking method. I just need to do more research and maybe come up with a method of my own. I think I might have a way. I just need to sit down and see if it'll work.

[OGmicromonster]

:::UPDATE:::

Well I got my test samples of upper link mounts so I can test fit them. They were cut in G10 fiberglass, but the final versions will be cut from 3mm carbon fiber. I'll use these for now until I get my final parts.



Here I've installed some Tamiya TLT link brackets for the upper links.



Then a pair of blue anodized Team Associated B4 servo mounts.



Once both fiberglass pieces were mounted I still needed to make some spacers to fill the gap between the mounting plate and the axle housing. This space is about 3/8 of an inch.



So I took some Associated (can you tell I like using associated parts? ) GT nose braces I had laying around and cut four spacers.
Here is a link to the method I used to cut the spacers.



With the link mounting plates installed I realized that the steering linkage needed to be revised. The servo arm was hitting the linkage and wouldn't work the way I had it set up. The solution? Flip the steering knuckles. This moved the linkage down just enough so everything fit perfectly and operated the way it should.



The nearly completed front axle.



And the rear axle.



Well that about does it for this update. Next I'll be drawing up the chassis and cutting some balsa wood test pieces before sending off the drawings to be cut. The chassis will be cut from carbon fiber as well.

[Skeeno]

That's looking really good.

I'm wondering if that steering link is going to cause any clearance issues on the rocks. I sort of looks like it's hanging out there a bit. Is that how far the TLT links hang out? I've never really noticed.