Tamiya TRF 501X WE Project

Page 1: Assembly

The model arrived assembled so the first thing I did was to disassemble everything down to the last nut and bolt (shocks excepted). Given the special nature of the model, and because I wanted to see what I was dealing with, I decided to knoll out the parts as shown. One issue with buying a model that has already been built versus one that is new in the box is the possibility that the previous owner has changed things. That certainly turned out to be the case here. Some changes are upgrades I'll keep, and others I'll revert back to the original design intent.

The left photo shows the parts for the first half of the center bulkhead which also serves as a motor mount. There are 3 bearing posts which serve to guide the belts and keep them from slapping. The original posts were straight and could not be adjusted. The new posts (on the right) are cam shaped so you can adjust their radial position by rotating the posts.

A set of blue aluminum steering posts attach to an angled plate. There is no kick-up angle for the front axle on this flat carbon chassis, but the steering posts are still tilted back slightly. The right hand photo shows both sides of the center bulkhead attached to the chassis plate. At this point there is only a single standoff connecting the two sides. There will be a laterally mounted motor. Note the groove on the chassis plate which will allow the motor to sit as low as possible.

The picture on the left shows the stack of parts that will make up the slipper clutch. The clutch is integrated with the spur gear, and the other end of the shaft contains the drive pulleys (right). The original TRF501X used 16 tooth pulleys, but the World Edition changes these to 18 tooth which will serve to increase the speed of the buggy. The slipper pads are also apparently upgraded on the World Edition compared to the original, but I can't tell what is different about them. I suppose they are intended to last longer.

Here is the completed slipper assembly. The original TRF501X used a 48p spur gear with 96 teeth. The World Edition changes this to 91 teeth which will make for more speed. My copy, which had already been built by someone else, came with the 96T spur so I replaced it later to stay with the spirit of the special edition. The slightly different sizes are compared on the right. This kit uses 48p spur and pinion gears, not the metric 0.6 mod which is unusual for a Tamiya kit. This is presumably to be consistent with standard racing pinions.

Here I've installed the slipper assembly which requires that the equal length front and rear belts also be installed. A pair of plastic bearing caps hold the shaft in place. This is a good time to adjust the cams which guide the belts.

Now I'll build the ball differential. On the original 510X the front and rear differentials were not the same. The front used a 35T pulley and the rear used a 36T pulley resulting in about a 3% overdrive of the front axle. Both pulleys integrated a ball differential with 8ea 3mm balls. The World Edition increases the ball count to 12 for more strength and standardizes on 37T pulleys both front and rear, eliminating the overdrive. The overall gear ratio changed from 12.7:1 to 11.0:1 for 15% more speed from the same motor rpm.

Here is the completed rear ball differential with drive cups installed. Note that the drive cup is much longer on one side than the other. This is to offset the pulley from the center of the bulkhead so it will align with the belt.

I came back later to add these dust shields to the ball differential. The standard design leaves the balls quite exposed, especially for an off-road buggy. The addition of the shields should tend to keep dust and dirt from sticking to the diff grease. These really should have been included in the baseline kit; they cost almost nothing.

Now the rear aluminum bulkheads can be installed along with the lower suspension mount. These attach to the chassis plate with a total of 6 countersunk screws installed from below. On the right I've installed the differential into the rear bulkhead. The rear belt spans between the pulleys. There are two plastic caps which capture the bearings of the slipper, so in theory only these and the aluminum stiffener need to be removed to access the slipper for maintenance. Things are somewhat more complicated in the rear. The rear upper carbon deck sits over the top of the differential bearings, so the ball differential cannot be removed without first removing the upper deck. This design was changed on the 502 by shortening the upper deck to allow for easier diff access.

This picture shows the eccentric cam used to adjust the belt tension. The bearings for the drive cups sit in these plastic toothed cams which index to the tang on the bottom of the bulkhead as shown. Rotating the cam to index to the next tooth tightens or loosens the belt. Incidentally, none of those holes on the side of the bulkhead are ever used for anything. If I had to guess, I'd surmise that they are used to support the part while machining.

Now I'll prepare the rear shock tower which is a carbon plate attached to an aluminum bracket. The blue screws shown in the image are not what originally came with the kit. These have been upgraded by the previous owner. I suspect they are aluminum which is not ideal for shock screws which are cantilevered and highly loaded, but they do look cool. The rear upper arm mount is also shown. Some subtle changes have been made between the 501X and the World Edition. The picture on the right compares the rear shock towers. The original part is shown in yellow and the updated part is shown in carbon laid over the top. You can see that the mounting holes are the same but the new part moves the shock holes slightly inboard and down. It also adds two new holes near the bottom which are not used for anything and a little extra material beside the mounting holes which does not appear to do anything. Presumably Tamiya made this change in response to some problem because they discontinued the original spare part and sold only the newer part as a replacement.

On the left is the same exercise conducted on the upper shock mount. The green part is the original and the dark outline is the updated part. It is by no means obvious what problem this part is trying to solve, but the description uses the word "strengthened" which would seem to provide the answer. The 3/4 view on the right shows that an additional gusset has been added between the lower and upper mounting holes which would help a lot with torsional rigidity. The old part appears to show a shock hole pattern which is angled, but this is just an artifact of the perspective used to draw the part in the manual. The true view on the left shows that all the holes are in a line both before and after the update.

Here the shock tower and upper arm mount have been attached to the the rear bulkhead. Note that there is no upper bulkhead cover, instead the tower mount and arm mount span the sides. The bumper which will be installed later will provide a certain amount of protection to the exposed belt.

While both the 501X and the World Edition came out of the box with a ball differential on the front axle, the previous owner of my model replaced the front ball diff with a front one-way. This is known to be better for cornering on high grip surfaces while racing, but if you are just driving for fun like me it is super annoying because you lose all front braking. Now let's get into some hidden details. On the far left is the front one way bearing (axles inserted into it can rotate freely clockwise, but are locked to the hub counter clockwise. Not so obvious from the image is the fact that the outer diameter of the hub is slightly larger on the left side of the flange that on the right side of the flange. The larger diameter is what fits the inside of the pulley, however the pulley needs to be installed on the right side to line up with the belt. You can't just flip the bearing around the other way because then it would lock in the wrong direction. The solution is the adapter on the right. It allows the pulley to be centered and securely attached to the smaller side of the hub. It took me a long time to figure out what the point of that was. Given that the front one-way (53951) was made specifically for the TRF 501X, why is an adapter needed? The part was made for the original 35T front pulley, but an adapter is needed if you want to use a different pulley. The previous owner used a 36T pulley from a TA-05 which results in a 3% overdrive in combination with the 37T rear pulley. I'll be replacing it with a 37T to get rid of the overdrive. I'd prefer to go back to a front ball diff, but I don't have the right parts to do so.

Time to build the dual bellcrank steering. Given the overall quality of the model, I was a little surprised to see that the steering bridge and cranks are plastic (aluminum upgrade parts were available). They do use full ball bearings for assembly though, and I found that the fit was extremely precise. There is virtually no backlash in the steering. The right hand photo shows the completed steering assembly, front shock tower assembly, front upper arm mount, and front upper deck with body posts.

Now the front bulkheads can be installed as shown on the left. It is interesting how different the lower suspension mounts are than what was used on the rear. The rear used a single bridging part that included both ball sockets. The front uses a pair of individual triangular mounts each with a ball socket. The right hand picture shows the installed steering system. The blue lock nuts shown are not original.

Here the shock tower and upper arm mount have been attached to the the front bulkhead with the upper deck stabilizing everything. You can't tell just from looking that a front one-way is present.

All four corners use CVD style universal axles rather than dog bones. The rear shafts are 64mm and the front shafts are 78mm which is a substantial difference in length, but the wheel axles are also different length so the overall length comes out the same. One subtle change for the World Edition axles is the length of the cross pin which has increased from 9.8mm to 10.5mm. This is not even a noticeable difference visually, but it was presumably done to keep the pin from popping out of the cup. The aluminum rear uprights shown are unique to the World Edition and replace the plastic parts of the standard 501X. The 5x8mm hex drive ball connectors shown are also longer and stronger than the 501X originals.

The uprights are installed into the lower arms with captured smooth hinge pins. The inboard hinge pins have metal spherical balls slid over the ends to allow for toe adjustment. The difference in width between the front and rear suspension mounts, if any, controls the toe angle of the lower arms. In this case, some quick geometry tells me the angle is about 3°. Additional toe at the axle can come from unique uprights. These uprights appear to be 0° toe and are identical left and right. The upper link turnbuckles shown on the right are not the standard steel Tamiya parts. I can't tell for certain since there is no label, but I'd guess that these are Lunsford titanium units.

The front aluminum caster blocks are also unique to the World Edition and incorporate a 10° caster angle, mirrored left and right. The front suspension arms assemble pretty much like the rears and are shown at the right.

Here the front suspension has been installed along with the titanium steering links (aftermarket). On the right you can see the rather conservative front bumper has been installed. It doesn't do much to protect the suspension from hits, but it does cover the front pulley. There is no rear bumper.

The World Edition includes front and rear sway bars which were optional with the original kit. The picture on the right shows how one of them installs, but you'll have to look closely to actually see it. It pivots on the end of the bulkhead, spans over the drive axles, and then connects to the lower suspension arms. Those blue aluminum countersunk screws you see supporting the sway bar are "upgrades". Strictly speaking they are much weaker than the stock parts, but they should be plenty strong for this application.

The previous owner of this kit had upgraded to "big bore" shocks from the TRF 503X to replace the original shocks. In case you are wondering, these have a 12mm bore instead of 10mm. That doesn't sound like a lot, but it results in 44% more volume. The old and new shock types are compared on the right. These shocks build exactly like the smaller set, but you may notice that they now include aluminum spring perches as well. Compared to the shocks included in the original 501X, the World Edition upgraded to Delrin pistons. The pictures shown here are actually from my TRF 201 build because I didn't want to tear down and rebuild the shocks that came with this model.

The installation of the shocks pretty much completes the rolling chassis. I can stare at this picture for a long time because of the beauty and quality of the chassis parts. On the right I've also installed the carbon plates which hold down the saddle pack batteries.

I decided to use a high speed Protek steering servo with an anodized blue aluminum case to match the rest of the chassis. The servo mounts you see in the picture are not original. The kit uses thin plastic mounts, but the previous owner upgraded to these much nicer aluminum mounts from the TRF 416. On the right I've installed my brushless motor and you can see a problem if you look closely. The motor shaft doesn't stick out far enough to reach the spur gear. This means the pinion has to protrude far off the end of the shaft. I searched for a very long pinion but couldn't really find one, so I ended up with a pinion that only overlapped the motor shaft very slightly. Not ideal. This chassis would have originally been intended to use a brushed Transpeed MS motor. I'm searching for one.

Now I've installed all of the electronics and done the wiring. I've cut all the wires to ideal length to make them tidy, then I've enclosed most of them in black wiring looms to make things as clean as possible. The saddle pack batteries shown actually came with my model.

This bottom view shows how nice and smooth it is with all the countersunk screws recessed into the surface. Even so, the addition of the under tray shown on the right serves to protect everything (especially the carbon chassis plate) and keep debris out. I left my tray clear so I can see the chassis.

The rear wheels do not use hexes, they attach directly to the drive pins which means unique wheels are needed. The narrower front wheels are more standard. I used a set of Tamiya dual block tires in competition compound for this buggy.

The TRF 501X uses a very low profile body which has almost no extra clearance beneath it. All electronics and wires must be very carefully installed. Despite looking the same, the 501X and World Edition bodies are actually very slightly different. They are compared in the photos above. The difference appears to be some extra room in the WE body in the "hump" which sits over the motor and electronics. The official Tamiya statement claims this is for "improved aerodynamics", but it looks to me like it is just providing more room for electronics.

This picture compares the two bodies I ended up with. The body on the right came with the model and uses a very nice black and white livery with a Lame spray effect giving it a sparkle. Despite the fact that this was a very nice body, I just had to have the classic TRF livery so I took the extra clear body that also came with the model and sent it off to a professional painter. The result is on the left. I decided to remove the body posts to avoid punching holes in the body since this body also attaches with Velcro anyway. I didn't have a 501X sticker sheet so I had to use some more generic TRF stickers. A close look will reveal that both of these bodies are the original 501X type, not the modified version from the World Edition.

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