Tamiya Porsche 911 GT1 Project

Page 1:  Chassis Assembly

This was an interesting project.  Because I took the Porsche GT1 body from a brand new Street Version kit, I was lucky enough to have both a standard TA-03 R-S chassis and a TRF version, both new in box, at the same time.  I decided to build both and take the same photos of each which allowed me to compare the two directly and highlight all the differences in the TRF model.  It turns out that there are a lot of them.  The TRF version represents a substantial upgrade.  For each step (except where noted), I've shown the standard chassis on the left and the TRF chassis on the right.


   

I was lucky enough to get this car new in the box.  The size and shape of the box is different from any other Tamiya I've owned.  It is much smaller than a buggy box, and not shaped like a typical touring car box.


   

These photos show the contents of the box.  There are no fancy internal boxes or blister packs here, just a single lateral divider.  As usual, all of the parts trees are individually bagged and the hardware is split into 4 bags labeled A-D.  The carbon chassis plates are loose in the box.  On the right I've knolled out all of the parts on my build table.  It doesn't look like much because most of the interesting parts come from the hardware bags.


   

All of the parts shown on the left are from hardware bag A and are unique to the TRF version of the chassis.  The picture on the right shows the parts from hardware bag B.  This begins to give you an idea of the vast number of special parts in this chassis kit.  The purple anodized aluminum parts are very unusual for Tamiya which hadn't settled on their signature electric blue yet.


   

The standard chassis uses open, unsealed gear differentials front and rear.  This is a standard Tamiya differential with sintered metal side gears and 3 spider gears.  The gears are lightly greased and provide very little resistance to differential motion.  The TRF model, on the other hand, uses a front one-way gear.  This means the axles are connected to the drive system when powered forward, but slip when braking or reversing.  Personally, I don't like these because it means the car only has rear wheel braking, but it is better for accelerating out of a corner.  Good for racing, bad for general driving.


   

Here are the two assembled gear diffs for the standard model which are the same in the front and rear.  The front one-way on the right looks nearly the same, but you can see a slight difference in the size of the output spline.  The external dimensions of the housing are the same (39T) meaning it can fit in the same bulkhead and use the same mating gears.


   

Now we'll assemble the front bulkhead.  The primary plastic housing parts are the same between the chassis versions.  The only difference is the use of shielded ball bearings on the TRF version instead of plastic bushings.


   

Here is the assembled front bulkhead.  Externally, you can't really see any difference between the two.


   

Here are the parts for the front counter gear assembly and upper suspension assembly which is combined with the shock tower.  Differences in the TRF model include ball bearings, sleeved upper arm pivots instead of step screws, and purple anodized aluminum ball studs instead of brass.  Note also how the hex on the counter shaft is split on the TRF model but continuous on the standard model.  I'm not really sure why this is different, but the hexes on the TRF shaft are 1/12 turn out of phase with each other.


   

Here are those same parts assembled.  The rectangular gap in the shock tower is for the belt to pass through.  The black plastic front drive gear (16T) is identical on both models.


   

Now we can prepare the lower front suspension assembly.  The lower plastic arms are the same between models, but the TRF model uses smooth hinge pins with E-clips instead of screw pins.


   

Here the counter gear has been installed, closed out with a cover, and the lower suspension assembly has been attached.  Only the purple ball studs reveal which assembly is which on first glance.


   

The standard model uses plastic pulleys while the TRF model uses aluminum.  The diameter and number of teeth (15T) are the same.


   

With the pulleys installed and the upper arm mount attached, the front bulkhead assembly is complete.  The appearance is very similar, but there are important differences inside.


   

You can see substantial differences in the front hub and steering assemblies.  Both models use the same plastic steering knuckles, but the TRF uses bearings, machined aluminum uprights, aluminum stepped king pins, and pre-assembled universal style axles instead of dogbones.  Completed assemblies and exploded views are shown above.  Note that the TRF model even has ball bearings on the king pins.




Take a look at these lovely steering assemblies. It is very easy to see that this is a special model with non-standard parts based on this photo.


   

It's time to complete the front bulkhead and suspension assembly.  Note again the hinge pins instead of screw pins.  Because the TRF model doesn't use a chassis tub, it needs to install the front bumper at this point as well.


   

Here is the completed front suspension assembly sans shocks.  Clearly very different, and there is no question which version is more expensive.


   

Both of these pictures are of the rear ball differential assembly of the TRF model.  The standard model uses the same gear diff in the front and rear so it was completed in Step 1.  The rear ball diff can be tuned much more easily for different amounts of resistance to differential motion. This ball diff uses a pre-assembled thrust bearing, five Belleville washers (disc springs), and eight 3mm balls.  The differential spur gear uses the same number of teeth (39T) as the basic differential so it fits in the same space and could be swapped if desired.


   

Now we can start on the rear gearbox assembly.  The plastic housing is the same, but the difference in the differential is very obvious.  Again, the TRF model uses ball bearings.


   

Here is a pretty big difference.  The lower part of the standard rear bulkhead includes a large curved section to attach it to the chassis tub and protect the motor.  Since the TRF model uses a carbon plate instead, it has a different lower panel.  The lower suspension arms are the same, but the usual difference in attachment hardware applies.


   

The lower suspension assemblies have been fastened.  There is no getting these two confused for one another.


   

Here is the hardware used to attach the lower suspension assembly to the gearbox housing.  The tiny bearings shown on the TRF photo are for a belt idler.


   

The lower suspension assembly has been installed.  The reason for the different geometry of the lower panel will be much more obvious once these are attached to the chassis.




The TRF model comes with two sets of gears.  The black gears are from the standard model, are made from regular ABS plastic, and use a combination of 0.8 mod gears for drive and 0.6 mod gears for the spur.  The white gears are Nylon and use a finer combination of 0.6 mod gears for drive and 0.4 mod gears for the spur.  Only the white gears are used for the TRF model.  The finer gears are quieter and offer more pinion options, but are also less strong.  The Nylon should offer better wear and friction properties as well.  It was a good idea for Tamiya to make these gears a different color because they cannot be mixed.


 Standard
   TRF 
Counter Gear
 25T
 33T
Spur Gear (inner)
 22T
 30T
Spur Gear (outer)
 43T
 63T


   

Here you can again see the difference in gears, the ball bearings, the purple ball studs, and the split hex on the counter shaft.  The white drive gear mates with the differential so it is the same pitch and tooth count (16T) as the standard gear.


   

Here are the assembled rear shock towers with upper suspension links and the gear sets for the rear transmission.


   

An exploded view with the gears, fasteners, and cover is shown here.  The other differences have been explained in previous steps, but the fasteners and gear cover are the same between the standard and TRF versions here.


   

Once the gearbox and suspension assemblies are complete, most of the differences are well hidden.  Only the purple ball studs and a peek at the white gears give away the TRF model.


   

Now the shock tower and upper suspension arm assembly can be installed.  This step also installs the rear pulley for the drive belt which is clearly different (metal) on the TRF version.


   

A few more little details differentiate these two assemblies if you know where to look.  Pretty subtle though.


   

You can't miss the difference between the plastic rear uprights of the standard model and the complex cast metal parts used on the TRF version.  A less obvious difference is the longer drive cup on the TRF which results in a mildly wider rear track.  More on that later.


   

Here are the parts needed to complete the rear suspension.  Apart from the uprights already discussed, virtually every other part is also different.  The TRF version uses lightweight aluminum dog bones, steel hinge pins with E-clips for lower pivots instead of screw pins, and flanged bushings for upper pivots instead of step screws.


   

Now that the uprights are installed, the differences between the standard and TRF versions are much more obvious.  They can't really be conflated at this point.  The difference in the lower housing is also pretty clear from this angle.  The standard lower panel connects to the chassis tub while the TRF lower panels connects to a carbon chassis plate.  This will be shown clearly later.


   

Both models use a C-shaped metal motor mounting plate, but while it is stamped on the standard model it is cast metal on the TRF.  While the standard models use a regular paper washer as a spacer, the TRF model adds the purple anodized aluminum plate shown.  The TRF motor is mounted with hex cap screws instead of JIS machine screws.  Finally, the standard plastic pinion gear cover is replaced on the TRF model with a nice metal heat sink version, though I'm not sure much heat is going to make it from the motor to this cover.

As previously discussed, the pitch of the spur gear is different between the models so the pinions must change to match.  The standard model uses a 14T 0.6 mod pinion (14T-25T possible) while the TRF model uses a 23T 0.4 mod pinion by default instead (20T-29T possible).  The standard model comes with the smallest possible pinion for the highest possible gear reduction ratio (8.51:1) which results in the slowest speed with the standard 27T silver can motor.  The TRF model does not come with a motor, but the stock 23T pinion results in a lower 7.34:1 ratio with the recommended 13T Dyna Run Super Touring motor, but the hotter motor more than makes up for the difference and results in a much faster car.  Interestingly, if using a silver can the TRF manual recommends using the largest possible pinion (which makes sense) instead of the smallest possible pinion used in the standard model (which seems odd).  (The Dynatech 02H motor shown is just a placeholder until the right motor arrives).

For those who like to see the numbers:

   

That's the completed rear bulkhead, gearbox, and suspension assembly.  Most of the mechanical guts of the model are right here.


   

While the standard model did come with decent oil filled plastic CVA shocks, the TRF model kicked it up several notches with these aluminum shocks.  Tamiya hadn't settled on their TRF shock design yet so these represent an early attempt, but you can see the aluminum cylinder with threaded caps, double o-ring seals with Delrin rod guide, Delrin piston, and volume compensation bladder.  The outside of the cylinder is not threaded for a preload adjustment ring so spacers must be used instead.  The front and rear are built differently with more internal o-ring spacers on the front (resulting in a shorter extended length) and different spring rates.  The pink and blue colored caps and springs help differentiate them and are unique to this model.  Even the very closely related TA-03 R TRF uses gold, red, and yellow instead.  This damper style originated in the David Jun Special Edition of the TA-03 F (silver and blue).  All 3 models share the same cylinder and spring part numbers despite being different colors.  Good luck getting the right parts if you ever need to replace them.


   

Here are completed front and rear suspension modules (with shocks) for both the standard and the TRF chassis.  Many external differences are pretty easy to spot with the TRF version being much more colorful.


   

Now we'll get to work on the chassis.  While the standard model uses a plastic bathtub chassis that is all one piece, the TRF version of the chassis needs to be built up from multiple parts.  We start with the carbon fiber base plate and work up from there.  The large plastic bits you see are for the battery holder and retainer.  I've used a low profile steering servo as a placeholder here, but a standard size fits fine and I'll be retrofitting one later.  Finally, the plastic spacer up front is just to adapt the forward bulkhead to the carbon chassis.


   

Both versions of the chassis use dual bellcrank steering, but while the standard version is all plastic, the TRF version uses aluminum cranks and a carbon bridge with ball bearings.


   

Here is a comparison of the completed steering linkage assemblies.  The overall geometry is the same, but the materials are quite different.


   

These photos compare the front end of the chassis with the steering linkages installed.  Note how much further the TRF carbon chassis extends forward to cover the bottom of the front bulkhead.


   

Now the front suspension and gearbox assembly can be installed onto the chassis.  The standard model also installs the front bumper at this point, while on the TRF model it was already part of the bulkhead assembly.  A close look at the picture on the left will also reveal an idler pulley which can be used for belt tensioning.  The TRF version doesn't have one.


   

The rear suspension and gearbox assembly now gets the same treatment.  This is also the time to install the drive belt which is NOT the same between the models.  Apparently the TRF belt uses aramid fibers which prevents it from stretching under prolonged tension.  This is why it doesn't need a tensioner.



The TRF version comes with front and rear stabilizer bars out of the box.  In keeping with the rest of the model, they are coated in bright colors.  The standard model has provisions for sway bars, but they are optional aftermarket parts.


   

These pictures compare the completed standard and TRF chassis.  External dimensions are the same, but there are a lot of external and internal parts which differ considerably.  You may notice that an upper carbon chassis brace has been installed on the TRF chassis while the standard chassis just uses an FRP bar.  The standard chassis doesn't need as much help with torsional rigidity since the bathtub is so much deeper than the flat plate used by the TRF chassis.  Also note the purple spacers on the rear axles.  Those will be important shortly.  I found it interesting how the sway bars just slip into holes on the uprights.  No connecting rods or ball studs are needed.


   

A couple of more pictures comparing the chassis varieties.  You can clearly see how the wheelbases line up exactly and how the batteries are positioned in the same place.  I've also installed the final items: the body posts.


   

Remember earlier when I mentioned that the TRF chassis uses longer rear drive cups (compared above)?  The Porsche 911 GT1 has a wider rear track than the front and also uses much wider tires in the rear.  The TA-03 R-S chassis does not have any additional track width in the rear, but the original 58193 model from 1997 used rear wheels with a different offset to obtain the wider track.  All four tires were the same size.  I figured I could do even better than that by using Tamiya's really nice chrome and gold two piece wheels which are wider in the rear.  This would achieve the tire and track width all in one step and be the ultimate in realism.  What I did not realize is that the TRF version of the chassis has the additional rear width built in.  That purple spacer we saw earlier along with the longer drive cups push the rear wheel outward.  That meant that I couldn't use the wider two piece wheels and I couldn't use the original wheels with extra offset either.  I couldn't even steal the drive cups from the standard model because the bearing seat is too short for the wider uprights from the TRF chassis.  I needed four front wheels to make it work.  The TRF chassis actually did come with wheels and tires, but they are a different style that didn't look right with the GT1 body.

The second photo shows the original Dyna-Run motor I managed to find.  I love the sound of an old school low turn brushed motor.


   

The final step was to install the electronics.  I'm using a regular NiMH battery because it fits perfectly and the car doesn't need to be any faster.  I was worried that my usual 60A ESC wouldn't be able to handle to 13T motor, but so far it is OK.  This model was never intended to use a mechanical speed controller.

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©2023 Eric Albrecht