The Hornet - Build with Wheel Nut's custom mods

[Wheel_Nut]

I'm starting this build with a number of upgrade parts.  I want to retain the character of this classic kit, while adding custom mods which represent my own preferences. 

[Wheel_Nut]

I will be upgrading to the wider Grasshopper 2 front suspension arms, so the front suspension has a similar character to what comes in the kit.   I'm trying a revised steering linkage that uses bellcranks instead of the directly connected steering servo.  To complement these modifications I have designed a 1.5mm FRP reinforcement plate that was sketched in CAD and cut out with hand tools.

[Wheel_Nut]

These are the parts I'm using for the bellcrank setup.

[toyolien]

Subscribed. Looks an interesting build. I do love creating stuff by hand rather than just printing it 

[Wheel_Nut]

Maybe in the future I would like to print some parts, but I have got used to making parts by hand.   Hopefully the injection moulded ABS parts are still strong enough after these modifications.   I'm trying to add some strength where possible.

[Wheel_Nut]

I bought the Grasshopper 2 "A" parts (Tamiya part no 10005340).   It includes suspension arms which are longer than the Hornet ones, but they are the only part from this tree I need to use.

I have the metal upgrights from Xtra speed.   These have 3mm threaded holes which makes it easier to install 5mm ball joints for the steering linkage.  The first pic shows a Tamiya fastener called a "3x22mm screw pin" which is supplied with both Hornet (58336) and Grasshopper 2 (58643), intended to secure the uprights to the front suspension arms.   Since the Xtra speed alloy uprights allow installation using a grub screw as in the original vintage Hornet kit, I decided to use this method with some 3mm pins I found in my spare-parts box.

[Wheel_Nut]

The Grasshopper 2 front suspension arms are not quite a direct fit on the Hornet chassis.   When attempting a direct fit, there is a point of interference at the front of the chassis on the extreme left and right sides.

I used a Dremel #196 carving tool to create up to 1mm extra clearance.

[Wheel_Nut]

I have installed the steering bridge and bellcranks.   These parts are intended for the Tamiya "M-07 Concept" M chassis car, where the bellcrank mounts on the left side and the idler crank is on the right.    Because I want to mount the steering servo on the right side of the chassis I decided to swap the positions, so the bellcrank on the right side.  As detailed in an earlier post, I have used steering posts with different heights on each side.  The bellcrank is mounted 2mm higher than the idler crank to provide enough clearance between the two.

The steering tie-rods use two different sizes of 5mm ball joint adjusters, the smaller size connects to the steering bridge where there is only limited space.   The larger 5mm reinforced ball link adjuster (Part 54257) is used on the outer end of the tie-rods.   The rods are 38mm titanium turnbuckles from 3-Racing (Part 3RAC-TR338).  The total length of the tie-rods are approx 59mm centre to centre, and is the same for each side unlike the stock Hornet steering linkage.

After the earlier photo of the suspension arms, I moved the linkage to the inner holes in the upright and it is now using Tamiya 5mm "ball studs" that screw into the upright rather than "ball nuts".   I changed it to get the tie-rods into the correct geometry for "Ackerman" steering.

[ChrisRx718]

Excellent, love this sort of ingenuity.

What is your plan for the front pogo-sticks? Always a weak spot on the Hornet / Grasshopper IMO.

[Wheel_Nut]

Front shocks are Tamiya CVA "Super mini" shocks (part 50746).   The "Super mini" length allows them to fit without changing the suspension mounting on the chassis.   

I've seen Hornets and Grasshoppers that have been modified for double wishbone front suspension, but that's beyond my plans here.   There won't be a large amount of front suspension travel, but I think it's ok for this design using a single wishbone (swing axle).   The negative aspects of camber-change and bump-steer are less than they would be with longer suspension travel.   I'm interested to see how well the CVA front dampers perform.  

[Wheel_Nut]

The CVA Super mini shocks were built using short rod ends (V1), two hole pistons, the springs and damper oil supplied.   They seem to be reasonably firm as they are intended for on-road cars.

The U-brakets supplied in the Hornet kit are normally installed on the front suspension arms, but I used them on the upper shock mount.

[Grumpy pants]

So much to like in this build thread 👍

[ChrisRx718]

Very cool. With the wheels on, does the chassis bottom-out before the suspension reaches full travel?

[Wheel_Nut]

Thanks for the kind words!

6 hours ago, ChrisRx718 said:

Very cool. With the wheels on, does the chassis bottom-out before the suspension reaches full travel?

As I wrote above, there is Not a large amount of front suspension travel.   I measured 22mm of total wheel travel.   I noticed some flex in the front suspension arms, so that is contributing when the suspension is loaded.

With Tamiya DT-02 front wheels and tyres there is 37mm clearance at full droop, and 15mm of clearance at full compression.   If I want to change the front suspension for more travel, I'd probably look at a double wishbone setup.

The DT-02 wheels are quite large for the Hornet.   Alternatives include the kit ones, Grasshopper 2 (Part 10445094), or the Kyosho Ultima (Part UTH001WH) if they fit.

[Wheel_Nut]

I have built the gearbox and rear dampers.  I took some pics of the various parts prior to assembly.

The Tamiya "Hornet Full Ball Bearing Set" as a nice Hop-up option (Part 54997).

There will be more custom parts in the following post.

[Wheel_Nut]

Probably anyone who owned a Hornet would know there is a "clunk noise" every time the car accelerates.  It is caused by a torque reaction when the rear axle is loaded and the gearbox moves in the slots of the "rear axle stays" as a result.  The Hornet kit includes two small "Axle springs" that are fitted inside the slotted rear axle stays in step 5 of assembly.   Unfortunately they are not strong enough to control the gearbox torque reaction force that occurs on acceleration.

Apart from being an annoyance, the torque reaction can cause a loss of performance.   Firstly there is vertical movement of the gearbox / rear axle which causes a variation in tyre loading.   Secondly, due to the movement of the gearbox in the slotted "rear axle stays" the rolling motion of the rear suspension is inhibited, so one rear wheel may become unloaded if the terrain is uneven.

To avoid these issues, I added a custom control arm that counters the undesirable movement of the gearbox / rear axle.   I call it a "Gearbox Torque Reaction Arm".   It is a linkage made from two 5mm Tamiya ball adjusters and a piece of 3mm threaded rod.  The upper end attaches to the gearbox, and the lower end to the chassis via custom mounting brackets made from 1.5mm FR4 fibreglass sheet.  When the rear axle is loaded due to acceleration torque, this control arm is in tension so it eliminates the undesirable movement of the gearbox.  In other words, the torque reaction is applied directly to the chassis by this control arm, instead of being applied via the rear axle stays.

Installation of lower mounting bracket requires modification of the chassis to cut 2 holes and a recess in the rear of the chassis as shown in the third picture.  Although I did not show a picture, I countersunk the two M3 x 8mm mounting screws on inside surface of the battery enclosure, so the battery will not get punctured or damaged.   I do not have any drill equipment that will fit into the space, so I used a 6mm drill bit in my hand to slowly countersink to the depth needed.

The upper mounting bracket is easy to install because it fits between the motor and the gearbox.  The 27mm motor mount screws from the Hornet kit are slightly too short, but 30mm screws from the Lunchbox screw-bag are a perfect fit.

[Wheel_Nut]

Once the lower mounting bracket was bolted to the chassis there wasn't much assembly involved.  With the rear suspension unloaded, the gearbox mounting shaft is slightly below the mid-point of the slots in the rear axle stays.  This allows the maximum articulation of the rear suspension.  As the rear suspension is compressed the gearbox pivots around the lower ball joint of the "Torque reaction arm".

[Carmine A]

On 12/6/2023 at 3:04 PM, toyolien said:

Subscribed. Looks an interesting build. I do love creating stuff by hand rather than just printing it 

So do I... I have a 3D Printer, that I haven't taken out of the box since I got it 2 years ago!  But I ll always love creating with Styrene, Aluminininium and Carbon Fiber. 

[Carmine A]

@Wheel_Nut That's brilliant!! I've been trying to sort out bellcrank steering for my Hopper II...  I did the double wishbone though. 

That "Gearbox Slapper Fixer", is a stroke of genius!!! 🏆✌️  Simple, effective, adjustable!

[jonboy1]

On 12/11/2023 at 6:28 AM, Wheel_Nut said:

Once the lower mounting bracket was bolted to the chassis there wasn't much assembly involved.  With the rear suspension unloaded, the gearbox mounting shaft is slightly below the mid-point of the slots in the rear axle stays.  This allows the maximum articulation of the rear suspension.  As the rear suspension is compressed the gearbox pivots around the lower ball joint of the "Torque reaction arm".

 

That's such a neat solution! Brilliant!

 

[Wheel_Nut]

Thanks for the positive comments.   I have not tried to run the car, but it gives the impression it should fix the "Gearbox slapping".

 

[Wheel_Nut]

People who owned the vintage Hornet in the 80s and 90s would know there was normally a second servo to operate the mechanical speed control.   That is the location where I chose to mount the steering servo.   The servo mount is still included in the Hornet kit as part 'A9', which is now a redundant part.   I decided to trim the servo mounting post to lower the mounting height of the servo for best alignment with the bellcrank.   Doing so means a low-profile steering servo is essential.  Here I used a Savox SC-1251MG.   I carefully marked and cut the front servo post with a junior hacksaw blade.   The pilot hole for the servo mounting screw was drilled to increased depth using a 3/32" (2.38mm) drill bit.   

The steering link is made from a 3-Racing 55mm titanium turnbuckle with Tamiya 5mm reinforced ball adjuster (Part 54257) on each end.  These ball adjusters are longer than most other ball adjusters from Tamiya.  The length of the completed steering rod is 84mm centre-to-centre.

It's hard to say if this work to install the aluminum steering arms / bellcranks has been worth it or not.   It probably has a bit less bump-steer, but I don't have a stock Hornet to compare it to.   I'm also thinking about a custom steering bridge and longer tie-rods that could further reduce bump-steer with the single-wishbone front suspension.

[JimBear]

@Wheel_Nut I have to ask, this "Gearbox Torque Reaction Arm" or rather the part that is the upper bracket - how thick is it? And what material - some sort of plastic?

If I understand you correctly, you are just substituting the standard motor plate, using the upper bracket instead, right?

I am thinking of using some of these ideas for the Rising Fighter (Grasshopper II chassis) - especially changing the steering rod setuo since it is so flimsy (we managed to bend one of the original rods on the first test run).

[Wheel_Nut]

@JimBear The upper bracket is made from epoxy fibreglass composite, which is the same material used for circuit boards (except for not having any copper).

It is possible to find on ebay if you search for "Fibreglass Sheet FR4".   FR4 is an industry code for this material.  I used 1.5mm thickness FR4 material in this build, including the upper bracket.

The Hornet does not have any motor plate as standard.   I just used the motor bolts to hold the bracket as there isn't really anywhere else to attach it.  Not easily anyway.

 

 

 

[Wheel_Nut]

I had difficulty choosing wheels for this project.  Perhaps I should be using the kit ones, but I ended up ordering a set of 4 wheels directly from the supplier "JC Racing Products UK".   They are Tamiya 2WD wheels for 1150 bearings on the front and 12mm hex drive hub on the rear with a 4mm axle hole.   The diameter of the bead is 61.8mm on the rear, 61.5mm on the front.  They are designed for 2.2" buggy tyres.   The rim internal width is 16mm front, and 32mm rear which seems slightly narrow.  The front rims are 3mm narrower than the Tamiya Astral Dish wheels.

(UPDATE)  WILD ONE WHEEL SET "JC-0058-W" is now sold at RCmart.  It may be more convenient for customers outside the UK.

The 12mm hex drive adapters I used are XTRA Speed (Part XS-TA29152).   They are almost identical to the "The Frog (2005) Alu. Wheel Adapter" from Tamiya that I used in my previous project.  However I came across the same issue to install either of them on Hornet / Lunchbox axles.  Both adapters have a step in the internal diameter where it changes from 5mm to 4mm, and unfortunately the 5mm section is not long enough to fit over the axles and correctly seat on the drive pin.  While they would probably fit on the actual vintage production axles, it appears Tamiya have varied the design of the axles in the re-release Hornet and Lunchbox kits.  The solution was to use a 5mm bit in a hand drill to take the diameter to 5mm all the way though.

Front tyres are Tamiya 51207 "Wide Grooved Font 60/19" from DT-02 series.

Rear tyres are Tamiya 54186 "Dual Block Tires K 62/35" from DF-03 series.