closer look

Street Jam

OTA-R31

by R&D Japan

With the Street Jam OTA-R31 you get quite a nice car for your money. The big box contains many plastic bags which contain the parts for the chassis, a clear polycarbonate body and a set of wheels.
Damper oil, diff grease, black grease and a bag of cheap tools are included.

People who want an aesthetically pleasing chassis probably will be disappointed. If you want a working horse which is fun to drift, you will like it.

The bags and the order in which you have to open them are a bit chaotic. You might want to have some small boxes ready to put the content of the bags in them. Or, if you are like me, just spill everything on the table. Mainly there are only two types of standard screws (countersunk and button head, both Phillips type) and only two sizes. So you will not spend much time searching the right-sized screws for each building step. Also all screws are M3 and no tapping screws. Nice!

Pulley Gears

You have to glue one side of the gears. Clean the surfaces with some break cleaner. The beveled side should face to the teeth of the gears. Press the ring on the gear and let thin super glue (I used glue for tires and glueing nozzles) run into the crevice between the parts. Do not use too much. You don't want any glue on the teeth of the gear. The ring for the big gears is a bit too small. So it is best to glue it step by step.

gluing gears

Excess glue can be wiped off or removed with sandpaper. Check if the parts are properly glued together before you start building the drive train later.

Chassis Plate

The plastic material has good quality. It seems to be fibre reinforced. It is flexible but not brittle or too soft.
On the rear a diffuser is on each side. Hardly of any aerodynamic effect. But it looks good!

chassis top chassis bottom chassis diffuser

Steering

The OTA-R31 uses no bearings in the steering system. The metal disks work OK, but bearings are nicer.

If you use the outer holes of the steering levers, you have parallel steering (no Ackermann) with an angle of 50°. The inner holes give you Ackermann with 45° on the inner wheel and 35° on the outer.

On the second photo you can see that vertical play is limited by two claws which protrude from the pulley gear holder.

steering arm steering arm

Motor Carrier

The motor carrier is a simple steel plate. Connect the motor carrier with the plastic part which later holds the pulleys before you mount the plastic part on the chassis plate (step 5 in the manual). Otherwise you can not access the lower screw anymore. See photo below for reference.
The motor cover (see photos at the end of this text) is prepared to carry a fan. Good idea.

Instead of the 8mm screws which hold the motor carrier on the chassis, you could use shorter 5mm screws; looks better. Apply some thread lock on them.

I used a shim on the nut-side of the belt tensioner. It is just cleaner than the nut directly on the long hole.

motor mount shim

Differentials

Out of the box the OTA-R31 uses two ball diffs. The diff gears have 12 holes, but only 9 diff balls are used. The bigger holes stay empty (that makes 3 balls in 3 groups).

Be sure to insert the nut completely into the t-shaped nut carrier (part P9), otherwise the diff screw can not be tightened. I compressed the spring with a plier a few times to minimize diff adjustment after a few rounds of driving.
The thrust bearing is only three parts, no single balls. The included black grease is nasty stuff. Be sure that the threaded part of the diff screw stays without any grease to avoid that the diff loosens.

When you insert nut and bolt, check that the nut goes into the diff joint with the longer cut (part P5).

diff balls diff joint

Pulleys

A 48dp 86T spur gear is used. The hole pattern looks strange, but gears from other makers fit.
The drawing on step 8 in the manual is a bit confusing. The hinged part consisting of parts N4 and N5 connects the gear which is closer to the spur gear to the pulley shaft. The belt gear further away is connected only with part P3 or P4 (they are the same). Check that the hexagonal side of the clips fit with the hexagonal side of the belt gears.

The clips on the shaft with the two small gears have been attached from opposite sides to have a better balance when the shaft is rotating.

pulleys pulleys complete

When you mount the finished pulley shafts and bulkheads on the chassis be careful not to get any twist in the chassis. Especially on the rear this might happen. Remove all excess little plastic parts which remained on the parts after you snaped them from the sprue. Tighten the screws gradually cross over and be sure everything is at the right place before you fully tighten the screws.

bulkheads front bulkhead rear bulkhead

Drive Shafts and Suspension

Assembling the drive shafts is done without any screw. The 2mm-pin is kept in place by the inner bearing.
The suspension arms have quite some play on the hinge pins (3mm diameter). Some shims solve the problem. Inner and outer hinge pins have a flat area which should face towards the small screws which secure the pins. The suspension arms are prepared for downstop adjustments. These screws are not included. 10mm long, M3 setting screws fit and give you an important setup-feature.
The rear wheel carriers and front c-hubs sit rather tightly in the suspension arms. If you want best performance from the start you have to use some sandpaper and remove a bit of plastic. Or you let time do the work; after a few batteries of break-in-time they should move smoothly.
C-Hub and steering knuckle fit fine. The kit contains two sets of c-hubs. One with 4° caster and one with 7°. The marking of the parts is not so clever, because you can not see it after the suspension is assembled.
Turnbuckles of the same size are used for upper suspension arms and steering.
The wheel hubs have lots of play. Other manufacturer's or clamped aluminium hubs should increase precision much.
The bearings are full of grease. They do not run so smoothly at the beginning, but should last long time.

rear hubs rear suspension knuckles steering top

Damper

The construction is standard. One o-ring plus spacers and cap at the bottom and a diaphragm plus damper cap (two parts) at top. The kit contains three different lengths of lower damper mounts. So it is easy to find the perfect damper length for your preferred ride height. Furthermore the damper's body is threaded. No damper clips!
Two damper plate sets are included (two and three holes). Oil (#400) is also in the set.
The dampers are as good as plastic dampers can be. The soft springs and good precision of parts make it a smooth working set of shock absorbers.
For the adjustment of the angle of the damper there are three holes at the suspension arm and six holes at the damper stay.

damper

Installation of Electronics

A 27T steel pinion gear is in the kit. If you use this gear, the final gear ratio is 1:6.9 (86T spur, 27T pinion, 39T diff pulley, 18T pulley). A gear calculator is here.
The servo is mounted upright. Mounting blocks for standard and low profile servos are included. Also a servo saver with four adapters. Opposite of the servo is enough space for the receiver and ESC.
The battery tray measures 141x48mm. The height is 27mm. The battery is kept in place by a clip-on plastic part. I don't have one for testing anymore, but there seems to be enough space for lipo-batteries with fixed cables at the front side.

Other Stuff

Street Jam has this huge diffuser on the rear which tries to do aerodynamic things to your chassis (and closes the ugly gap the body keeps open behind the rear bulkhead). On the front they also have a small spoiler lip under the bumper.
The upper bumper plate acts also as a stabilizer carrier. The rear is also prepared for a stabilizer.
A set of tires (45°) and TE37SL wheels in black and with 6mm offset are included.

chassis top chassis to chassis bottom full steering angle rear diffuser front spoiler wheels and tires
This kit is fun to build and great to drive. The construction is not beautiful (I often find Street Jam's design of parts a bit odd), but it works great. The quality is good, the price is more than OK. Already some aluminium parts are available, so optical and performance upgrades are possible.
Counter steer is also possible. Just get the pulley set and a front one-way.
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