Tamiya's Gravel Hound (Page 2)

Let's go off-roading

Went racing with my friends today and the Gravel Hound did pretty well. We weren't really being competitive - and the field consisted of 3 other Gravel Hounds, an old Schumacher Cat, an older Tamiya buggy and a HPI RS4MT. However, my GH handled great and was easily one of the top-handling cars in our little make-shift track. It understeers, but it's nothing some suspension tuning can't fix. The brushless ran great and the car sped around like a rocket.

To it's credit, it even survived in the hands of some 10 year old kids that I passed the controller to (I adjusted the EPA for the thottle though...)

We had 2 jumps on the track and the buggy was quite a pleasure to fly. It took off well and really held level flight easily as the pictures show. Landings however, were pretty harsh. I'll try some harder shock oil (already running 60-70 wt!) to see if it helps. The harsh landings though, did not really seem to hurt the buggy in any way, other than scratching up the chassis a little and my ESC mounting tape coming loose.

Any damage?

Well, after I got my buggy back from the kids who had bashed it into everything possible, there was a whine that was coming from the gearbox. After stripping the transmission down, it showed that my pinion gear had worn off pretty badly. This was a surprise because the plastic spur didn't show much signs of wear at all! Here's a pic of the pinion.

I cleaned out the powdery remains of the pinion and installed a new one. All fixed. It did seem that a little dirt did get into the tranny although nothing that could cause much problems. My friend also had a worn pinion in his GH so I wonder if we need to hunt down some hardened ones for our running.

All in all my buggy was a competent basher and great fun. A friend running with us commented "I gotta get me one of these" and that was compliment enough for the budget basher GH. Some of the other GH's suffered some damage though... one worn pinion & spur, and another had a twisted dogbone - but that's actually not too bad if you saw what these buggies had to go through today! lol!

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Night bashing...

These pictures were taken at a night bashing session. 5 guys showed up and we set up ramps and a track to run our buggies. The third pic shows the GH going for some distance - and this wasn't even at full throttle.

The good news is that with the stock pinion installed (after I destroyed two Tamiya AV pinions) there appears hardly any wear on the gear. Seems the stock 19tooth pinion is hardened. I just have to hunt around for some hardened 48 pitch metric pinions to gear down a little for the brushless to run cooler.

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Rear Suspension Mod
As mentioned in earlier testing, the Gravel Hound bottoms out very easily. Even with heavy damping and springs, it doesn't take much to get the chassis to slap the ground. While it held up to the hard bashing even with this issue, it just didn't seem right for a off-road machine to do this.

I set out to see if I could improve the suspension geometry to handle jumps better without compromising handling of the car. In the rear, I suspected that the mounting points on the lower arms were simply too far "in" towards the center of the car - as the suspension arm is essentially a lever pivoting on the inner hinge pin, this resulted in an overly "soft" suspension.

I machined out some simple mounts to test out my theory. The mounts not only moved the mounting point outwards, I decided also to raise the mounting point upwards to further increase the suspension travel. The more severe angle caused some binding where the dogbone and the drive cups met, but I simply used my dremel to elongate the slots in the drive cups and that solved the problem.

Lastly, a set of 4" Associated shocks were installed with 2hole pistons and 60wt oil. These are longer than the stock shocks allowing the suspension to drop further at full extention.

All this was done to achieve a few things:

• Move the mounting point outwards to get more positive damping
• Allow the suspensiont to drop further so that full stroke of shock can be used to absorb impact
• all this while maintaining ride height through use of shock spacers

Front Suspension Mod
A simple drop test seemed to indicate that the rear suspension was functioning better now but the front suspension was still bottoming out even with single hole pistons and 70wt oil. I didn't have any 3" shocks to play with so I decided to fashion an entire new front shock tower to accomodate the stock shocks in a different orientation.

I concluded that the softness in the suspension was largely due to the lay-down design of the front suspension. Laydown shocks are great for handling on smooth tracks but they are less able to cope with jumps and rough terrain. As such, I decided to make a new shock tower to stand the shocks up more.

The pictures above show the tower that is machined out of 3mm thick 6061 aluminum. Although the shock tower can be installed without the original shock tower, I designed it to still allow the stock tower tp be attached for an even stronger setup. Together, the entire assembly seems very solid - I suspect that if anything breaks in this it would be the tab on the gearbox holding the shock towers.

The pictures below compare the shock position before and after the new shock tower is installed.

Before	After

I've got to test out these mods on the track but a drop test confirms that they definitely improve the car's ability to handle jumps. No doubt the car might still bottom out on hard jumps, but at least it's good to know that there is more effective damping to reduce the impact of the chassis hitting the ground. Track testing will tell how these mods have affected the handling of this buggy.

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Performance
Took the truck bashing last weekend and tested out the modified suspension. The good news is that handling doesn't seem to have been adversely affected. On the contrary, on the surface where we run our buggies the buggy seems a little more responsive now. The not-so-good news is that we didn't set up our ramp for this session so no real opportunity to see how much the landings have improved.

Here's a great shot of the Gravel Hound taken from this session.

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