Double or Nothin'

[HenryCrawllins]

Seems like I'm on a bit of a kick right now with building rigs two by two. We're just going to say it's for science. I decided to make a revision on one of my chassis designs, and it sort of snowballed into building a second rig, so that I could test them side by side with no need for downtime to tune or swap parts. I figure there's some benefit to running a line back to back, while I'm in the same zone, and while conditions are identical.

Anyway, this is Purple Rain. So far, it's my best performer. It has flaws, however. I've found some perceived drawbacks with the super short front center, and I may have gotten a little lost in the sauce with chasing a low CG. I think it's pulling around too much brass. The core of the truck is a Fusion 1800kv, driving a set of Pro45 axles through an Exo gearbox. Right now it's sitting at a 330mm wheelbase. It has 27% OD, and 60/40 weight bias.

And this is the new hotness. No name yet, and it's still probably a couple weeks away from hitting the rocks. All the parts are here as of yesterday. Early estimate puts it at 500g lighter. Going to try my luck with the mini Fusion. Same gearbox, but Capra Axles (Dlux built), so this will be a good comparison of high scrub vs low scrub, as I've matched the track width of the Pro45s with more positive offset on the wheels. Same OD, but with 67/33 weight bias. I have plenty of weights I can add if I want to match the corner weighting between the rigs.

I still need to swap out the shock caps, and add a small spacer to the rears. Hopefully that'll be done this afternoon.

 

[HenryCrawllins]

Got everything buttoned up, pending a shakedown. Already thinking I might need softer inserts, but I'll test these ones out a bit to see if they settle in at all. I also went over my goal of sub 5lbs, just barely. I'll go back through it and figure out where I can cut some weight.

Some shots of it with its older brother, since it's only gonna look this pretty once.

There's also a little teaser of another potential summer project in there, which is digging out that hillside in the yard to see what kind of test course I can build.

 

[Gula]

Those look SO SICK !!!!! now i needs one too.

 

[HenryCrawllins]

Those look SO SICK !!!!! now i needs one too.

I'm always happy to share the CAD files for my chassis, if you have an affordable spot to get them made. If there's enough interest. I could even do a larger order from SCS, and get that volume discount. I'd still want to do a good bit more testing on this latest build though. This is my third revision, but in my mind, there's always room for improvement. So far, my friends are the only ones running my chassis. I'll have to be a harsher critic of myself if I'm going to send them out into the world, haha.

 

[Gula]

Neat. Really? I could probably get one made for free then. I'm a jack of all trades, and
"machining " is one of them. Maybe my old job can waterjet one out for me........

 

[HenryCrawllins]

Neat. Really? I could probably get one made for free then. I'm a jack of all trades, and
"machining " is one of them. Maybe my old job can waterjet one out for me........

For sure! Shoot me a DM and we can exchange emails. I'll send you the chassis file, and a basic parts list. It will be cool to start getting some outside opinions on the chassis.

 

[HenryCrawllins]

The weekend got away from me before I had time to take the trucks out on the rocks, but I was able to spend an hour in the garage this morning testing their limits on the incline board. Very interesting results. The new build is a definite improvement.

Limit 1 - Front end begins to unload, front overdrive can no longer hold shocks under compression
Limit 2 - Front wheel begins to lift, torque twist begins to overtake front grip
Limit 3 - Failure point, rig can no longer pull itself up incline

Old rig, limit 1 - 49 degrees

Old rig, limit 2 - 62 degrees

Old rig, limit 3 - 63 degrees

New rig, limit 1 - 63 degrees

New rig, limit 2 - 65 degrees

New rig, limit 3 - 67 degrees

The interesting parts are at the first and third limit points. The old rig would unload the front more rapidly with any steering input, whereas the new rig wouldn't. Both have the same shocks, with same spring rates. Maybe this is a function of the low scrub radius on the new rig, or the greater caster angle on the old rig. At the third limit, the old rig would flip, whereas the new rig was simply traction limited. The new rig remained stable at the failure point, even with back and forth steering input. It would just hang there under cruise control, swaying side to side. So it's got more in it, if I can increase the mechanical grip.

Anyone have insert recommendations for the Ruptures on a 5 pound rig, or a different 1.9 tire/insert combo that could out perform the Ruptures? So far, I've ranked the top five tires I've tested on my local rocks as follows:

1 - JC Ruptures
2 - JC Tusks
3 - Extreme Route Lamprey
4 - Power Hobby Grabber
5 - PL Hyrax Predator

 

[cul8tr]

Dang I thought I had a bicycle problem. I have 7 mtn bikes myself. I think I counted 11 hanging on the back wall.
Awesome builds. No place near me for type of build. I wish. I'd build a couple also but work doesn't allow. I'm back to 6 days a week.
We are 90% trail trucks.

 

[HenryCrawllins]

Dang I thought I had a bicycle problem. I have 7 mtn bikes myself. I think I counted 11 hanging on the back wall.
Awesome builds. No place near me for type of build. I wish. I'd build a couple also but work doesn't allow. I'm back to 6 days a week.
We are 90% trail trucks.

That's what 20 years working in bike shops, the last six of which owning my own shop, will do to a guy. For that entire 20 years, I've collected, rebuilt, and restored vintage mountain bikes. I've got 15 classic mtbs, plus a handful of modern bikes.

Sorry work's got you so tied up. I started a new job at the beginning of the year, and am just starting to balance out my schedule again. I've always gravitated to the technical side of crawling, but I am working on my first trail truck design. I'll have to start a thread for it soon.

 

[Gula]

Hmmm, this just gave me an idea of running a motor in the front, transverse in the chassis, hanging over front axle or further forward..... Are you allowed to do that?

 

[HenryCrawllins]

Hmmm, this just gave me an idea of running a motor in the front, transverse in the chassis, hanging over front axle or further forward..... Are you allowed to do that?

I've only seen that done once, with a Johnson 540 motor. Legend has it, the guy had a clean sweep of podiums at EVERY sorrca event he entered that year. Even painted it up to look like a Warn winch for extra scale points. Now if you really wanted to make some waves, you could try the same setup, but with the Big Johnson 550 motor. That'd really be something.

 

[Gula]

Outrunners alowed? You could have it, ( an outrunner) mounted on two standoffs on the chassis, and run a belt drive to the "skid" where the "t-case" is a funky design (input from top, ie vertical with a set of bevel gears), but have the belt leaving horizontal from the motor. You could have a huge 120t gt2 pulley on the t-case, sitting like a helicopter and a 12t on the motor. Make sense?