Savage XL TVP length and center diff gearing
 

I have been prepping for a center diff XL length Flux project for a while now. Just starting to protoype the layout with wooden TVPs. I have a couple of questions before i really get underway. I'll probably create a thread here for the project btw.

1) Simple question, a double check really. What length are normal HPI Savage XL TVPs? Can someone measure between the top bulkhead mount points for me please (should be about 356mm). AFAIK the XL wheelbase from axle to axle should be 55mm longer than the Flux. i.e the TVPs are 55mm longer.

2) Gearing. Arithmetic and more arithmetic.
Think i have it right but i have no experience with setting up gearing.
Gearing for Flux gearbox from tooth count is (44/18)*(29/32)*(44/20) = 4.874 (20 pinion for 4s battery)
For 6s a 25 tooth pinion is used = 3.899
I want to run 5s so this should require a gear ratio between 4s\6s = (4.874 + 3.899) = 4.387

For my build i will need to run a 46t center diff so that the center dogbones sit at similar angles to the original setup.
The pinion i would need for 5s = 46/4.387 = 10.486. Which is pretty small (will wear faster?).

So i was thinking i would use a different kV motor, around 1500kV.
To achieve the same speed with the slower motor i would need a porportionally lower gear ratio.
So gear ratio for 1500kV = 4.387*(1500/2200) = 2.991
Pinion for 5s with 1500kV = 46/2.991 = 15.380

To sum it up. For 5s battery with 46t centerdiff and 1500kV motor i need a 15t pinion.

Does the above checkout ok? Does using a lower kV motor seem reasonable?

Cheers

1) Er, yes they are 55mm longer. I have them at home, could measure them tonight.
2) 46/15 is probably still a bit high even on 1500kv. Probably want to run like a 11-14t pinion. 13t is ~40mph. 14t is ~45mph. (From BrianG's calc).

Lower kV with 5S or 6S has the benefits of running cooler and being more efficient. Which is a good thing. I'm thinking of swapping my RC8T to a 1800/1900kv motor and my Savage Flux 5B thingamabob to either 1500kv or 1800kv myself.

Downside is that you need the smaller pinions due to the 43/13 gear ratio of the front/rear diffs. If you swap out to 43/10 truggy diffs (which do fit, I checked with my RC8T diffs and they just needed a tiny amount of shimming) you won't need to run as small of pinions. I.e. 16t to 18t on the 46t spur, although the rollout is quite a bit away from 1:1.

Thanks.

Running cooler is definitely a goal as i live in Australia.

What did you mean by 'a bit high'?
Around 80kmh (~50mph) would be what i would have expected, speedwise at least, from 5s considering 4s is ~60kmh and 6s is ~100kmh

Really? I trief fitting some RC8T diffs in the stock X bulkheads but I needed like ~4mm of shimming, which is quite a lot. I also had plans for a centerdiff in my XL Flux:

http://lh4.ggpht.com/_oXx8nzDDQME/S6...0mount%201.JPG

Tekin make a 1350kV. Maybe i would be better off with that. How bad could running a small pinion be?

Well maybe I need to go try it and reshim again... I only did a quick demo of it to see if it'd work.

Why not use the http://www.rc-monster.com/proddetail.php?prod=rcmcdmm Unless of course you want to fab your own. :) One of my three center diff designs uses Mike's universal mount; biggest downside I see is that it does raise the CoG. Btw what CAD are you using?

1350 on 6S, using bulletproof diffs and dirt bonz tires, would need min. of 44/12 to do ~45. In my Flux conversion I'm actually running the 47t spur gear without a problem. Switching to that allows you use slightly bigger pinion gears. With the 47, you'd use a 13 rather than a 12.

Center diff planning I'm thinking of going with a 50t spur to be able to use a larger gear to target around 45. Plus since I'm also thinking of running larger tires, like 6.8 rather than the 5.8 or so of most MT tires... it gives me room to gear down with the pinion. I was looking to use the 1600kV CastleNeu.

He's not using Mike's because ; Sorry, this product is not currently available.. Could you link it again?

That is no CAD design, it's Google Sketch-up :lol:

About the gearing. A higher spur indeed gives you more flexibility, but it also raises the CoG more and more. I will have to give this project some good thinking before I will start it.

He has an extra question mark on the end of the URL http://www.rc-monster.com/proddetail.php?prod=rcmcdmm

Fixed the link, but here it is.. http://www.rc-monster.com/proddetail.php?prod=rcmcdmm. Its the Universal Center Diff/Motor Mount.

Ah, may have to check that out... I've been using a combination of cheap ones because they do some things well, others crap.

Yes, very true about the CoG. However if say going with the something akin to what you drew up or Mike's Universal mount, the size of the spur hardly matters as the motor is up above the CD so the CoG is going to be much higher due to the weight of the motor of course.

Here's what I've drawn up, at least one of three. Its based on the RC8 center diff (because I have one to measure in my RC8Te!) and is as low as I can get it. This uses custom dogbones, I've got another one that uses the tCS Crawlers 5T conversion front dogbone (12") but I like this better as the other one has some higher CoG and some chassis issues. These are about 11" and 5.4" dogbones. I think HotBodies may make a 134mm dogbone, so adjusting the cd back about 4mm would mean only one custom dogbone.

http://lh4.ggpht.com/_dcy74CG15Qk/S7...ge5B.2.png.jpg

The other that I've drawn up uses Mike's Universal mount with custom dogbones. Its a little nicer in that it all fits within the TVPs and no cutouts on the side for clearance. You can also get to the engine, adjust the mesh, swap pinions, etc. much easier with it. Its just that the CoG gets jacked.