TriadicRC

I just recently purchased a HPI Trophy 4.6 truggy and i have to say i am absolutely in love with the truck. I know it's not the best race vehicle out there, but i am slowly replacing parts to get it there.

When launching from a dead stop the weight shifts back, the front tires balloon just a tad, and loose a lot of traction. All of the power moves up front where theres not enough traction to really get the truck moving. I'm attributing this to the stock grease filled diffs.

I've read that F:7K, C:10K, R:3K is a good balanced starting point. I tend to prefer just a bit of oversteer , so would a F:7K, C:10K, R:5K give me what I'm looking for? I've tuned the brakes to about 70% rear 30% front and i really like it. I would like to get a similar feel while under power.

nitroexpress

Some of your statements seem to be in opposition. If the front balloon just a tad, then isn't necessarily bad. But then you say that you loose a lot of traction. This would seem to indicate more than a little ballooning of the front. You also state that all the power moves to the front. This would seem to indicate a more serious front ballooning problem. So do the front tires balloon a lot or just a tad?


With a stock vehicle, the shock fluid weights and spring stiffness may need to be adjusted. Excessive rear squat under power may need to be addressed. Try 7/10/3

TriadicRC

The weight of the truck shifts to the rear which raises the front end. It raises the front end enough that the front wheels have less traction than the rear wheels and all of the engine power gets transferred to the front. I am still running the grease filled diff's, so power is going to be delivered to whichever wheels have less traction.

As far as how much the front tires balloon... If 0% ballooning is when the truck is parked, and 100% ballooning is about to shred the tire, then i would say it's about a 35% - 45% ballooning.

These tires bite into the dirt really well, thats why i was considering a thicker rear diff oil such as 5K. I would like to get just a small about of oversteer under power. I've already tuned the brakes to do this under braking, which works really well for me.

Maj_Overdrive

My Feedback: (11)

When you see the front tires balloon that is the center diff unloading. This can also happen side to side with the front or rear diff. The diff fluid resists unloading whether the tires are in contact with the ground or not. Finding the right balance to prevent excessive unloading while still allowing the outputs to spin at different speeds is the goal. Believe it or not increasing the weight of the fluid in the rear doesn't always promote oversteer, in fact a heavy fluid or locked rear diff can promote on power under steer. I'd start with the 7/10/3 as well.

Edit: In case it wasn't clear, making the fluid heavier in the center diff will transfer power more equally between the front and rear tires resulting in less ballooning of the front tires. Right now the front tires lose traction during weight transfer, power unloads to the front tires which are spinning instead of accelerating the truggy. Increase the weight of fluid in the center diff and more power will go to the rear which has traction increasing acceleration even when the front tires are off the ground. Making the rear diff fluid heavier will transfer power more equally between the rear tires. Try 3k in the rear first and if you're spinning the inside tire during turns then make the fluid heavier, no more than 5k should be needed though.

wwit

I'm running 5k front, 7k center and 3k rear and am getting great hook up on both my 4.6 Trophy Truggy and new D8T.

This is my D8T on its first shake out run. Check out my channel for video's of the 4.6 TT as well. Now with that said, I don't use the stock TT tires. I never did and have only run a set of Proline Badlands on 1/2" offset Desperado's and this set up works great for me. Probably the best off road bash tire I have ever used.

http://youtu.be/Sx-E_AUVuE0

wwit

Oh and just another thought. My TT had pretty heavy weight transfer too. Changing the watery shock fluid out for AE FT 50w shock oil cured many of the handling woes for me. I put the 50w in at all 4 corners. As I ran the truggy more and more, I noticed I was having to keep adding preload shims as the springs got gradually softer with use. I was researching heavier springs with lots of difficulty as HPI didn't make any other rates in the same size for the TT. I then starting thinking about just stepping it up a notch and find some big bores that had easy to come by variety of springs. After learning that I would be spending $100+ to go to decent big bores, I figured that's more than a 3rd of a new D8T....so then I ordered a D8T and have put my TT on the back burner. I think it will become an on road nitro car this winter while I'm stuck inside waiting for warm weather.

EXT2Rob

My Feedback: (1)

So it seems the majority of you run a heavier center diff fluid than you do on either end. Having a heavier center fluid makes sense in the previous post, that it'll keep either end from unloading. I'm pretty new to truggies, and mine is smaller than most of y'alls (it's an Ofna/HoBao Hyper 10TT, about 4.5lbs) and I have been using 5k front and center with 3k in the rear. Theoretically, what would be the effect in your truggy if you were running the same weight fluid front and center? I ask, because I wonder if I should go with a heavier center fluid. Thoughts?

Maj_Overdrive

My Feedback: (11)

Are you trying to establish a relationship or ratio between the fluid weight of front and center diff? I honestly don't think there is one. If anything there is a relationship or ratio between the front and rear. The rear is usually somewhere around 50-75% the weight of the front in most cases, whether nitro or electric.

IMO Center diff (CD) fluid weight is totally dependent on the amount of power, traction and amount of weight transfer under acceleration. With more power you need heavier fluid. I've had two 8ight-t 2.0 truggies (should've never sold first one) and the first one had two different setups. With a Castle 1515 2200kv on 4s my CD was ok with 50k but when I went with a much more powerful Castle 1520 1600kv on 6s I was burning up the front tires with 50k and needed 100k.

Diff fluid weight depends on a lot of factors. The physical size of the diff and the spider gear count (Traxxas Revo/Maxx diffs need much heavier fluid than 1/8), amount of power and traction as well as driver preference. I personally like a looser cd to retain some diff action as it helps turn in and is more predictable powering out. I would think with 5k center you have enough diff action. The only reason to go thicker is if the front tires are ballooning or you'd like to experiment and find better handling. If you're going to experiment I'd pickup some 20k, you can always mix in some of your 5k to lighten it up if 20k is too thick.

EXT2Rob

My Feedback: (1)

No wasn't trying to figure out a ratio or anything. Just trying to get a feel for what fluid weight where does what, ya know? I can totally see how diff fluid is dependent on the factors you stated. And how to judge what my rig is doing. I'm starting to get it. I actually got a recommendation from the Hyper TT guys over at URC for the 5K/5K/3K starting point. I had noticed that a lot of them were running heavier variations, and some with heavier CD, like 10K/15K/8K. But then, they were also running larger motor systems than my little EZrun. So your advice makes total sense to me. Thanks Major! Indeed, especially since I lowered the front end a bit via the droop screws, removed the rear sway bar, and lightened up the shock oil, the 5K seems to give me decent turn-in, and powering out of corners is no problem. I do find I have to stab the brakes a little mid-corner to get the back end to rotate, tho. (BTW, my track is kinda bumpy compared to the tracks you guys prolly run on.)