How to reduce drag in CRT.5 diffs...
 

I have and E-CRT.5 with the RCM extended chassis. The handling is very good, but I'd like to get it better. I have two related areas I'd like to address.

1. The weight of the truck lurches forward when I let off the throttle at the end of a high speed straight. This makes the truck very controllable, but I loose a lot of ground to other standard 10th scale vehicles that free coast better.

2. The rear of the truck drives a bit too square. In most tight turns the rear tends to slide out too much. It is very controlled and usually only goes 90 degrees, but it looses a lot of speed when this happens.

I think both are related to the setup of the 3 diffs. I'm currently running 2K F, 3K C, 1K R. This gives a really nice on power feeling, but it seems to have a bit too much drag causing the vehicle to slow too quickly when off power and the rear to slide out when turning.

I don't have much experience with 3 diff vehicles and I'm wondering if I'd be better off with closer to the stock diff oil weights. Stock is 1K/1K/500. I did not even drive the vehicle with these weights because everyone was recommending 3/3/1 or 2/3/1.

Q1: If I change my oil weights to 1K/2K/500 would I maintain a similar on power feel, but reduce the drag I'm getting from the system?

Q2: What affects drag the most? The front and rear diffs or the center diff? Or, are the all the same?

Q3: In my quest to reduce drag will I be giving up too much in accelleration?

Thanks for the help,

I have experienced the same lurching after a fast run down the straight...
neither of the diffs should have any affect with coasting though so I have found it difficult to identify as well.

The diff's should mostly only affect on power handling. Any lurching would seem to indicate either binding somewhere in the drive line or the esc/motor braking. I used a metal pinion and it was eaten up after just two runs (Metal to Metal). I just picked up some hardened steel pinions today, from Mega Motors, so I'll let you know how the hold up and if the lurching issue is resolved. I am also working on a very simple plastic spur solution so I'll report on that as well.

The coasting issue might be due to the addl weight of rotating components compared to a similar sized electric vehicle. Do you have any drag brake set in the esc? What motor/esc combo are you running?

There's definitely more drag from the front diff than from a 2wd vehicle with no front drive mech. When ever you have drag or braking from the front wheels you will have more weight transfer to the front than a Rwd vehicle. I hoping that by lightening up the diff oils I can reduce drag and get a better off power corning vehicle, but I don't want to loose much of the good on power handling it currently has.

I'm using a hardened steel pinion. It shows very little (if any) wear, and is properly engaged with the spur.

Keep us informed on your plastic spur. I am tempted to try that, but the hardened pinion is so easy and reliable.

Drag brake was my first thought too, but the esc is set to zero.

I'm running a brushless Novak GTB esc and 6.5 motor.

I think I'm going to try setting the rear diff to 500 to see if that frees up the rear for better off power cornering. If that helps I'll try the front diff at 1000. I'm hoping to keep the center at 3000 to maintain good power transfer from the motor.

I'm really new to the oil filled diffs, so I'm just guessing.

I think this is typical of a 4wd vehicle. Both my 18T, G2R, and CRT .5 do this. I find the easiest way to cope with this is to always give some amount of throttle. Even just enough for the ESC to register is enough to prevent a significant amount of the "drag" brake.

I've been thinking of turning up the throttle center position until the truck creeps forward even at zero throttle. This should help when coasting into a turn. I've heard of an esc that allows you to do this but has a sensor override so that when the car does come to a stop under braking it will allow the car to go to true nuetral.

Id just put the throttle trim up so it coast's more, but not enough to actually go when your not on throttle

I dunno what vehicle you use, but on mine, I can easily switch between profiles. So if I were to have a profile where I set the trim to apply throttle during neutral, I'd also set an identical trim where neutral was true. So, right before the start of the race, I'd switch to the profile with the "powered" neutral, but otherwise leave it under true neutral, as I would suspect its HORRIBLE for the ESC (as well as the motor and battery) to have the truck always under some sort of throttle, no matter how little).

However, if you do this only when you are lapping, I can see no real issue - just be sure to hold some level of brake unless you are actually driving, and it should be fine.

I don't really understand why it's a problem, can't you just let off the throttle a bit later or get back on the throttle a bit earlier.

The actual problem is the rear of the crt.5 slides out too much. The car drives too square and looses too much speed in the turns. On lower speed corners I can drive deeper in and get on the throttle quickly if not leave it on a bit through the whole turn. But if I stay on the throttle too long down the back straight (into a tight 180 turn) the rear of the car will slide out every time. I just can't carry the same speed through that turn as a true 10th scale vehicle.

I believe this is caused by three things:
1. Not enough weight in the back. I'm working on a new layout for the car and will have the new batteries by the end of this week to do it.
2. Too much drag by the diffs. I'm running 2/3/1 (front - rear) in the diffs, but the rear still seems to thick. I'm thinking of going back to stock 1/1/500 to see how that handles.
3. The front has too much steering. Some of this should be cured by moving weight from the front to the rear. I think there can be some improvement by working with toe and camber on the front.

I don't know how much you can adjust on CRT.5's, but on my truggy, caster, front and rear anti-squat, droop, and ackerman position are other things that affect steering response and how the car enters and exits corners.