How much top speed would you expect 4wd to suck away vs rwd??

[marlinspike]

Long story short - I have an old TA03R-S PIAA Porsche I built years back when I was a kid that I pulled back out to play with with my kids. Nowadays skyrc exists so ive become obsessed with tuning the speed. Back in the day I ran it as a RWD conversion, and with a new ball-bearing modded Super Stock RZ I was able to get 38 (once 39) mph out of it (the old 14 turn triple Mighty Motor I had died when I put the car away as a kid, so not able to get hard numbers but this is a lot faster than that was). Unforunately, as a RWD conversion this speed was just unmanageable (those old TA03R-S were great, but the steering is full of slop), especially on braking. So I converted it back to 4wd. This dropped my top speed all the way to 35mph. That seems like a big hit. Is this the amount of additional drag one would expect from going to 4wd? I wouldn't think the belt drive would cost that much, especially with all the bushings involved replaced with bearings.

[Kowalski86]

It depends on the drivetrain setup, something like a TT02 probably wouldn't be effected much, but a TL01 would be effected quite a bit as their stock setup uses over 20 bearings/bushings.

With on-road cars, I wouldn't mind a 4mph hit if its already hitting the 30mph range (anything quicker becomes a blur), and having 4wd braking helps to keep it from crashing into things.

[CMshooter]

With ball bearings, I wouldn't expect much.  3 mph in the top 30's isn't a lot.

[Juggular]

3 mph down from 38mph is 8% down.  That seems reasonable for doubling the gears.    

Having installed bearings, you've already raised top speed by about 20%.  You could increase a bit more if you want. 

Ball-tinkerers like me would remove shields off of bearings, wash them in alcohol, re-lube with very light machine oil.  You could reduce the rolling resistance close to that of ceramic bearings.  I even sort them out so that the most free-rotating ones would be closer to the motor (because they get the most rotation).  Naked ball bearings should stay inside the gearbox, obviously. And the gearbox should be sealed with grease on both halves, so dust won't get in. The ones that will hold drive shafts should have shields, as well as wheels.  

If you stick a pencil through it, and roll it with a finger, untreated rubber shielded ones may turn once after your finger is off.  Washed no-shield would turn a dozen times. With light oil, it would turn for several times (oiling takes a performance hit, but it's better than rust-seizing and premature wear.)  You may think: "turning once vs 6 times is 600% improvement!"  Well, most of the work the motor does is fighting the momentum.  Rolling resistance is a small % of it.  Improving 2% down to 0.4% will only improve 1.6% (these are random numbers, I haven't measured or anything).  

The other thing is the grease.  Light teflon grease can reduce the rolling resistance of the gears.  The sidewalls of teeth are rubbing and slide against each other.  It's more like sandpaper action than humans pushing revolving doors. Which is why the teeth wear like they are scooped from the side. 

Above is Labelle 106 grease, which I applied too much here. Tamiya ceramic grease is quite decent, but not the best. People spend money on all kinds of grease. But rarely do they choose something faster, because most grease aren't designed for small plastic teeth.  Less is better, especially if it's teflon. 

If 8% hit is big, these are the things I would try.  Perhaps you can gain a couple of mph back.    Oh, you can put bearings in your Superstock. That alone can give you about 2% improvement, if I remember correctly.  By itself a motor going from 20,000 to 20400 RPM isn't noticeable. But combined with other things, you might just be able to break the 40 mph barrier with your current setup.   

(As you can imagine, thick grease would not make it run faster.) 

 

[marlinspike]

Thanks for all the info. I in fact already put bearings in the super stock, though looking at the brushes I am starting to think one of the reasons I've lost as much as I did is the motor needs new ones. I managed to tinker my way back to 37mph (adjustment to toe, cleaning everything that rotates and relubing as appropriate, and surprisingly setting the rear wing on the PIAA Porsche body to create more downforce all helped a bit each. I just wanted to break 40 with a superstock, but I think it might be impossible - don't know that I'm devoted enough to start disassembling bearings. I could go back to RWD and with all my tuning I might get there, but as a RWD car it was unusable at these speeds.

[marlinspike]

Turns out I had put the idler pulley above the motor on backwards, so the belt kept slipping off then rubbing in the motor case.  Fixed that, and with and with the tweaks I had done in the meantime to get speed back...

 

I hit 40mph with 4wd!

[Wooders28]

(Just to throw this in..).

Shaft drive 4wd are less efficient than belt drive 4wd, so a belt drive 4wd is marginal ,where a shaft drive is quite significant.

(Brushless and modern batteries kind of notice less though, where bitd, it could be decided if your battery finished the race or not)

 

[rich_f]

On 2/28/2025 at 6:36 PM, marlinspike said:

I could go back to RWD and with all my tuning I might get there, but as a RWD car it was unusable at these speeds

Could you convert it to front wheel drive? It would be easier to control than rear, but I'm not sure how much of an improvement in friction over 4wd it is compared to going to rear.wheel drive, in terms of the number of rotating driven parts. 

[Wooders28]

3 hours ago, rich_f said:

Could you convert it to front wheel drive? It would be easier to control than rear, but I'm not sure how much of an improvement in friction over 4wd it is compared to going to rear.wheel drive, in terms of the number of rotating driven parts. 

Fwd is shocking in cars not designed for it, there's just not enough weight over the front wheels, so the car just sits with the front wheels spinning.

[Saito2]

Pretty drastic, but this gets the idea across.

 

Kyosho's Maxxum FF was less extreme but still clearly had front weight bias to make it work.

[rich_f]

Sorry I though the op was just doing top speed runs and wanted something that didn't spin out at the slightest steering input. 

[marlinspike]

On 3/5/2025 at 5:31 PM, rich_f said:

Sorry I though the op was just doing top speed runs and wanted something that didn't spin out at the slightest steering input. 

I was, but now that I managed 40mph with a superstock rz and 4wd, 2wd is not worth the trade of. Also, a testament to the superstock, I tried a Thrust B-Spec 8 turn and it just gave me 3mph more and a TON more heat.