I'm fairly confuzzeled here.
I have my 2007 rx8 up on jack stands to bleed the brakes and change the oil before the next track day. After bleeding the brakes, I started it up to make sure that the pedal felt good.
When I did that, in neutral, I noticed the digital speedo indicated I had a speed of 3 or 4 mph. WTF? I checked, and sure enough, the rear is spinning slowly. It's slow enough that I can stop it by hand with no noticeable idle change, but still..
I was under the impression that if I was in neutral, the rear end is physically disconnected.
Any idea what's going on?
That's weird. It should be physically disconnected when it is in neutral, my 2004 was. Something is wrong.
Cold fluid causes things to move when there's no external force to overcome that.
Perfectly normal, there is always some transfer even when in neutral
Okay, thanks for the reassurance. I'm still not sure how that works technically, though..
I think I'll need to rear apart one of those transmissions some time to get my head around it.
Swede nailed it, gear oil flowing around in there.
There is a bearing between the input and output. It has friction.
Streetwiseguy wrote:
There is a bearing between the input and output. It has friction.
That's what I was missing in my mental image of what was happening... Thanks! I guess I'll just take it as a good thing that with all the fresh fluid, my rear end has so little drag that it'll spin in that situation. Bearings must be good, right?
Bad ones have more friction than good ones.
Appleseed wrote:
Bad ones have more friction than good ones.
Which would result in less motion, due to increased drag?
Basically a manual gearbox in neutral is a very inefficient torque converter. :)
Perfectly normal and nothing to worry about.
start worrying when the car moves forward when it's in neutral: this happens with older trucks that have thick gear oil in the granny gear manual trans when it's somewhere around -30 degrees or so... you have to hold the gas and clutch for a couple of minutes for the oil to heat up enough to thin out and not kill the engine. this assumes a functional parking brake, of course, which means that you don't also have to hold the brake pedal down with your invisible third foot..
jstand
HalfDork
6/10/15 5:38 a.m.
The input shaft shaft and main shaft/output shaft are inline, with a bearing in between to allow them to spin at different speeds during normal operation.
The shafts are typically locked together when in the highest non-overdrive gear (1:1), and unlocked in all other gears.
The drag in the bearings and synchros in the trans must be higher than the force required to rotate the rear.
Since you can stop it with your hand it sounds normal.
As the bearings and synchros wear they will get looser until the contact surfaces degrade enough to start to increase friction. An example of this is the idlers for timing belts, typically the used idler will spin easier than a new one due to wear (and less grease), unless they've worn to the point of degrading the friction and are rough or bind.
It makes me wonder how many times it's happened to me before, but I never noticed until now because the 8 has a digital speedo, and my Miatas and Rx-7s all have mechanical ones.
yamaha
MegaDork
6/10/15 11:20 a.m.
Nick_Comstock wrote:
My bike does that.
Yep, both of mine do as well.....if you rev the engine without a load they'll spin the rear faster yet. The important thing is that they don't move you while in neutral.
My RAV4 will do that when it is cold. I have some heavy gear oil in there, and when it was like <10F out with it in neutral, it wanted to go forward in the driveway.
Look closely and you'll see the same thing happening on this million dollar race car:
Porsche 908
In reply to novaderrik:
It also happens on my DD Echo when we have winter. Even w/fresh fluid.
Streetwiseguy wrote:
There is a bearing between the input and output. It has friction.
There is also friction between 1st, 2nd, 3rd, 4th, and 6th gears and the output shaft from oil drag.... Ever so slightly... just enough that if your brakes are working correctly and not binding, the rear wheels can be turned by the engine even in Neutral.
Pilot bearing. The worse it wears, the worse this will be. Ever have a crunch when shifting to reverse? Probably a bad pilot bearing.
In reply to Kramer:
Every Toyota in the 80's needed a pilot bearing then.
Reverse is unsynchonised in most (if not all) transmissions, that's why it crunches.
You get a lot of this when it gets really cold. Below -20*C in Canada STIs frequently stall out in neutral on cold start because the trans oil is so thick it acts as a fluid coupling.
