I installed the Rennshift last night. Nice piece of kit to be sure. It spring loads the 4/5 plane just like the factory shifter spring loads the R/1 plane. My question is, what are the springs pushing against in the tranny?

The stock 915 (in a 911) uses a little tab on the 5/R side to keep the springs from pressing the shift linkage against stuff in the tranny. Here is a pic from Seine systems (he makes a 915 kit that also springs load 1/2 and adds the tabs to both sides).

Click to view attachment

You can see the little tabs and the hook on the shift lever that relieves the spring pressure when your anywhere other than the center plane (3/4 in a 915, 2/3 on our 901s). The reasoning is that you can spend alot of time in 5th gear in a 911 and its putting pressure on the linkage & couplings. The stock 914's stock shifter doesnt have it, but you'd never spend alot of time in 1/R either.

What is the spring loading in the 4/5 plane doing to the tranny? Are the 901 shift fork internals that much different from a 915 (doesn't look like it in the pics I've seen)?

Just looking to keep the tranny healthy,
SMD

The 915 is the only transmission that I know of that uses those tabs. Nearly every car in the world uses a spring loaded shifter with no ill effects on the transmission and linkage, including the G50 in Porsches.

James,
But the G50 has an internal gate mechanism that relieves the pressure. (I understand its spring loading is internal too.) You can add the Wevo gateshift to do the same to a 915 but they dont offer one for the 901. I've never taken apart a honda/vw/etc tranny to know how they handle the problem. I've seen the 915 guts and I partially buy into the argument that the tranny wasnt designed to handle the constant pressure.

SMD

Hey man,
The shift forks inside the 901 are held in place by springs and balls that rest in groves on the rods. I will try to get some pics for you. The rods are actuated by a comb looking apendage that sits atop the shift console shaft. If one were to put pressure on the shifter it is likely that it would be in the left/right plane of the car which is equal to the in and out of the shift comb on the console rod. This would not effect anything negatively. Putting too much pressure fore and aft on the shifter would make the internal gear selector rings push harder on the synchro teeth and synchro band (not too big of a deal) but would also cause the shift fork to constantly put pressure on teh groove of the synchro sleeve causeing premature ware of both parts, friction, and lots of metal fragments over time.

So, in short, side to side pressure is no big deal in a 901, fore and aft can be.

1/R is spring loaded only to keep one form hitting R accidentally.

As far as I know, Wevo does not make a 901 unit because the Renn shift is already out there and a perfectly fine unit.

What are you thinking is going to fail from this pressure?

I don't really see those other devices as "relieving" the pressure, they are simple taking it up in a different spot, and in the case of the factory 915 shifter it is a poorly supported, imprecisely produced, non-lubricated device that can easily be out of sync with the transmission location due to the linkage and the trans/engine mounts.

To be specific, the 901 rides against the neighboring shift rail. This is very much up to the job, covered with gear oil, and is out of plane with any other motion (i.e. it won't cause any other parts in the trans to be loaded). You could put another part in the trans to do the same thing, but you don't gain anything over resting it against the already well supported rail except more parts to potentially wear and have to align.

I built my first sprung 901 shifter in the mid-80's, and I have never seen a wear problem from their use. If it was to wear after many, many, many miles, I'd rather replace the one shift rail with a good used part rather than replacing a custom unit that may no longer exist 20 or more years later (my projection as to how long it would take to see any difference).

There will be more load on any plastic bushings, but I would say the shifting loading will push them beyond service before the side pressure loading.

Also, from what I have seen of the 915, the arrangement is basically the same as the 901.

A minor hijack, but a related topic. One thing I'm wondering is: why spring the shifter? If you spring the gearbox instead so the shift rod is sprung to stay in the center plane, you can accomplish this much easier inside the gearbox with one spring. Alfa transaxles work this way, for example. Seems to be a better solution to me. This puts no wear on the linkage parts while it's in gear, and the various shafts inside the gearbox are bathed in gear oil and fairly hefty, so the wear rate will be very slow.

I understand by the Rennshift is made like it is. Adding the spring to the gearbox would be a much harder installation. That's not really my question. I'm just asking a general design question.

James/Mike - Thanks guys. I thought side loading the 915 pushed one of the selector forks against the slider/syncros/etc. Maybe it was the fore/aft that you were mentioning Mike that I remember. It has been years since I've seen one apart and played w/ it. Its why I said I partially bought into the tab idea and was asking the question.

Mike, I wasn't talking about the wevo shifter, but the gateshift - the gate that goes in the tranny.

James, you should do a FAQ w/ pictures on why sideloading the levers in the tranny doesn't wear the sliders. I figure Im one step ahead of most in that I've actually played w/ the internals of a tranny, but there must be others out there who thought the extra tabs were also a good idea. I wouldn't target starting a war between companies, but it would dispel some myths.

All that being said, I went out to the garage and shifted the 915 (in the 911) w/ its weltmeister shifter. There is no mistaking youre in 5/R. If you engage those little tabs, the shifter got the right gear. You can both feel it and look down and see it. It may just take practice, but when I shift into 5th (or 1st) in the 914 - I *know* I made the correct movement, but the lever moves back towards 3rd when I let go. When I put my hand back on it or even look down at it, it seems like its in 3. Just a little disconcerting.

I ran out to the car and measured (from knob to the door), Even though I can push the shifter >2" (3rd->5th) it springs back >1" until its only 3/4" over from where it would be in 3rd. Is that normal? Maybe my shift rods are slightly worn on their sides if it isn't? Some sort of tabs/gates would definately help the perception of hitting/keeping the right gear though.

James, Im in no way knocking the rennshift. I mentioned in my first post its a fine piece. When I get the 911 back together, I'll likely buy one for it too.

SMD