Learning to drive in the 1960's,as I did, few (if any) cars had synchro on first gear - most were geared low enough that second could be used from very low speeds, but on 3-speed Fords it was almost essential to learn double declutching if you were balked on a hill and wanted to keep moving smoothly.

And the first few cars I could afford (well used ones!) had synchro so worn on second that DDC was often the only way to change down without crunching noises.

The reason why second gear synchromesh is the first to wear out is simply that the gear ratios on any vehicle are spaced as a geometric progression to match the drag curve. This means that the ratio steps become closer together as you move up the gears.

First is not often selected on the move at higher speeds and hence wear here is minimal, but a change from third to second is frequently made and is a large ratio step. The consequence of this is that the synchroniser has to dissipate an appreciable amount of stored energy in order to match the gear/dog speeds. This equates to wear.

Not only is 3-2 the widest ratio step frequently used, but of course, the energy dissipated (as heat) in the synchroniser is a function of the square of the rotational speed difference. This makes the 3-2 change doubly vulnerable to wear.

Better gearbox makers equip the lower ratios with larger synchronisers. The Volvo transverse transmission is a bit of a bodge (they had to leave room for a 5 cyl engine and still ended up with no steering lock) and the double layshaft box they designed has a higher moment of inertia than a standard transmission of similar torque capacity. The synchromesh suffers accordingly.

659.

Edited by 659FBE on 30/08/2012 at 11:21

There was certainly nothing wrong with Volvo (pre-Ford) designed and made transmissions of conventional layout. They were conservatively designed with SKF rolling bearings and lasted well - if not the nicest to use.

In any engineering project, problems arise when you stick your fingers in the fire. Volvo presumably did not want to buy or tool for V6 engines for their larger models and thought a compromise might be a transverse 5. In my engineering experience, this sort of thing happens when a Chief Engineer stakes his name to a project, stands on the table and won't get off. The poor sods under him have to make it all work (been there).

Whatever one's feelings about 5 cyl engines (not for me - I like perfect primary balance), the transverse fitment of such an engine to a normal car poses real problems of power unit length. Marketing demands 6 speeds for a manual transmission (questionable with a high torque diesel and a 70 mph UK limit) which places further constraints on the transmission design - especially its length.

Volvo's solution was a twin layshaft box (I wonder if the designer ever had a SAAB 96) which will accomodate the required ratios in a very short space at the expense of complexity, number of rolling bearings and inertia.

OK, it fits, but the vehicle for me is unusable on account of its restricted lock. Try manoevering a trailer with the thing. Volvo have suffered from a lack of variety of in-house engines (compare VAG) and that mythical Chief Engineer...

Not a complete disaster by any means but in my view, not a very good compromise either.

659.

Edited by 659FBE on 30/08/2012 at 18:48

Thanks 659 - glad to have an explanation of why modern Volvos have such an awful turning circle. The old 240s turn on a sixpence - RWD of course but some FWD cars, like Skodas, are perfectly respectable in this regard. My Octavia has a better turning circle than SWMBO's Mini.

The Audi A4 I had was excellent - could that be because - if I remember right - its engine isn't transverse?