I've had my 2012 Bonneville for some time now and realised pretty early on the suspension was amongst the worst I'd ever experienced, even the 1978 Cossack 175 I used to take my test on was more comfortable on a bumpy road! I changed the rear shocks which made a big difference in that I was no longer being catapulted out of the saddle if I hit a pothole at speed but the front was still very stiff and really jarred if you hit any lumps at motorway speed. I had noticed on here and other forums that plenty of people complained and even the guy in the Triumph dealers assured me they were all like that but they did seem to me to be exceptionally bad. I got some progressive springs and preload adjusters and finally got round to fitting them a couple of weeks ago and decided that while I was at it I might as well replace the fork oil at the same time. Imagine my surprise when I tipped the fork legs up and discovered that each one held just under 500ml of ATF!!! No wonder the damping was bloody stiff!!! I've no idea who filled them with ATF or why but I can say that with my new springs and with proper fork oil it's like riding a different, and much better, bike
ATF is a good fork oil ! But is very lightweight low viscosity , equal to about 2 in fork oil damping rate , most bikes nowadays like 5+ up to 15 weight.
Its not about quantity but air gap. The air when compressed (forks are air tight too) acts like a spring. Most bike now quote air gap rather than oil volume.
You surprise me there as I was always told that ATF wasn't good for forks, something about how it behaves when under compression.
I did take note that the manual gave both quantity and air gap, I did check both when refilling and they did correspond-which is what I would expect anyway.
I've changed my rear shocks for Hagon 2810 which has transformed the rear. I'll do a complete front end rebuild in the winter with new headstock bearings, fork oil and springs. ATF shouldn't be a problem, it's just very thin. 500ml seems like quite a lot. I recently bought some new Fox mountain bike forks for just under a grand and they came from the factory dry! I thought they were a bit crap so opened them up to change the oil and found there was none.
