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Managed to finally get a road ride in, my first since August.
I've found today that I'm constantly shuffling back on the seat ?
Only things I've changed since August are a different saddle and new pedals and shoes ?
Any ideas ?
Saddle too low because of different saddle and pedals?
If you are measuring bb centre to saddle top, pedal thickness affects it.
Any ideas ?
Bike fit. Well worth it.
Saddle & pedals / cleats in different positions?
Saddle angle wrong since replacing the old one?
When you say shuffle back do you mean you keep finding yourself at the back and having to move forwards. Or you end up forwards and have to keep shuffling back?
I tend to end up forward which is great for an aero tuck, less great after 100 miles perched on my taint. I also prefer the narrowest saddles and probably should try one of those stubby TT ones.
If it's your first ride in a long while I'd not read too much into it. I go through a phase every year where my bike feels really cramped, then as I get stronger the bars feel further away. They're not I'm just better at tucking myself into the bike. Bike fit is a wired thing, if you're not consistently riding then it may well evolve.
I’ve found today that I’m constantly shuffling back on the seat ?
I went for a bike fitting and discovered that my shuffling back habit came from having the seat at the wrong angle. It was slightly nose-down, and I was constantly shuffling to find the back. So I adjusted it to be slightly more level, and also raised it a little.
Try putting sandpaper on it. Worked out really well for erm.... that tester pro guy....back with an edit in a second!
Tony Martin

Get the tape measure out between bikes - all my five are set up at the same saddle height, even the MTB's, but the FS has the dropper as I'd fall over the bars otherwise on steep stuff.
I'd guess that the angle of your saddle is most likely to be the culprit, if you are slipping forward.
Try moving the saddle back a bit. Or maybe spend £200+ on a bike fit. I know what I'd try first 👍
Weak core
lift the nose of the saddle.
To pointlessly echo the above, it sounds like one or more of: the new pedals/shoes position your foot further from the pedal; the new saddle isn't as tall as the old one; and/or the saddle wants moving or angling up just slightly.
Sorry for the delay in replying.
Halfway round the ride I did raise the saddle and whilst it helped with my leg extension it didnt help with the shuffling.
Its feels like Im slipping forward and need to constantly shuffle back. The saddle is flat as measured with a spirit level.
I did have a charge spoon on there but swapped to a Fizik Aliante.
Shoes Ive gone fro wearing my Shimano Spd's and pedals to proper road pedals and Fizik carbon soled shoes.
The saddle is flat as measured with a spirit level.
Spirit level be damned, adjust it and experiment.
A spirit level would probably say my saddles are slightly nose-up, but the rear half of them where I actually have my sit bones is flat. But that’s the shape of the saddles I use. If I changed to a different shape then it’d have different curves in all directions and I’d be starting from scratch with seating position.
you've changed the saddle and the cleats at the same time. put the spoon back on the bike and see if you still do it, if so you know it's the cleats. if not it's the saddle.
a cleat set up can be done instead of a full bike fit (less costly) and these can be as much use as the rest of the bike fit