Cube Attain SL Disc in 2018 weighs 9.8 kg according to the maker, and has disks front and rear. The brakes are
Shimano BR-RS505, Hydr. Disc Brake, Flat-Mount (160/160)
The Felt is 9.54 kg and has rim brakes. Sadly is a little vague.
Dual pivot w/cartridge brake pads, Sora STI levers
- Shimano hydraulic calipers are 151 grams each, compared to 175g for dual pivot rim brakes.
- Brifters are 200g heavier each than the comparable non-hydraulic brifter
- A rotor is about 112 g plus bolts
- Front hub for discs is 353g compared to 210g for the same thing without a disk mount
- Rear hub for disks is 368g compared to 350g for a rear without disk.
- Hose and Oil is probably on-par with inner and outer cable weight.
- Caliper pads and hydraulic pads are probably about the same
RESULT: The cube's disk brakes weigh approximately 850 grams more than the felt's rim brakes weight. That the whole bike is only 350g heavier will be down to weight savings elsewhere.
Another point is that the disks are both slightly further forward and further aft than the rim brakes, if everything else was equal. The lengthening of the weight on the Cube will help on a fast straight, but will slightly resist turning more than the Felt.
But I feel there's something more to it than "heavy back end weight" Lets look at the other significant difference.
Here's a blinky between the felt and the cube. I've tried to position the two wheel axles together, and the ground as the consistent third point.
Break that down more - here's the older 2013 Felt with a wire on the centerline of the frame:
And the same for the more recent Cube
Finally, here's the two wire frames superimposed, kinda.
From that, we can see the seat tubes are pretty much identical. Downtube and chainstay are very similar (allowing for distortions in two different photos)
The seat stays are significantly different line, but that's probably going to have more impact on compliance over bumps rather than handling.
Sherlock Holmes once said something about eliminating all the possibilities and whatever remains must be the cause. So if its not the backend, perhaps its the front?
Notice the head tube on the felt is steeper. And quite significantly at 72.5 degrees on the felt vs 73.5 degrees on the Cube. (assuming a 56cm frame on both)
This leads to the Cube having just about double the trail. And we know that:
More trail equates to slower steering. Making a bike feel more stable like it’s steered ‘with your hips’ (leaning).
The newer bike's geometry at the front makes for more stable and solid steering. You'll be more planted and confident at speed and on descents. The trade off is slightly more effort is needed to turn, with slightly more commitment required to initiate the turn. But the bike will want to straighten up by itself.
I suspect the Cube is much easier to ride no-hands than the Felt ever was.
Go with your gut - either return it now if you think you'll never like it. Or give it a 500-1000 km of riding to get acclimatised. I know I take double that and more to get really at home on a bike. Check out some hill descents that you may have logged in Strava or whatever from the Felt, and compare them with efforts on the Cube.