I live in an area with brick roads. This means it's really bumpy and sometimes with small potholes that hurt when I dip into them. I'm looking to buy a new ebike but I can't afford anything expensive. Should I look for a front Fork suspension, or one with a seat post suspension? I don't know much about bikes. Is front Fork mostly for off road or does it absorb vibrations to my arms? It seems like a seat suspension would be ideal for me since it hurts my bum the most. Thanks for any help!
Instead learn to stand up when expecting bumps. Your legs will comfortably take the force of the impact, and it makes you more stable on the bike as you can adjust your position to balance.
A good seat suspension will help, especially if you have long periods of riding over rough stuff and you don't want to stand for that long. But the simplest and cheapest solution is just to use your legs.
If you're worried about rough roads causing numb hands or sore wrists, front suspension is what will give the greatest benefit. For smaller, higher frequency, vibrations, the biggest tyres that will fit, run quite soft, will be more effective. Cobblestones could need both, though the brick paving in my street is fine with neither. I don't much like the short travel front suspension used on a lot of e-bikes and some hybrids, but this is where it's useful. With a motor you won't really notice the extra effort it demands compared to a rigid fork.
A suspension seat post doesn't help your arms, but it helps your backside, and back. These are easy to retrofit on most bikes - just match the seatpost diameter (perfectly) and length (roughly). I've fitted one to the back of my tandem, as the stoker can't see to anticipate small bumps or time them right if warned by the captain. Note that they're generally, even the simpler sort with a spring in a tube, adjustable for rider weight. Only the most basic have no adjustment at all - avoid those.
You note that your bum is uncomfortable. Skin discomfort (chafing) is often caused by a saddle that's too wide and soft, so it presses against you and moves. A bruise-like feeling can be caused by a saddle that's too hard, but also by one that's too soft - you sink into it and pressure builds up on tender parts rather than the sit bones. Saddle shape plays a part too. A (deeper) recess or a slot in the middle can be a big help, and some people get on better with a flatter saddle while others prefer a more curved top.
To make your neck and shoulders more comfortable on rough roads, both sorts of suspension, soft tyres (Silca's calculator is designed for efficiency but is good for comfort - but watch your minimum tyre pressure), and a fairly upright riding position all help.
Before answering strictly the question, two very important criteria:
- tires: large supple tires require less pressure at equal rolling resistance than narrow and rigid ones, and that can make a huge difference in comfort. Entry-level bikes often have rigid tires (only notable exception are Schwalbe Big Apple/Big Ben ranges). If you puncture protection is not a priority in your area, you can also compromise on that.
- saddle: saddles are highly personal, so if you plan to use the bike on shortish rides/utility cycling, a saddle on the plush side can also make a big difference.
But that would deserve separate questions.
To answer the question now: seat post without hesitation, but not all of them. There's a common understanding that suspension needs to damp a vertical motion, but the shock that are coming from the saddle is actually a rotation around the front wheel axle: there's a vertical and an horizontal component. A good suspension seat post needs to damp an oblique motion, not a vertical one. It's then better to have a "parallelogram" design, such as the CaneCreek Thudbuster (given for reference, there are other brands that do that as well - Redshift, Suntour, XCM). Their price may seem high, but if you value comfort, it's a good investment. Very compliant carbon seat posts work also surprising well. I also noticed that such suspension also lower wrist pain, what would be logical if shocks are transmitted from the saddle through the back and arms. I recommend this article if you want to learn more about seat posts.
As you mention, front suspension are mostly useful offroad, and are not good at damping small and fast vibrations - especially the entry level ones. Also another point: good suspension need regular maintenance to keep their efficiency.
A suspension fork is likely not what you want here. They tend to be very well designed to deal with hard impacts of the front wheel (such as going off of a sudden drop or landing from a jump), but are poorly designed to deal with vibrations. They also tend to eat some of the power you would put into pedaling (the physics behind this are a bit complicated to explain, but the short version is that part of the energy goes into compressing the suspension a tiny bit), and require a nontrivial amount of routine maintenance.
A good suspension seat post is probably a worthwhile investment for you though, but the good ones are not cheap. ‘Good’ in this case generally means a parallelogram design (Redshift’s Shockstop suspension seat-post is a good example of a very good design, I’ve ridden them before and can attest that they work excellently). Other designs generally only move parallel to the seatpost, which is decent for dealing with hard landings, but almost worthless for dealing with the type of things you’re having issues with.
Aside from that, you may also want to consider:
- A proper bike fit. If you’re already looking at getting a new bike, spend a bit of extra money to go through a proper bike fitting. This sounds kind of stupid, but it actually can provide a huge improvement to comfort. Even just getting the saddle and handlebars in the right place for your body size and shape has a huge impact on how it feels to ride a bike.
- A good saddle. Just like a proper fit, the saddle has a huge impact on how the bike rides. A lot of bikes come with either generic saddles that don’t really fit anyone very well, or with oversized ones with excessive cushioning that are absolute crap for any kind of serious riding. Good saddles are not cheap, but if you can find one that works for you, it will be zero maintenance long-term and will likely significantly improve how you feel after a ride.
- Better tires. Larger volume with lower pressure is generally preferable here, as well as less rigid tires when possible. Wider tires (which is how you get higher volume) also have a secondary benefit of handling better in poor (wet, muddy, icy, etc) conditions, so they may be worth investing in for other reasons as well. Better tires will almost always be less expensive than most of the other options here, though may not be as effective. Ask at your LBS and they can probably point you at some good tires to help here.
- In addition to suspension seat posts, you can get suspension stems. These let the handlebars flex up and down enough to dampen vibrations, but unlike a suspension fork they don’t generally require lots of maintenance and they also don’t really eat too much out of your power output. Just like good suspension seat posts, these are generally not cheap though, and it may also be challenging to find one that isn’t designed for a very hunched-over riding position.
- A carbon-fiber seat-post and/or carbon-fiber handle bars may help significantly. Carbon-fiber parts flex and deform a more than aluminum or steel, so they actually can absorb a surprising amount of vibration before it gets to you. They’re not cheap though.