Today as I was riding I tried to breath only through my nose, I wasn't really pushing it, probably around 16-18 mph but I couldn't maintain it. I felt as soon as I exhaled I needed to inhale after a few cycles of these in and out breaths I felt like I was going to hyperventilate.

So I was wondering if this (breathing only through the nose) would be a way to simulate training at altitude?

It's hard to make out what you are interested in from your post. Do you want to train to perform better at altitude, like climbing or biking over passes? Or are you trying to modify your whatever so you can perform better near sea level?

As I understand it (and someone correct me if I'm wrong) athletes train at altitude in order to increase their aerobic capacity; this is because the oxygen is at a lower concentration than at sea-level. Therefore, the body must become more efficient at using avaliable oxygen. So if you only breath through your nose during aerobic activity the thinking is that your lungs will receive less oxygen forcing your body to become more efficient.

It's about improving my aerobic capacity at sea-level.

Atheltes don't train at altitide to get these benfits, they live there.

My understanding is that the oxygen density is much lower at higher altitude, forcing your body to work harder the entire time you are there, both on and off the bike and the measurable benefits are to be had by living there for weeks on end, not just training there for a few hours.

No. Some (not all) athletes train there, albeit for an extended period of time; not everyone can live at altitude. So I'm thinking if I start breathing through my nose on some of my bike rides week-after-week it's going to have a positive effect in my body's ability to convert oxygen into energy.

Looks like I'm going to just have to experiment with this and get back to you'll. BTW, I was just researching this and seems there is some basis in my thoughts; it's called Hypoxic Breathing
https://www.livestrong.com/article/39...s-for-cycling/

OK. If you were training to climb at altitude, you'd need to spend a lot of time up there, a week or so for every 1000 meters, because there are many phsiological changes which occur to adapt to high altitude. However if you are just trying to improve your sea level performance, what you are after is increased hematocrit. You get that from the release (or injection) of erythropoetin (EPO).

The kidneys release EPO when they sense below normal blood oxygen saturation. There are several ways to create this lower saturation. Sleeping at high altitude or in a hyperbaric tent is one. Hypoxic breathing is another. However the most common and most useful is simply anaerobic intervals. They not only create the hypoxic state, they make you stronger, both mentally and physically. So that's what most cyclists do.

A set of 3X3X3 intervals well above lactate threshold once or twice a week will do the trick. In 2 weeks to a month, your body will respond to the stimulus by creating more red blood cells, thus increasing your hematocrit and your performance.

You should have a good bit of base and LT or sub-Lt interval work before doing these harder intervals.

True altitude simulation would be done in a swimming pool, riding underwater with a snorkel.
You'll get the added benefit of the water pressure on your chest, plus it'll keep you cool on those hot summer days.