When I do rides led by a bot-pacer, for the 1st 10min. or so the bot is clearly at higher than the stated power level (e.g. often 3.0+ for a stated 2.6 w/KG) and as I continue with the ride the bot at some point thereafter begins riding at the designated power level and gets left behind. What is going on with this?
Robo Pacers do not ride at a fixed power level. On an incline the power is higher, on a descent the power is lower. Exactly where on the route you show up when you join determines how much power the pacer is doing.
This is not new, and Zwift HQ calls it a feature. Take a loot here for an ongoing discussion: Feature Request: Bring back static pace bots please
How much variability is there from the stated w/kg pace in the higher-powered uphills and lower-powered downhills and does that remain consistent throughout the ride?
Up to 10% power increase uphill. Up to 20% decrease downhill. It remains consistent.
From vague memory, once the slope is over about 4% the RP will be at their max or min power.
I was on a ride today and doing 2.1wkg while being overtaken by the bot doing 1.8kg, all on the flat. Go figure.
I’m guessing you weigh significantly less than 75kg.
or was out solo riding and got passed by the huge drafting blob?
If you’re asking me I’m 79.45kg as of this morning. Pre ride.
I (second) guess it was this one 
The bot might put more power briefly when going up a hill, but the speed of the bot is influenced by how many people are riding ahead of it.
If there are many ahead of it putting out slightly more w/kg than the bot and staying right in front of it then it will go faster. And everyone else chases to avoid being dropped.
Rapha 500 last year proved that. Jacques group moving at Constance speed, etc, Constance group going significantly faster than normal. Then when everyone finished the 500 efforts the speeds dropped back to normal.
None of this is related to rider weights, it’s looking at the average speeds per lap of Tempus Fugit or Tick Tock.
I completely understand that larger groups with the pacers will ride faster than smaller groups. What I struggle with is why it requires a higher average w/kg to keep up with a larger group vs a smaller group with the same pacer. If the pacer is at the same average w/kg regardless of group size and a large group is riding just ahead of it making it go faster, why can’t I just apply the same average w/kg (as with the smaller group) and get the same benefit from the group riding just ahead. Instead I can need up to 0.3 - 0.4 additional w/kg to keep up with a larger group.
There may well be drafting inefficiencies etc to consider, but if anything the overall amount of draft benefit in a small group should be less than a large group where you can stay in a max draft area more easily.
Agree in real life, huge groups give you a big draft to follow that is very easy to stay with provided you know how to corner and have confidence of riding in groups. You barely have to put out much power. Same with smaller groups, just not to the same degree, but still easier and much smoother than Zwift. In Zwift you also don’t roll the same as in real life. I might not know how to ride in Zwift, but I do know about riding in fast bunches in real life.
I think this is probably to do with the blocking of the pack dynamics making it more difficult to overtake people, among other things related to the top secret pack dynamics behaviours.
Also notice that other riders behind can push you out of the draft, that should not happen, they should be forced to go around you and overtake without draft, but it doesn’t work that way. That doesn’t Make Racing More Exciting™ does it?
It just seems sometimes like the pack behaviour is bandaids upon bandaids and enormous groups of 100-300 expose the shortcomings. Other things to notice in the enormous groups is when you are teleported into the middle of a group I have seen the view shaking - I presume because the avatar is being blocked but has nowhere else to go.
Probably your best bet for staying in groups in normal rides (without sprinting) is to use a Garmin to control your trainer and then tap the arrows on the Garmin screen to adjust power. You behave more like the robopacer with a steady fixed power.
Go watch the front of the Constance group at certain times of the day, they all use this trick with power set at between 4.3-4.5w/kg and increasing it ahead of hills. They very effectively stay ahead of the robopacer and most of the other riders.
Agree, there might be lots of reasons why extra power is required in larger groups. However, my understanding is that the pacer (other than being a bot) is in every respect treated like a regular rider and subject to all the same pack dynamics, bandaids, blocks, push and pull, and draft idiosyncrasies in Zwift. So if the pacer average w/kg is set at exactly the same level regardless of pack size, then why shouldn’t I be able to apply the same w/kg to keep up, regardless of pack size. If I am treated to extra stickiness, or pushed out of the way into a less favourable position more often, then presumably so is the pacer.
[I’m obviously not saying I would need the same w/kg as the pacer (different weight, height, bike etc)]