Proper Posture for Preventing Painful Posteriors on Penny-farthings?

Penny-farthings are known for their relatively harsh ride, and lack of stability. There is no option on many frames to replace the saddle, although there were some that you could. I would get a sprung Brooks saddle, or similar, which might add a bit of comfort without significantly sacrificing the vintage look.

It will add a bit of suspension effect as well.

Something like this:

I am not sure about Penny-farthings, but on a normal (what is normal?) bike you aim to sit on your sit-bones which are the two bones right in front of your tail bone. Sit those on the rear of the saddle where it is comfortable. This may require seat adjustment with attention paid to your riding posture. Eventually you will find your own sweet spot for where to position your saddle. Being as upright as you are on a Penny-farthing I imagine you would want a nice flat position with a small but comfortable seat.

If it is back pain that you are experiencing being too tense while riding could cause that. Especially when exerting a great amount of effort you want to be relaxed as to be able to move easier without straining your muscles.

I ride both Penny-Farthings (UDC 50" MK3 & MK4 models) and "safety" bicycles- those odd things with symmetrically-sized wheels ;-) - so can say this is not an issue particular to Penny-Farthings, but any type of bicycle really. My experience on the below advice offered is based on riding my Penny-Farthing at least 20 miles/day most days, but at least 2-3 times a week 30 miles/day.

(3) Big Factors Affect Cycling Comfort:

• Saddle Choice > Sprung Saddle: @zenbike 's recommendation to use a sprung saddle is right, but for the wrong reason: the stiff ride one experiences riding a Penny-Farthing is not down to stability issues, but lack of pressurized tires. Penny-Farthings have solid tires, so there's no dampening of shock when you ride over bumps. A sprung saddle will definitely make your ride more enjoyable. I actually bought both a Brooks "B17" and Brooks *Flyer" (pictured in @zenbike 's post) to do a head-to-head test. The sprung Brooks "Flyer" model saddle is hands-down the winner. You want a sprung saddle on your Penny-Farthing to compensate for the stiff ride resulting from lack of pressurized tires. A final note on saddles: although it seems counter-intuitive that a bare leather saddle would be more comfortable than a padded plastic saddle, my experience is that the bare leather saddle once broken-in, is superior. YMMV. Plastic saddles are also going to be more sweaty and can increase the chance of getting saddle sores which will not merely be uncomfortable, but can bench you for a least a few days, possibly more.

• Saddle Adjustment: @chris-belsole is right: correct saddle adjustment is just as crucial to a Penny-Farthing as it would be any "safety" bicycle. Incorrect saddle adjustment has a huge impact on your comfort.

• Padded Cycling Shorts: Something not mentioned by the other posters is wearing padded cycling shorts that incorporate a chamois. Combined with a good quality sprung saddle will significantly improve riding comfort. Choosing a pair of padded cycling shorts is a huge rabbit hole I won't go down here. What I will say however though is that if you ride with them, wash them after EVERY ride, or you risk getting saddle sores- you definitely don't want those!