I rode all last summer and autumn on an old mountain bike that was too small for my physical size. I'm 5' 10" with a 33" inseam, and the bike had a 16" frame. Other than some knee pain caused by not being able to raise the saddle high enough, I was pain free. I had a great time.

That old bike is gone now, and I've purchased a new mountain bike with an 18" frame. The size feels perfect, and the knee pain is gone. But now I'm confronted with a new issue. I'm getting rubbed raw between my scrotum and thighs. These are not saddle sores, as they're not around the buttocks or perineum. I've never in my life had an issue with saddle sores. These raw spots are up higher, to the left and right of my penis.

Truth be told, I don't wear any sort of specialized bicycle clothing. Just underwear and either jeans or cargo shorts. I'd blame the clothing, but it's the same exact clothing I wore last year. The clothing hasn't changed, and my mileage hasn't changed either. I just take it out for nightly trips to Dunkin' Donuts and back. A total ride of about 4 miles.

I thought it might be the new seat at first, so I tossed on the seat that I used last year. The problem persisted. I have no clue as to what is causing this. The only changes that I can think of are the bike itself and me aging 6 months.

Any good ideas? Saddle tilting? Saddle height? Different distance between saddle and handle bars? Has anybody else experienced excessive rubbing this far up?

Bike shorts and creamy petroleum jelly.

So you've had this same exact problem? I've read lots of posts about saddle sores, but never posts about chafing that is up so high?

I don't understand why this never happened last year.

Never had the problem you have.
I have always used CPJ to prevent rubbing.

If the new bike is bigger then the upper part of your legs are now making more contact (rubbing) with the nose of the saddle because your legs are more extended than before. Maybe a narrower saddle will help. Or just wear bike shorts. Underwear is the bane of cyclists!!

liners or lined shorts - no underwear. put a dab of something where you have the chaffing. consider another saddle or a road bike.

I'm guessing shorts will fix it, too. The main reason to wear them isn't for the padding, which is too thin to do much good in most cases, but to eliminate chafing. If you don't want to wear Lyrca (I don't), then mountain bike shorts work fine, or get some cycling underwear, which fit like Lycra bike shorts but can be worn under regular shorts. REI carries them for sure. One brand name is Andiamo.
Jeans are death on long rides. i started mountain biking in the early '80s, when it was pretty much mandatory to wear jeans and a flannel shirt. wore my thighs OUT...

You're recommending a road bike to get rid of chafing? Wow, that's a new one.

Buzz, there are a few factors you might want to consider. Your saddle is probably fine, but I find that a level or very slightly nose-pointed-down saddle helps with pain in the nether regions. Furthermore, don't ride long distances or over difficult terrain with just regular shorts. The seam going down the middle, and for us guys, another seam often crossing over, is what's rubbing you the wrong way. Bike shorts are a very easy fix for this issue because they don't have that seam by the crotch. If you're anxious about the idea of tight, short pants, they also have shorts that have a baggier, cargo-shorts-like shell and a chamois on the inside. You get the best of both worlds.

Take a look at the difference at these links:
https://www.rei.com/product/797381

https://www.rei.com/product/796171

Also, I don't recommend chamois cream right off the bat. It's a little funky your first time, and I find that most people don't really need it on rides under 60 miles. But yeah, don't wear underwear with bike shorts. It's very counter-productive.

chamois lining is for moisture management
you can wear lycra bike shorts under anything

Definite possibility, are you spinning more than mashing now that the bike fits better? If you can't handle Lycra, get some liner shorts as underwear while on the bike. You will probably have to do something to let the irritated skin heal as well.

Thanks for all of the excellent comments. I finally broke down over the weekend, and bought a pair of cycling shorts. To be specific, I stopped by Sports Authority a bought a pair of Canari Velo II shorts. They're nothing fancy, I'm sure. They were just the cheapest pair I could find at $29.99.

I wore them under a pair of ripstop cotton cargo shorts for a ten mile ride yesterday morning, and could tell after the first 1/4 mile that all of my problems were solved. It was an unbelievably comfortable ride, even under 85 degree heat and 52% humidity. I didn't want to take them off when I got home. My daughter was running under the sprinkler to stay cool, so I just took off the cargo shorts and used the Canari's as a bathing suit and joined her. They're awesome.

My wife also likes the way I look in them.

Weird. I ride 30 miles with boxers and regular shorts, no problems. I do prefer to wearing wicking boxers from REI, but have used regular cotton. Especially with you talking about 4 miles or 10 miles? Should be able to wear jeans and flip flops for that distance and have no worries.

I agree. I did it all last year with nothing special. It's just this spring with the new bike that's giving me problems. But for what it's worth, the cycling shorts have cured all problems.