Why not just come out and tell her that you cum like crazy when riding? Is this just a friend that's a girl or a sex friend?
I have always done the same ever since i first got on a horse I grind into the saddle and push my clit hard into the saddle, I love having my legs either side of this big Hunter of mine and when he snorts I can feel it vibrate through my saddle which he rarely does it so delicious if he does,I was 13 the first time it happened my pussy was tingling like mad as my pony slowly walked and i found i was enjoying how the saddle felt between my legs i had my first proper orgasm that day, Over the years I've become so conditioned to it that just the smell of a horse and I start to get wet.
My cousin tried my motorcycle which was a bit broken. It vibrated quite a bit and she sat on it and moaned how she was going to orgasm. I told her to get one and gave a bit of throttle in a rhythm and she got one. After it she told me how she have got orgasmn on horse back too.
So glad I saw your posting, now I don't feel so perverted when that happens to me whenever I go horseback riding.