Can anyone help explain why my hands start to go numb after riding for a short time and what I can do to stop it. I ride a mountain bike at the moment
your hands are most likely going numb because you're putting pressure on the nerves in your wrist: the ulnar nerve is the one that's the usual culprit, it's fairly easily compressed and you can quickly lose sensation in your fingers.
the first thing to do, if you haven't already, is to get some cycling mitts: go for ones that have specific padding to counter compression of the nerves in your hand. there's loads to choose from, specialized ones are generally very good for the ladies.
secondly, check your bars and controls. The lower your bars in relation to your saddle, the more weight you'll bear through your arms. so a riser stem can help alleviate wrist issues. secondly - and this is a very common one - check that your brakes and shifters aren't set too high. the should be pointing down at the same angle that you arms are; if they're flat you have to angle your wrists and this can cause problems.
It's worth looking at your grips too: ergonomic ones like the Ergon range are good at spreading the load through your hand and relieving pressure points.
Classic sign of ulnar nerve compression is that your little and ring fingers go numb.
One of my bikes has handgrips which bulge a lot in the middle. If I wear padded gloves with them I get ulnar nerve numbness, but not if I wear unpadded. But I need padded gloves for my other bikes with straight grips or drop bars.
I have similar issue except the pain/discomfort is at the base of my thumb. I am new to road biking from mtbiking, so wonder whether I am trying to ride in the more upright position.
I'll have a look at whether the bars/hoods are at the right angle.
I can't work out whether the frame is the wrong size but don't think so and hope not.
Any other tips for identifying and eliminating the cause?
the base of your thumb is an unusual place to be getting pain, presumably you're putting pressure on that area which would certainly be down to your position but i'm not sure exactly what would be causing it. one possible culprit is bar width: if your bars were too wide and mainly ride on the tops the angle of your hand might cause pressure there. Conversely, if your bars are too narrow and you mainly ride on the hoods then the same thing might happen.
more of a guess than anything though
I have noticed that often people will set the saddle, nose down, to avoid numb nuts, but this tends to mean you are forcing a lot of weight on your hands, as you are constanly pushing yourself back onto the saddle.
Pain at the base of the thumb could be connected with riding on the hoods.
I was advised to try riding with my hands in a neutral "handshake" position when on the hoods. Imagine holding your hand out to shake hands with someone and try and keep your wrist straight when you rest on the hoods. It's easy to "sag" and let your wrists turn in (so you start seeing the backs of your hands) when you get tired, but I've found that keeping my wrists straight has sorted out the numbness and soreness that I used to get.
Also, just try moving your hands around on the bars - drop bars offer a variety of hand positions and even briefly moving your hands up to the flats or down into the drops for a few seconds can alleviate compression from holding one position all the time. It's also worth noting that when your hands are in the drops, you should be able to exert more force through the brake levers, if you need to.
I was having this problem commuting, but I threw my cheap and nasty gloves away and got a decent pair, and also fitted the Ergon grips that someone mentioned above. Haven't had any trouble at all since.