Mountain Biking - Noobie Bought The Wrong Bike ???

Hi all,
I'm new to the site and this is my 1st post so be kind :-)

I've just bought a GT Agressor XC3 from Halfords which seemed like an exceptionally god spec for the price, I've got it home and ridden it though and I hate it !!!
I have a very old Free Spirit which was falling apart but the new one it feels as though my legs are coming up to my chest and the only way to stop it is by raising the seat so high it's then uncomfortable on your hands and I can't touch the ground whilst seated.
I measured how far the middle of the crankshaft is from the ground and the XC3 is a good two inches higher than my old bike.
My query is does frame size affect how far the centre of the crank is from the ground or does it only affect the distance from the bottom of the cranck to the top of the seat tube ???
Not quite sure what I can do with this, I'm not damning the bike it's a great bike BUT it just doesn't seem to be the right size for me.
I'm 5"7 and my inseam / inside leg is 29" and it's a 18" frame.
Do you know if I have any right to change it, I've taken it our for about 5 miles of cycling, does that mean I have to live with it ???
Any help / advice appreciated and thanks in advance!

A 16" frame would be the ideal size for you. I'm 5'10 & 18" frame is good for me, though I prefer 17" on some bikes. If you bought it from a local bike shop (LBS) go back & see if you can swap it out for a 16". If you haven't taken it out on any trails yet or at least give it a good cleaning if you have, they might take it back. If you ordered it online, same thing but try shipping it back.

Sam, from the description, it sounds like he's saying it's too small. Plus, he already said he got it from Halford's, not the LBS.

To the OP, I would think an 18" frame would be plenty big enough for your height, in most cases, so maybe the geometry is just not a good fit for you, and you should try a different bike altogether. Definitely try a larger frame at least. As far as whether or not they'll take it back, you'll just have to ask them.

I measured how far the middle of the crankshaft is from the ground and the XC3 is a good two inches higher than my old bike.

Sounds to me like the problem is the frame is too tall for this person. They're saying they lowered the seat so they were a comfortable distance from the ground, but that made their knees come up too high. It sounds to me like the solution would be to get a smaller frame so that they could be a comfortable distance from the ground with the seat height in proper proportion to the pedals. You can adjust seat height on any bike; you can't adjust crank height without changing the frame.

Sam, from the description, it sounds like he's saying it's too small. Plus, he already said he got it from Halford's, not the LBS.

To the OP, I would think an 18" frame would be plenty big enough for your height, in most cases, so maybe the geometry is just not a good fit for you, and you should try a different bike altogether. Definitely try a larger frame at least. As far as whether or not they'll take it back, you'll just have to ask them.

Think I'll try and see of they'll change it, I think though it's too large not too small.
If the crank was nearer the ground I could put the seat higher in relative terms and then my legs wouldn't bend quite so much and I could extend my leg fully when furthest from the seat ie at the bottom if that makes sense.

My mistake. I took "legs are coming up to my chest" to mean when the saddle was at a normal height because of the frame being too small (versus lower than normal for proper stand-over), thus having to raise it.

My mistake. I took "legs are coming up to my chest" to mean when the saddle was at a normal height because of the frame being too small (versus lower than normal for proper stand-over), thus having to raise it.

I was very clear in the original, the only choice I have is to move the saddle up so high that you then can't get your feet to the ground and you're right over the handle bars and acrrying the weight on your hands.
I not sure where frames differ in size, will a smaller frame take the crank closer to the ground or is it just how different models are designed that'll make the difference ??

I don't think it's normal to change the BB height with different sizes, but definitely not for those GTs. Check out the geometry charts on their website, and you'll see that they're all the same.

I'm 5'7" and ride a 15" frame. Your 18" frame is waaaaaaay to big for you. Go to your local LBS and try out 15" or 16" frames.
>> my legs are coming up to my chest
Is this because frame is too big for you and you are stretching to hold on to the handlebars? Again try a smaller frame.

What is the return policy? It's new, so I wouldn't ride it anymore and return it for a bike that fits! I'm 5'10" and ride an 18" bike just like a previous poster.

Absolute result, complete refund, must admit impressed with them for that.
Now just have to find and try something in a better size !!
Thanks to all for your input !!!