One is bigger than the other?

A frame that takes 1.5 can generally take a 1 1/8. Usually. Via adapters.

So we’d need to know the frame and fork. And ideally if you’ve an expensive headset you want to continue with you should mention that too.

one is 1.5″ across. one is 1.125″ across
There also steerers that start at 1.5″ ‘at the fork end’ and narrow to 1.125″ where the stem bolts to (aka tapered steerer tube)

If you want to swap the steerer tube, you’ll need a complete new Crown-Steerer-Unit (CSU) to fit your fork. You’ll need the spring and damper assemblies swapped across to the new CSU.
The CSU will likely be costly
A Fox 32 CSU can be had for ~£135 from mojo
iirc RRP 32mm Rockshox CSU’s are a bit over £200 and 35mm over £300
Maybe you can get an RS one for less
and ime TFtuned won’t sell you one

Trevon – we need to know the frame and what fork. Then we can help rather than speculate.

How old is the bike? i.e. what is the year of manufacture? I’m just looking at the current SL4 on the cannondale site and it has a 1.5head tube on the frame and a fork with a 1 1/8th steerer, hence it’s already set up to accept 11/8th forks…so…if I’m reading your original post correctly…you shouldn’t have a problem fitting a new fork!

You’re changing the stem? Ok let me help you out a little here, click on this link:

Cannondale Trail SL4

and then click on the ‘SPECS’ tab on the left hand side.

This will then bring up alot of information about the components fitted to your bike.

For example, as we’re talking about the stem, the stem is described thus:

Cannondale C4, 1-1/8″, 31.8, 7 degree

Which initially seems quite confusing, but to break it down for you it actually becomes quite useful when you get to buying new components.

Firstly it tells you the Manufacturer and model of the component, in this case Cannondale’s own C4 stem.

Then it tells you the size of the steerer tube is attaches to: 1 1/8th

Then it tells you the size of handlebar it accepts: 31.8mm

(NB: When you buy your new stem these two measurements must be the same as teh existing component or it won’t fit!)

Finally it tells you how many degrees of rise the stem has (i.e.how many degrees the centre of the handlebars are above the centre of the steerertube clamp.)

The single measurement missing from all this is the length of the stem, but this is easily measured with a tape measure, again from the centre of the handlebar clamp to the centre of the steerertube.

Hope all that helped!