Measuring steerer length

mistertommyt

I am going to get the fork cut down for my new road bike this weekend. It's not a cheap fork and I'm feeling under pressure to get the measurements perfect. Having measured everything to within an inch of it's life I'm fairly confident I'm good to go, but wanted to ask if there was a more simple way to measure this first.

Having been fitted to my old bike I know exactly where my bars need to be. Therefore can I just stand both forks up next to each other and mark the new steerer where the old is cut? The fork lengths and rakes are different on each, so the amount of steerer tube would be different once cut, but surely if the overall length from mid axle to top of cut steerer is the same on both then I should be ok?

Sorry for the long post, just don't want to mess this up.

Cheers.

Ben6899

How long is the head tube? And is the stem the same type?

For example, if you cut it the same length as the 'old' fork and the 'old' bike has a headtube of 130mm then it might not be long enough for the 'new' bike if it has a headtube of 170mm. So don't use the method of simply comparing forks.

Put the new fork in the bike with all the spacers, stem etc in place. The final cut end of the tube needs to be ~3mm below the top spacer or stem so base your measurements on that.

smidsy

Unless it has a carbon steerer then it needs to be longer.

nicklouse

fit it to the new build. use lots of spacers and adjust to suit then get it cut.

Ben6899

smidsy wrote:

Unless it has a carbon steerer then it needs to be longer.

Beyond the top of the top spacer?

smidsy

No above the stem, even if spacers not required.

mistertommyt

It is a carbon steerer. Back to marking the cut point on new steerer using old cut point on previous fork, I'm struggling to picture why this wouldn't work. The new frame does have a 15mm shorter head tube, but surely as I need the bars to be at the same height as previously set up I'll need to compensate for this shortfall with spacers. No?

mistertommyt

To be clear, I'm not suggesting cutting the steerer to the same length as the old fork. I'm suggesting measuring from mid axle to where the old fork was cut and applying the same total length to the new fork. Does this make any sense? My brain hurts.

Ben6899

smidsy wrote:

No above the stem, even if spacers not required.

Aah, we agree. That's why I said top of- stem or top spacer.

Ben6899

mistertommyt wrote:

To be clear, I'm not suggesting cutting the steerer to the same length as the old fork. I'm suggesting measuring from mid axle to where the old fork was cut and applying the same total length to the new fork. Does this make any sense? My brain hurts.

It doesn't make sense to me, but it's been a long week.

Fit everything together and then see where that leaves you. Ignore the old bike and fork.

smidsy

mistertommyt wrote:

To be clear, I'm not suggesting cutting the steerer to the same length as the old fork. I'm suggesting measuring from mid axle to where the old fork was cut and applying the same total length to the new fork. Does this make any sense? My brain hurts.

You can not rely on this method. You said the rake of the forks is different so that will scew the measurements.

Also the carbon steerer needs to have about 3mm above the stem (even if you are not intending on placing any spacers above it), like this:



So the steerer extends above the stem clamp point and then sits flush with the spacer (top cap does not sit inside).

This is to allow for any shear that may occur on top of the steerer.

This differs from metal steerers where you leave the steerer short and the top cap sits inside.

mistertommyt

Understood. Thanks all.

Velonutter

Simple thing, measure from the floor to the top of the steerer on your old bike, that then is the height that you want your new bike to be at.

Then transfer the measurements to the steerer, remember, measure twice, cut once!

For safety, allow say an extra 10mm spacer when you first do it, which you can easily cut off once you have gained some confidence.

I have got to cut all my steerers down after allowing a 10mm spacer, but haven't got round to it yet.

velojay

Surely just transferring the measurement from your old fork does not take into account any difference in BB height or top tube length or seat post and head tube angle! You want your bars to be in the same position compared to your BB and saddle position and not your front axle or the ground?

Best method is to get saddle position the same compared to BB by setting at the same saddle height and setback then getting the bars in the same position relative to the saddle using the same saddle to bar drop measurement and tip of saddle to middle of bar clamp measurement. That way you take away any difference in frame geometry and top tube length. You also may require a different length or angle of stem and different spacers to achieve this. Once everything is the same and you are happy then cut steerer to length.

It's a long way of doing things but the only accurate way of replicating your position on two different frames.