Mountain Biking - Fork and geometry question.

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mjw16
10-03-05, 06:41 AM
I'm thinking about going from a 100 mm travel fork (Marzocchi EXR) to an 85 mm travel (Marzocchi MX Comp). Is this new fork effectively 15 mm shorter in total length or, is the overall lenght the same? Should I add 15 mm to the steerer tube length to maintain the same geometry?

Dannihilator
10-03-05, 08:09 AM
You won't get the same geometry, the bike will be notably twitchier due to the loss of 15mm. As long as the steerer is the same length as the old one, the cockpit geometry should be the same.
C Law
10-03-05, 08:18 AM
It would be imperceptibly shorter in my opinion.

Leaving 15mm in additional steerer tube will not compensate for the lower axle to crown height of the reduced travel fork.

you could add a spacers above the head tube if you wanted to maintain the exact handlebar position you had before.

But adding spacer above the head tube will not change your axle to crown height as stated above.

The front end of the frame will still be a bit lower and the headtube a bit steeper. Neither would be an issue from such a small change. It will make the bike ride slightly twitchier.

stapfam
10-03-05, 03:27 PM
Take into consideration Sag when you put your weight on the bike. The actual loss of 15mm, will be less if the fork is set up to have minimum sag, and an 85 mm fork will naturally have less sag than a 100mm. So your 15mm will not be.

Going the other way-- I went from 135 mm to 150mm, and did not even notice the difference. What I did notice though was the plushness of that extra 15mm. Going down to 85mm may give you a firmer but harsher ride.
seely
10-03-05, 08:22 PM
I'm thinking the MX Comp is a significantly lower end fork than an EXR... correct me if I am wrong as I don't know Marzocchis real well.
duckliondog
10-03-05, 09:23 PM
I'm thinking the MX Comp is a significantly lower end fork than an EXR... correct me if I am wrong as I don't know Marzocchis real well.

Yeah, you had it backwards, the MX Comp is higher end than the EXR. I have an MX Comp w/ ETA, the 120 mm version. It's an awesome fork! The MX series comes in lots of different sizes, why don't you just get the 105 mm version? I'd much rather be 5 mm higher than 15mm lower.