I think it is the same spacing front and rear, so you can have a different gearing on the two wheels.
From the surly website
Fork: Suspension-corrected (100mm travel). 447mm axle to crown, tapered straight blade, 4130 CroMoly. 51mm IS REAR disc mount. 135mm-spaced dropouts, 17.5mm offset
@dancing james started
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I think it is the same spacing front and rear, so you can have a different gearing on the two wheels.
From the surly website
Fork: Suspension-corrected (100mm travel). 447mm axle to crown, tapered straight blade, 4130 CroMoly. 51mm IS REAR disc mount. 135mm-spaced dropouts, 17.5mm offset