What do the different sizes mean, like 90mm? Is it a measurement on the bike I need to take? How do I know which size to order for my bike?

It is the length of the stem, center to center. Measure your current stem from center of steering tube to center of (diameter of) handlebar. The longer the stem, the longer the reach. A small change can make a big difference in comfort level. Measure carefully. This is a very nice suspension stem. It takes vibration away from hands.

Is there a sku for ordering one with shims on amazon? my handlebars are 1" (25.4mm)

Shims from Amazon https://www.amazon.com/MUQZI-Handlebar-Aluminum-Bicycle-Conversion/dp/B09BQB9PC1/ref=pd_bxgy_sccl_1/144-7925088-0949638?pd_rd_w=x1LpD&content-id=amzn1.sym.26a5c67f-1a30-486b-bb90-b523ad38d5a0&pf_rd_p=26a5c67f-1a30-486b-bb90-b523ad38d5a0&pf_rd_r=WVEC5RMXZ7MATFBA233V&pd_rd_wg=HciQV&pd_rd_r=d2e06161-e947-4344-b136-a5d96558c876&pd_rd_i=B09BQB9PC1&psc=1

frist of all will this fit a domane alr 31.8 handle bars also do not understand the measurements 6×100 mm

Yes, it will fit. I just installed mine on my Domane SL5. 6 = is the angle of the stem, either 6 degrees rise or 6 degrees lower depending on the way it's installed. 100 = is the length of the stem, from center of steerer tube to center of handlebars.

Color	Black
Size	6 degree x 110mm
Style	Stem Angle: 6 Degree
Height	3 inches
Length	4 inches
Weight	0.85 Pounds
Width	8 inches
Shape	Fits 1 1/8in Steerer Tube (Standard)
Number of Items	1
Batteries Included?	No
Brand	REDSHIFT
Department	unisex-adult
Manufacturer	Redshift Sports
Item model number	Redshift Sports
Product Dimensions	50 x 50 x 28 cm; 285.76 g
ASIN	B01MU2HUWW

ShockStop Suspension Stem for Bicycles, Shock-Absorbing Bike Handlebar Stem for Road, Gravel, Hybrid, and E-Bikes, fits 1 1/8in (28.6mm) Steerer Tube

Description

Specifications

Common Questions