Threshold ramps for wheelchairs are typically shorter ramps designed to bridge a small step or doorway threshold. Threshold ramps are ideal for transitions less then 2 inches.
Most threshold ramps are between 8" and 36" in overall length. This will typically accommodate most thresholds transition requirements.
Threshold ramps are available in aluminum or rubber styles. The rubber styles are more easily modified for installation.
Wheelchair ramps typically need to be 1in in lengh per each inch in rise. This means if you have a 1in step, you would need a ramp with a length of 12 inches. ADA requires a slope of no more than 1:12 ( 12" length per 1" rise )