I personally have employed several of the methods mentioned here myself and found some to be effective. it really does all depend on the physical challenges you face.

Every chair i’ve gotten since that young age i’ve -always- gone after the largest metal foot plates my chairs could hold and ramming is a good, last ditch move.

with ramming you run the risk of your adversary falling on you so the angle of attack is important depending if you have a structural defect issue effecting you.

if you do, instead of a head on collision,
try to do a single side impact, target only one leg in your hit so the automatic reaction of your adversary is to balance or fall away from you with the good leg.

another important factor in ramming also depends on the strength in your arms and the configuration of your anti-tipper devices if present.

if they are present and depending the height and your skill level you should strongly consider tipping the chair yourself while engaging at maximum speed.. this will raise your angle of attack above the shin to the weaker knee area inflicting more damage to your adversary and making hostile pursuit more difficult.

an additional tip..
(applicable depending chair configuration)

someone grabs the handle bars of your chair,
Use all strength and force -sustained- forward..give it everything you got so your adversary has to use more force to try to hold you where you are, then.. Full reverse full speed.. use there own hold on you as your method of escape..

once your back or back handles impact on the mid region of your adversary you should either
A) Run like hell

(if you feel that your adversary still presents a danger to your safety..)

B) with your adversary theoretically doubled over from the hit, back hand them while he is bent over you from behind.. -then- run like hell.

Thank you Glenda for posting this article,
Wheelchair self defense ~is~ out there but the resources are limited and hard to find.
I think your writing just may help save some lives.