The T-cog gives the body instructions on how to rearrange itself into its alternate form or how to change into inbuilt weaponry etc.
In some rare cases like triple changers and shifters the t-cog can contain multiple instructions but most are limited to one alt-mode and a few minor transformations like weaponry.
They can scan and transform into forms similar to their original shape.
So if some alien species used mechanical spider's as car's, original airachnid could have scanned it and adopted it as her alt-mode.
But a bot/con cant just sprout wheels or wings from nowhere. So Airachnid for example can't just scan a jet, grow wings and turn into that.
Radically changing your alt-mode requires either major surgery changing your t-cogs structure and altering your vehicle / beast mode kibble, or transplanting your spark and brain module into a new body (which come's with a whole host of risks as a spark can sometime's reject a new body).
In airachnids case the rust plague damaged her original t-cog and beast mode kibble beyond repair.
The only suitable* 'organ' donor available was a two-wheeler. So she was repaired by having a two-wheelers t-cog and vehicle kibble transplanted into her, hense the bike mode she has today.
*A transformer of the same spark type and of a similar size (as t-cog from a truck former or some other large bot just wouldn't fit into her small protoform)
Shattered Glass Reference Sheets -> www.deviantart.com/soundbluste…