Better known as a jumps jockey, John Allen gets his chance for a major win on the flat aboard Howard Be Thy Name in the Group One South Australian Derby.

The Irishman rode four of Darren Weir's jumpers to victory at last week's Warrnambool carnival with the trainer regarding him as one of his best assets in an ever expanding stable.

With Group One races in Brisbane and Adelaide this weekend, Allen has been rewarded with the ride aboard Howard Be Thy Name in the South Australian Derby at Morphettville on Saturday.

Allen landed his first black type victory when Howard Be Thy Name won the Listed Port Adelaide Guineas at Morphettville last month.

He was replaced by Brad Rawiller when the three-year-old took out the Chairman's Stakes at that track on April 30.

Rawiller will be in Brisbane on Saturday for the Weir stable which has Stratum Star and Stellar Collision nominated for the BTC Cup at Doomben.

Howard Be Thy Name drew barrier six in the 16 horse plus one emergency field and is the $5 second favourite for the Derby over 2500m.

Charlevoix, a Mick Kent-trained stablemate to last Saturday's Australasian Oaks winner Abbey Maree, holds favouritism at $3.70.