Roger Maris 1961 home run splits.

Roger Maris hit 61 home runs in 1961, breaking the old record of 60 set by Babe Ruth in 1927. Ruth set the season HR record four times:
1919 29 Red Sox, home park: Fenway Park
1920 54 Yankees, home park: Polo Grounds
1921 59 Yankees, home park: Polo Grounds
1927 60 Yankees, home park: Yankee Stadium

In 1961 the American League expanded by 25%, increasing from 8 to 10 teams. The National did the same in 1962. The schedule increased five percent from 154 games to 162. Five percent of 60 is 3. In a general sense to equal 60 HR in 162 games the total should be 63 HR.

In 1961 the Washington Senators moved to Minneapolis and became the Minnesota Twins and were replaced by a new expansion team called the Washington Senators. The other expansion team was the Los Angeles Angels.

The two new teams were stocked with players drafted from the 8 AL teams. The dilution of talent of the 16 major league teams was about 12%. Weaker pitchers and weaker batters were playing in the AL and NL who would not have been playing in 1960. There's no getting around that.

Did Maris specifically take advantage of the two new expansion teams? It doesn't really matter as all teams were impacted.

Rk	Player	HR	Year	Age	Tm	Lg	G	PA	AB	R	H	2B	3B	RBI	BB	IBB	SO	HBP	SH	SF	GDP	SB	CS	BA	OBP	SLG	OPS	Pos
1	Roger Maris	61	1961	26	NYY	AL	161	698	590	132	159	16	4	141	94	0	67	7	0	7	16	0	0	.269	.372	.620	.993	*98/H
2	Mickey Mantle	54	1961	29	NYY	AL	153	646	514	131	163	16	6	128	126	9	112	0	1	5	2	12	1	.317	.448	.687	1.135	*8/H
3	Orlando Cepeda	46	1961	23	SFG	NL	152	637	585	105	182	28	4	142	39	11	91	9	0	3	21	12	8	.311	.362	.609	.970	*379/H
4	Jim Gentile	46	1961	27	BAL	AL	148	601	486	96	147	25	2	141	96	5	106	11	0	8	12	1	1	.302	.423	.646	1.069	*3H
5	Harmon Killebrew	46	1961	25	MIN	AL	150	656	541	94	156	20	7	122	107	6	109	3	0	5	11	1	2	.288	.405	.606	1.012	*35/7
6	Rocky Colavito	45	1961	27	DET	AL	163	708	583	129	169	30	2	140	113	2	75	2	2	8	14	1	2	.290	.402	.580	.982	*79/H
7	Norm Cash	41	1961	26	DET	AL	159	673	535	119	193	22	8	132	124	19	85	9	2	2	16	11	5	.361	.487	.662	1.148	*3/H
8	Willie Mays	40	1961	30	SFG	NL	154	659	572	129	176	32	3	123	81	15	77	2	0	4	14	18	9	.308	.393	.584	.977	*8/H

Including Cepeda and Mays in the NL, eight players hit at least 40 HR in 1961. All but Killebrew and Mays had career highs in HR in 1961.

Maris hit only three HR against the Orioles but he also had his fewest at bats (AB) against them, so let's look at his HR rate.

Player	HR	AB	AB/HR
Roger Maris	61	590	9.67
Mickey Mantle	54	514	9.52
Orlando Cepeda	46	585	12.72
Jim Gentile	46	486	10.57
Harmon Killebrew	46	541	11.76
Rocky Colavito	45	583	12.96
Norm Cash	41	535	13.05
Willie Mays	40	572	14.30

Among the HR leaders, only Mantle had a better HR rate than Maris. HR rate for AL teams:

Tm	HR	"AB"	AB/HR
BAL	109	4,240.30	38.90
WSA	131	4,104.00	31.33
NYY	137	4,173.00	30.46
KCA	141	4,085.00	28.97
LgAvg	153	4,170.00	27.25
CHW	158	4,206.60	26.62
MIN	163	4,146.30	25.44
BOS	167	4,156.60	24.89
DET	170	4,230.30	24.88
CLE	178	4,187.30	23.52
LAA	180	4,160.00	23.11

The Senators pitching staff had the second best AL HR rate but the Angels had the worst. Maris had his fewest HR against the Orioles and they allowed the fewest HR. Here are his HR Rates against AL teams.

Split	HR	AB	AB/HR
Chicago White Sox	13	71	5.46
Cleveland Indians	8	61	7.63
Washington Senators	9	75	8.33
Detroit Tigers	8	74	9.25
Boston Red Sox	7	65	9.29
Kansas City Athletics	5	67	13.40
Los Angeles Angels	4	59	14.75
Minnesota Twins	4	65	16.25
Baltimore Orioles	3	53	17.67

It's not clear that Maris took particular advantage of the two expansion teams: Senators and Angels. Here is how Maris did by ballpark.

Park	HR	AB	AB/HR
CLE-ClevelandStd	5	33	6.60
DET-Tiger Stad	5	37	7.40
CHW-ComiskeyPk 1	5	39	7.80
BOS-Fenway Pk	4	35	8.75
KCR-Munic. Stad	4	36	9.00
NYY-Yankee Stad	30	280	9.33
WSH-GriffithStad	4	38	9.50
LAA-Wrigley (LA)	2	27	13.50
MIN-Metrop. Stad	1	30	30.00
BAL-Memor. Stad	1	35	35.00
Park	HR	AB	AB/HR
Road	31	310	10.00

Maris had an overall HR rate of 9.67, so he was only slightly better in Yankee Stadium.

	Year	HR	AB	AB/HR
AL	1960	1,086	41,838	38.52
AL	1961	1,534	54,904	35.79
	Year	HR	AB	AB/HR
Maris	1960	39	499	12.79
Maris	1961	61	590	9.67

Maris	Year	HR	AB	AB/HR
home	1960	13	232	17.85
road	1960	26	267	10.27

In 1960 Maris had a much better HR Rate on the road, about what he had on the road in 1961. He was on pace to hit about 55 HR in 1960 when he was injured. In the first half of 1960 in 72 games Maris hit 27 HR in 310 AB. In 1960 Maris batted mostly third and fourth. He and Mantle had these PA in those spots:

	Mantle	Maris
3	296	173
4	187	288

Mantle was not protecting Maris in 1960 as he did in 1961. In 1960 Yankee manager Casey Stengel moved Maris to number 3 and Mantle to 4 in September and in all seven World Series games Stengel batted Mantle lower in the Yankee batting order than Maris. It was Stengel, not his replacement in 1961 Ralph Houk, who switched them to have Mantle protecting Maris. The pattern was largely in place in early September 1960 and for Yankee games 147 (9/23/1960) though 155 (10/2/1960) it was Maris 3, Mantle 4 in every game. Roger Maris had become a Home Run hitter in 1960.