EXPLANATORY NOTE: [*] INDICATES THE PORTION OF THIS EXHIBIT THAT HAS BEEN OMITTED AND SEPARATELY FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT.

Exhibit 10.21

December 10, 2014

Caterpillar Inc.

Building Construction Products Division

400 Regency Forest Drive, Suite 400

Cary, North Carolina 27518

Attn: Joseph Hellrung

Dear Sir,

This letter spells out commercial terms related to the purchase of MTL undercarriages and tracks (“Products”) by the Building Construction Products Division of Caterpillar Inc. (“Caterpillar”) from ASV Corporation. Caterpillar and ASV are each a party (“Party”) to this letter and collectively may be referred to as parties (“Parties”).

Duration. The Parties agree to a firm commitment to purchase for the next three (3) years from date of signing this letter (“Execution Date”).

Pricing. A straight forth structured method to adjust for the major cost drivers (steel, rubber and the motor).

[* Indicates portions of this exhibit that have been omitted and filed separately with the Securities and Exchange Commission pursuant to a request for confidential treatment.]

Or

Pricing will be weighted and adjusted according to the content of the bill of material. Tracks and wheels will follow the SMR20 and Butadiene indexes. All other steel components will follow Bureau of Labor Statistics US Producer price indexes of (1504) and ductile iron casting and (101703) hot rolled steel sheet and strip.

Price Reduction. In addition to ASV’s aggressive pricing concession commitments we would also include a volume discount bonus. The volume discount only applies only if the entire three year program is agreed to.

Tracks – Cost Savings. Low cost track identified: ASV will provide an affordable track option to Caterpillar to supply their low cost replacement tracks. When ASV’s affordable track becomes available Caterpillar will then purchase all their replacement tracks from ASV. Caterpillar will discontinue purchasing low cost replacement tracks from their current supplier. During the interim, Caterpillar can continue to purchase low cost tracks from their current supplier. ASV will perform the validation process on the entry level Alpha and Beta tracks. We’re looking at an estimated introduction time frame for the new track of 4-6 months which includes the validation process by ASV.

Parts. Parts pricing will be capped at [*] per year over the life of the agreement and will remain at previous years pricing if 2000 per year or more units are purchased on the whole goods side. ASV will provide input on the cost drivers if the [*] price increase is required.

Delivery Requirements. Caterpillar will provide ASV with a forecast of its needs for equipment, and parts for the next twelve (12) months. The first three (3) months of each such forecast shall be firm and binding on both Caterpillar and ASV.

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[* Indicates portions of this exhibit that have been omitted and filed separately with the Securities and Exchange Commission pursuant to a request for confidential treatment.]

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EXHIBIT A

PRODUCTS

2015 Pricing reflects a [*] discount on U/C’s, [*] discount on primary tracks, and [*] discount on the low volume tracks.

[* Indicates portions of this exhibit that have been omitted and filed separately with the Securities and Exchange Commission pursuant to a request for confidential treatment.]