Shares in Harley-Davidson are down about 8% in New York after the motorcycle maker slashed its sales forecast for this year.
The Milwaukee-based firm now expects to sell between 241,000 and 246,000 bikes this year, down from 262,221 last year.
Demand has been dropping for years. A decade ago it was selling almost 350,000 annually.
Harley is also facing competition from cheaper bikes made by rival US firm Polaris and Japan's Honda.
In a BBC interview earlier this year, the boss of the firm, Matthew Levatich, said his iconic motorcycle company was "emblematic" of what the Trump administration thought a home-grown US company should be.
But in its second quarter results, published on Tuesday, the company said sales would be down sufficiently for it to need to cut production, and workers' hours, at some of its US plants.
Harley-Davidson demand has weakened among its aging baby-boomer customers and fewer millennials are taking to motorcycling.
Part of the reason for the fall in demand is that its loyal customers are ageing, and, as they do, they sell their bikes, dampening the appetite for new vehicles.
Despite this, the company said it was not looking to cut new bike prices in the face of these lower used-bike prices.
Harley has tried to counter its image as a high-cost, older age group brand and has introduced a number of models at the lower price range aimed at younger riders.
Across the US, big bikes are falling from fashion. Figures released last week show registrations of large motorcycles fell by about 7% so far this year.
Harley-Davidson's second quarter net income was down almost 8% at $258.9m (£198m).
Revenue per motorcycle rose about $437 to $15,530 in the quarter.