Why Indoor FTP Is Generally Lower
According to TrainerRoad’s statistics on 120,000 users, indoor FTP averages 5-10% lower than outdoor FTP, with a few individuals seeing differences as large as 15%. The reasons are not singular:
| Factor | Impact | Quantification |
|---|---|---|
| Insufficient cooling | Elevated core temperature, heart rate drift | Heart rate 5-10 bpm higher |
| Lack of coasting | Non-stop pedaling, no muscle recovery | Power reduced 3-5% |
| Lack of road vibration | Stiff upper body, poor muscle activation | Power reduced 2-3% |
| Psychological monotony | Staring at a screen, dullness | Affects RPE by 1-2 levels |
| Trainer measurement error | Direct drive ±1-2%, wheel-on ±3-5% | Data drift |
Calibration Process: Establish Two FTPs
The professional approach is to manage outdoor FTP and indoor FTP separately:
- Indoor workouts → use indoor FTP
- Outdoor rides → use outdoor FTP
- The gap between the two should be fixed at 5-8%; if it exceeds 10%, it indicates a problem with the indoor environment
Standard Indoor FTP Testing Protocol
- Environment preparation: Room temperature 18-22°C, two fans (front + side), 750ml of water within reach
- Warm-up: 15min progressing to 70% FTP
- 5min All-out (wake up the nervous system)
- 10min recovery @ Z1
- 20min Test: Steady pedaling, with a small surge allowed in the final 1min
- FTP = 20min average power × 0.95
Sources of Trainer Measurement Error
- Direct drive trainers (Wahoo Kickr V6, Tacx Neo 2T): ±1-2%, most accurate
- Wheel-on trainers (Saris H3): ±2-3%
- Magnetic resistance trainers: ±5-10%, recommended to pair with a power meter
Calibration Actions
- Spindown calibration: Perform after any significant temperature change or moving the trainer
- Warm up for 10min: Cold readings can be off by 3-5W
- Avoid pointing a fan directly at the trainer flywheel: This can cause temperature drift
Practical Recommendations
- Don’t chase an indoor FTP that “matches” your outdoor FTP; this is a physical limitation
- Use RPE (rate of perceived exertion) as a supplementary gauge—if indoor 4×8min Threshold feels like outdoor 4×8min @ Threshold, that’s good enough
- Retest indoor FTP every 6-8 weeks, and record ambient temperature and fan setup
Indoor training is a highly efficient tool, but the price of precision is accepting a 5-10% power discount. Once you understand this, your training prescription will be accurate.
Related Reading
- The Physiology of Virtual Racing on Smart Training Platforms: Why Indoor FTP Is Often Higher Than Outdoors
- Outdoor vs. Indoor Training Power Differences: Why Outdoor Feels Harder
- How to Effectively Improve FTP: From Testing to Practical Training Plans
- A Full Comparison of FTP Testing Methods: Accuracy and Applicability of the 20-Minute, 8-Minute, and Ramp Tests
相關影片
訂閱CT的頻道 RAMP FTP Test 直播 究竟能撐到幾瓦 訓練台 Gravat Zwift
6 年前
西進武嶺 8000名單車友數據分析 PART1 | 從新手到高手數量/瓦數/推力比/FTP推力比/功率計使用率 大解析 | 公路車 | CTYeh
5 年前
Fitting訓練兩用功率車 ! Thinkrider ST900 完整實測！/ 坡度升降 / 鄰居測試/ smart bike/公路車/ CT Yeh
2 年前
ThinkRider 智騎 XXPRO 訓練台，高階機種功能 只要 1/3價格！？ 實測功率對比、噪音實測 / 公路車 / CT Yeh
1 年前
西進武嶺配速器 3.0更新 / NEW 免功率FTP估算！ / 大數據xAI體能模型 / 如何模擬人止關前集團效應 / 配速儲存功能 / AI全台自行車賽事行事曆 / 公路車 / CT Yeh
1 年前
沉浸室內單車練功房開箱! 100吋大畫面，最便宜的花費? 居然還能K歌!? | 公路車 | CT Yeh
3 年前
車友來我家測FTP...好硬啊🤣 #cycling #公路車
2 年前
單車AI教練！全新 ChatGPT4o 幫你分析訓練成果！排武嶺課表，分析騎車姿勢！ 太神了！ / 公路車 / CT Yeh / feat. 緯緯
2 年前