Research has reported cyclists incurring up to 10g forces due to hitting pot holes and lower g forces from road humps, manholes covers and situations where the road/path is not smooth and even. A recent article details g forces from slow cycling speeds incurring up to 6g acceleration forces . A lightweight helmet at 0.25 kg incurring a 4g acceleration would involve a force of about 10N or 2.2 lbs force imperial. In general, helmet use results in extra forces per typical hour cycled and, on some occasions, they may add to problems in maintaining balance. Therefore, the increased "other" rate, mainly falls, is what could be expected from helmet use.
Zang, K.; Shen, J.; Huang, H.; Wan, M.; Shi, J. Assessing and Mapping of Road Surface Roughness based on GPS and Accelerometer Sensors on Bicycle-Mounted Smartphones. Sensors 2018, 18, 914.
The research reporting a 10g acceleration came from 1986 when hitting deep potholes. it said forces were in random directions. The time to cross a pothole at say 15 mph, about 24 km/hr, 6.6m/sec, say 0.3m wide, would be about 0.045 seconds, less than a persons typical reaction time. the out of balance forces will occur before a person can react is the likely situation, assuming the surface defect has not been noticed prior to impact.
Assuming helmeted and non-helmeted ride identically except for wearing a helmet, both may encounter say, 1g - 3g during a typical 2 hour ride. The out of balance forces will be higher for helmeted on occasions. Riding on rough surfaces and gravel will also lead to g forces of perhaps 2g-4g accelerations.
page 11 of the paper refers to;
.Other explanations for increased accident and injury occurrence for helmeted cyclist may be increased head diameter, impaired vision, impaired hearing, sideways wind shear and forces, reduced riding stability and loss of "safety in numbers" due to reduced cycling participation following helmet law enforcement
Reference to wind forces are
Fintelman, DM, Sterling, M, Hemida, H & Li, FX 2014, 'The effect of crosswinds on cyclists: An experimental study' Procedia Engineering, vol 72, pp. 720-725. DOI: 10.1016/j.proeng.2014.06.122
Brownlie L, Ostafichuk P, Tews E, Muller H, Briggs E , Franks K, The wind-averaged aerodynamic drag of competitive time trial cycling helmets, 8th Conference of the International Sports Engineering Association (ISEA), 1877-7058 c 2010 Published by Elsevier Ltd. https://www.sciencedirect.com/science/a ... 5810002638
Side wind forces may be extra for a typical cyclist wearing a helmet and combined with extra out of balance forces from g forces results in a higher fall off rate. It may well be possible to measure the degree of movements but not easy to compare riders who in practice will all be slightly different. Reproducing the 1986 research and measuring forces from helmets when encountering road surface defects should be feasible.
At the same time measuring the bicycle stability may be possible.
Robinson 1996 refers to the Wasserman data that detailed the incidence of cyclists hitting their head/helmet during an 18-month period was “significantly higher for helmet wearers (8/40 vs 13/476 - i.e. 20% vs 2.7%, p 0.00001)." A bare head width of approximately 150mm may avoid contact compared to a helmeted head at approximately 200mm wide (Clarke 2007 ). Assuming the 20% and 2.7% figures are typical, on a yearly average for helmeted and non-helmeted the risk of hitting their helmet or head would be 13.2% and 1.8% respectively. The increased risk of impact for helmeted is about seven times higher. A degree of protection could be expected plus a degree of risk from the extra impacts.
Extra falls and more helmet impacts compared to not wearing helmets, a general statement, based on available evidence