Let me preface this post by saying I do not represent Mike Reed, Voyager Outdoors nor Bill Burke's 4-Wheeling America. I took a class from them last weekend but am neither an employee nor representative and the below is my own commentary and opinion, and does not represent those of the event organizer.
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I just completed Bill Burke's 4-Wheeling America's Trail Leader course in Grand Junction, CO and Mike Reed is one of his instructors. We had five Grenadiers: one stock, three with mild lifts and tires and Helga, Warner INEOS Grenadier's race truck. Over the four days, we took some pretty decent obstacles and the general consensus was that the stock Grenadier with no rock sliders had slightly more clearance due to the way the rock sliders bolted to the underside of the Grenadier. Not much, mind you, maybe like an inch. I'll be leading the two easiest trails (Geyser Pass and Shafer Trail) due to my having factory step bars and losing some clearance due to them, despite my lift and larger tires. I did all the obstacles the other Grenadiers in the class did, but we were extremely careful with my line and took it slow. Basically, with the steps (-), lift (+) and 33" tires (+), I have about the same clearance as a stock Grenadier. Hopefully Mike will chime in, but I know he said he wasn't going to have a day off between last week and the Rally, so he might not even log in to the forum to see this. He's a busy man!
Also, if you don't have them, feel free to get What Three Words and On-X Off-Road apps on your phones, as we will be using them during the event. (The Trail Leaders, at least are required to have them because the SAR and EMS systems there use W3W for geolocation and coordination in case of an emergency.)