From MTB to Triathlon there is a fine Balance Between the Art and Science of Bike Setup
There is an important balance between power delivery, comfort, stability and aerodynamics. The best position is the one that gives you the best speed over your event and distance. If you're hill climbing or riding behind a strong team then power is more important, if you are time trailing then aerodynamics becomes more important. If you have to ride 180km after a swim and before a marathon, you had better be comfortable and then if you intend to bomb down some insane dirt track over boulders the size of small houses balance and stability are going to be paramount.
To get the best set up I follow a simple 5 step process:
1) Measure the rider. Inseam, thigh length and body plus arm length are the most important as these determine seat height, seat for-aft and seat to handlebar distance. I also measure forearm length for aero-bars, shoulder width for handlebar length and a few others.
2) Measure flexibility. Hip flexors, hamstrings, lower back are the key measurements. Too tight in the legs and you will need a lower seat, too tight in the back and shoulders and you may need to raise your handlebars or shorten the distance from seat to handlebars.
3) Measure the bike. The key points are those that you can't change top tube length is the most important. This is especially so these days with radical frames with very short seat tubes on 'compact' frames. But seat angle also plays a part.
4) Determine 'theoretical' best position. Positioning a rider on a bike involves getting the contact points between a rider and the bike set correctly, these are a) seat position, b) foot pedal position and crank length, and c) hand positions.
5) Once I have determined these distances and positions the science ends and the art takes over. I check out riding position by eye. First this is done on a stationary trainer, I always look at the position and ask the question "is this optimal?". Hip and lower back movement is one key area to watch. Too much (2cm max spine movement). The job must be completed with some real riding and feedback, I always ask riders to go ride and report back as to how it is going.
The other day I was doing a bike fit on a mountain biker. They had the latest XTR equipped X-country Beast with nice light carbon wheels etc. The rig was just outstanding. This rider also had talent and good understanding about training. Upon completing the bike fit I asked if they did any road cycling. The answer was of course yes and that they had a road bike for road cycling. Upon further discussion I discovered several important setup differences between his road bike and his mountain bike.
I reckon that in a two hour event having your road and mountain bike set up EXACTLY THE SAME is worth 5 minutes. Think about how much better your training needs to be to get 5 minutes- improvement and you'll get some idea as to the magnitude of how important I think this is.
So why is it SO important? Well it's all about specificity, cycling is about the rider and bicycle combination. Specificity is all about showing your body exactly what will happen on the day. That means that you need to know exactly how hard to push the lever across to change a gear. Your muscles are use to the exact amount and time of the contractions when pedaling or when pulling on the handlebars going up a steep hill or when tucked down on the aerobars holding an extreme aero position for a 40km TT. Show your body something even slightly different and you loose specificity and the amount of your training that actually makes you go faster is less. It's a concept that those with limited time to train really need to sit up and take note off. If you have 20 hours a week to cycle it's probably not going to make a great deal of difference to your cycling if some of it is done on a bike with a different saddle or a slightly different seat height or different shoes and pedals. So that's why if you are a professional (as in full time) mountain biker some road cyclings not going to hurt you. But if you only have 3-8 hours to train a week you need to make it all count.
OK so you can't get to the Forrest every day. The next best thing is to put on some slicks and ride your MTB on the road. OK so you like to mix up some road cycling with some off road cycling. The most important thing to do is to do more of the race specific training closer to the event. I remember a few yearsback when a very good road cyclist got cramp within 5km of the finish of a 40km TT and finished 4th, he should have won but having not done ANY riding on aero-bars prior to the event his body just couldn't take it
Another common mistake is to have a different bike on a windtrainer for those indoor workouts or to have a different bike for training (I don't want to get my race bike dirty). If you want to do this you need to ensure that your bikes are set up as close as possible.
Shoes are also important. Buying a second pair look for the same make and model set the cleats in the same fore-aft position and also check that the "Q" distance is the same. "Q" distance is the distance that you feet and pedals are apart. The further apart your feet the closer your seat needs to be to the bottom bracket The ideal would be if you have the same width bottom bracket, same pedals (with same length axles) and cleats the same distance from the inside side of the shoe. If your "Q" distance is different between bikes you need to take this into account when getting the seat height the same. Generally speaking feet are set wider apart on a MTB bike than on a road bike and this will make the effective seat height higher on a mountain bike. To get a typical road and Mountain bike set the same you need to make your seat about 5mm lower on a MTB bike to have the same bend in the knee.
Another possibility for those Mountain bikers out their with road bikes is to set the cleats on the road shoes so that the shoes are further apart and to set the mountain bike shoes as close in as possible (although this might have a negative impact on stability and balance).
If you feel stiff after training on a different bike it is likely that the small differences in setup are at least partly to blame.