Bit of a hypothetical post this one… hope it generates some discussion, am certainly interested in what people have to add.

The bike that I mostly use for xc riding at the moment is a 120mm travel, 25lb full sus bike (including a superfast 1385g wheelset), it certainly seems pretty zippy and I've used it for the odd xc event doing fairly well.

What I'm wondering is is how much faster I'd be if I transferred the parts making a 20lb 100mm travel hardtail or even a 22.5lb 100mm travel full sus(e.g. SC Blur XC Carbon)?

Now I'm aware that the type of xc track, conditions etc obviously add a large number of variables to a race but could I reasonably expect to be significantly faster if I transferred to one of the two above?

Nope not significantly faster I dont rekon…

I might add I have been forced to ride my 27-29lb do it all full sus as my race bike, do it all, 22lb hardtail was knicked, and I havent really noticed much difference…. My own performance varies so wildly its hard to tell mind.

12.4%

OK… You'll be faster on a bike suited to the terrain but it's really impossible to say how much. How much faster you feel on a 'faster' bike may well make more difference that the bike itself…

From the two bikes you've described, I'd doubt you'd be a lot faster (assuming a decent propedal/lockout shock on your current bike) though unless there's a lot of short/sharp climbs or long fireroad type ones.

Hmmm, I'd say 4, maybe even 4.2 quicker.

It's absolutely impossible to quantify it, you may not be any quicker!

Edit: dammit, Clubber beat me!

LOL @ Clubber! '12.4%' – that's v precise! 😉

average rider will be no faster – the bike is not the limiting factor in most people at races times ….

faster, perhaps.

SIGNIFICANTLY faster, nope.

Well I was going to show my workings and give the result to the full 3 decimal places that I calculated (12.378%) but I thought that would just be silly 😉

I've noticed a big difference from a 28lb steel hardtail to a 22lb carbon HT but it's probably got more to do with wheel weight and the fact that the carbon frame transmits every bit of energy I can put into the pedals.

When looking at what you're proposing I'd say that it won't be as noticable as you already have very light wheels, you'd probably notice over a very long distance as ultimately you'll use less energy getting the lighter bike from A to B.

Speaking as a fat middle aged bloke, I'd have to say none at all. I'd have to lose a good two stones of my own weight before I started thinking about the benefits of a lighter bike, but that rarely comes into the equation does it?

A light bike might feel "faster" but actually be going slower. Due to lack of traction or being bounced around on decents.

Just have a big poo and get your body fat percentage below 8!

bennyboy, 25lb for a full suss seems pretty light anyway, I'm not sure how much lighter you could reasonably get without spending silly money.

Hard to quantify. I raced a 30lb odd Pace RC405 with Pikes on at two of the Lee Quarry races and did okay by my standards. I guess my much lighter – 7lb lighter – ML7 would have been faster on the climbs and might have gained me a place or two, but being fitter and faster would have a bigger impact and the Pace would probably be faster on other parts of the course as well. How competitively wired are you?

It will be quicker on the climbs, even if you're a 20 stone chipper, saving 2.5lbs off the bike will make you quicker. Even if this is by 0.01 seconds over a 5 minute climb, there will be a difference.

Then whether you'll lose twice as much time on the descents is another point.

If you want a race bike, and the opportunity arises to get something like the Blur XC Carbon then go nuts, but don't expect to suddenly be quicker than Liam Killeen.

My only MTB is a 21lb Epic, and it's perfectly capable. If I was riding trail centres 90% of the time with the odd race, I'd have a beefier bike, but I'm not!

I reckon you'd be between -2% and -5% faster on the 20lb hardtail.

Unless it's an extremely bumpy course with a very technical descent and our OP is not a good rider technically, when it may be 20% slower.

I guess it all depends on how fast you are already.

You've already got a light wheel set as reducing rolling mass is the best way to "speed things up," that and equally fast rolling tires.

other than that you are going to be spending big £ to reduce weight so you have to consider if it's worth it.

I've recently dropped my hard tail to around the 20lb mark by upgrading the wheels and the difference that has made is noticeable.

It's more to do with stiffness and power transfer, but as many people have already stated, the fitness level of the rider is going to control how fast a bike is.

I think it would be faster, but in all relatively it depends what bike you ride all the time.

If you ride the same bike all the time then personally I dont think you will feel much difference come race day. I jump on my race bike and I feel like I'm flying. I would say I am faster on it, but more so its in my head. But dragging 5-6 lbs extra around in the long run you would be faster on a lighter bike.

But going from a bike you are used to, then jumping on a superlight new race bike, its not like jumping from a 1.2 punto to a porsche. You will notice more benefit by getting fitter!

I have an Enduro SL and also built up a lightish Pace Hardtail this summer convinced that I was only slow because I was on an All Mountain bike.Ive compared times on my routes and although I feel quicker on the Pace my times are more or less the same 🙁 Time to admit I need to shift some weight off of the rider 😳

For perspective, the best bang per buck I've ever got in performance terms was spending a smallish wodge on a Dave Smith 12-week training programme a couple of years back. The amount I spent wouldn't have saved me 200g on a crank-set, but training properly and specifically made me much, much faster even if it also turned me into a slavering psycho for three months. Assuming you've already got the basics sorted – fast wheels, tyres etc – you'll get a lot faster for your money by just training properly.

Bear in mind that I'm a very average rider, so when I say faster, I mean faster for me, not fast in the bigger sense.

Its got to be all about acceleration. Given f=ma and all that then on a lighter bike using the same amount of effort you'll accelerate (marginally) quicker. Once you stop accelerating then the top speed is likely to be very similar. Therefore how much faster you'd be would depend entirely on how much time you are accelerating, so probably how bendy the course is.

More likely you'll put in more effort to accelerate at the same speed and then it'll come down to how many times you can keep doing that before you get tired. So more likely you'll be the same speed but for less time.

Or maybe you'll be quicker. Who knows.

You might be able to squeeze an extra lap or two in if it's a 12 / 24 hour endurance type event if your not lugging so much weight around.

A 5lb weight saving is about 2.5 kilos.

Total weight of you on your bike with all riding kit is probably about 100 kilos.

2.5% weight loss in speed terms?

http://analyticcycling.com/ForcesLessWeight_Page.html

About 30 secs faster over a 9 km climb if you believe the figures.

Essentially, stop trying to buy performance in the bike shop because it isn't going to happen….

A mater of mine used to ride a 24 lb hardtail. I couldn't keep up on the climbs. He then bought a 30 lb hardtail – then I could even overtake him on climbs. I was on a 27 lb hardtail

Heavier wheels and wider tyres as well on his new bike

We were both suprised how much difference it made. He is a skinny whippet tho but the difference seemed rather more than you would expect from gaining a whole 6 lb, especially as I weigh 20 lb more than him

Not a racer but over a 10 mile loop
24lb yeti asr-sl 1hr 3 min 24 secs
35lb titus supermoto 1hr 4mins 02 secs

just come back from the pryenees so reasonable condition for me, but i am no racer or athelete felt i could ride the loop a few times more on the yeti, dead on the titus.

As BWD says spend the money on a training plan.