1.5kg lighter bike

The bike will feel lighter, yes.
You will not be much faster.

As they already said, all other things being equal it won't make a big difference- as already said the weight difference is only 2.1% so once air resistance is taken into account it would be difficult to see any real difference in times.

However, if the lighter bike has significantly lighter wheel rims then this could make a slight difference as you accelerate faster when the gradient slackens at points.
Stiffness would make a little more difference- if your wheels and frame are stiffer then you'll loose less energy to deforming each and put a little more into the road. The steeper it gets the more important it'll be as the torque on the wheel can become quite high as you start to run out of gears.

When I swap from my heavy winter bike to my summer bike - I notice it for about an hour or two. After that it's the norm.

I'm the same weight as you. My bike is 6kg.

A lighter bike can only bring positives as long as you are not overly compromising stiffness (and today's light bikes do not).

When I have hired an 8.5kg bike in Majorca it feels a bit dead by comparison.

markhewitt1978 wrote:

robbo2011 wrote:

Yes, you will feel the difference and yes, you will be faster. On a long climb of over an hour, there could be a couple of minutes difference in your time i guess.

Yup, in a race, that matters, in just a ride, well it's up to the OP if that matters or not.

What's often ignored is that the lighter bike is just nicer and more fun to ride. I have an average road bike, about 7.5kg without anything on it, and I went Mallorca and hired a bike, no idea how much it weight but it was more than the one at home and I didn't really enjoy riding it, it just felt sluggish and that the bike was holding me back.

I highly doubt that feeling would be coming from the weight difference, but rather from geometry and sizing not being ideal. While weight differences are nice, I've climbed the best on the heaviest bike I've owned due to it fitting perfectly.

You'll notice some differences, but in the words of everyone I used to MTB with in Austria, it's not the bike that makes the biggest difference, it's the rider. You may be more mentally motivated when riding a shinier/lighter/newer bike, and that will have a huge impact on your effort and speed.

What about if I lose 12kg by dieting?

"Its not about the bike", someone said..... :ducks for cover:

bernithebiker wrote:

I'm the same weight as you. My bike is 6kg.

That is very light - as I understand for racing, bikes are required to be a minimum weight of 6.8kg!

Simon Masterson wrote:

ForumNewbie wrote:

bernithebiker wrote:

I'm the same weight as you. My bike is 6kg.

That is very light - as I understand for racing, bikes are required to be a minimum weight of 6.8kg!

At events held under UCI rules, yes. But bikes get lighter still; head over to Weight Weenies...

There are some seriously light bikes on there!

As far as I know, UCI is possibly revising the weight rules. The weight regulations were introduced in a time when frames under a certain weight were extremely unsafe, but with recent advances, they are looking to revise it to reflect how light and strong modern frames are!

Put my numbers into the local hill climb time trial. If I lost just 2kg from me and the bike, I'd gain nearly 30 seconds (5km at 7%). I can't afford 2kg off the bike, so I really shouldn't of had the extra portion of salmon risotto this evening. If the numbers in the model are correct, then I'd be interested in losing 30 seconds in this years hill climb, not sure how much I'd pay for it though.