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Still on the search for a new bike and was wondering what I wanted in terms of geometry. I have searched on line and found loads of sites with calculators for geometry and sizing but most are old school and don't take into account modern 'long low slack' developments. So, does anybody know of a source that will explain the terminology and the actual riding effects of twiddling the various numbers especially looking at new style geometry?
Bit of a long read but it covers everything http://www.mtbiking.com.au/how-to/bike-tech/mtb-frame-geometry-explained
Basically, the slacker/longer the bike (reach, chainstays, wheelbase) the more stable at speed and downhill but the less nimble it will feel, and vice versa. There's a nice middle ground on some bikes.
Lift leg over bikes and ride, it's the best idiots guide
Do you know anyone with a MTB? Have a go on it, remember what it is and look up the geometry chart. Do that with lots of bikes and you’ll understand how the variables affect things.
Look at some manufacturers websites, I think Transition & Cotic have some info to share on this.
For every geometry tweak to "improve" the handling, it'll "worsen" another characteristic of the bike elsewhere. At the end of the day, geometry number tweakage is all a compromise.
Well yes, riding bikes is the best way of deciding but not all makes do test rides so a method to narrow down the possibilities would be useful. I appreciate numbers are not everything - I have ridden my son's bike which on paper isn't that slack but rides as if it were a lot slacker than some bikes with 'better ' numbers.
qwerty
For every geometry tweak to “improve” the handling, it’ll “worsen” another characteristic of the bike elsewhere.
hat’s probably true of everything except seat angle. I can’t think of the downside of a steep seat angle.</div>
hat’s probably true of everything except seat angle.
I agree that the old school bikes were too slack, but at some point going steeper will be detrimental. If you steepen the seat angle, the saddle moves forward, which helps with climbing, but moves the rider forward in relation to the front wheel and bars. You either need to run a longer stem or lengthen the front of the bike, which will lengthen the wheelbase unless you shorten the chainstays. There's a limit to how much you can shorten the chainstays without running a smaller back wheel, so the question is how long can you make the wheelbase before it makes the bike too slow turning in tight turns, and so on. It's all a compromise.
It can play havoc with your pedalling if you can’t get seat back far enough. Won’t affect everyone the same but some bikes annoy the hell out of me as it puts me in a terrible position for seated pedalling, great on steep scrabble uphills but for gradual fire road climbs, long flat mile munching and flat swoopy bits it’s horrible (to me)
its one of those things you can adapt to though, but if you ride other kinds of bikes as well then it’s hard to adapt to wildly different pedalling positions.
“Well yes, riding bikes is the best way of deciding but not all makes do test rides“
Surely some other people ride MTBs where you live? If you ride flats it’s dead easy to swap bikes on a group ride.
The problem with most idiot's guides to geometry is that they are written by idiots spouting advertorial and no understanding of why certain geometry decisions are made.
Suspension means there are dramatic changes in the geometry figures anyway; eg take a 160mm travel fork on a bike with a 69º HA (we'll assume the HA is measured at about ⅓ travel - it varies).
At full dive the HA has changed to 71º, and at top out it is now 65º. This also changes the trail and flop characteristics.
For this we'll assume a fork offset of 51mm; at sag it is 85mm/29mm respectively, full dive 72mm/22mm and at top out 114mm/44mm.
All these figures suppose a vertical bike, but the moment you turn the bars you have changed the effective HA. Also with wider tyres becoming more popular, the tyre profile begins to have a greater effect on things like flop.
There are reasons slack angles are popular at the moment, but they are largely to do with the smoother operation of the telescopic fork.
The remarkable thing is that we humans who are seeking certainty in the numbers on our bikes can quickly adapt to riding while the geometry is changing quite wildly.
If you want some of the science behind it, "Bicycling Science" by David Gordon Wilson is a good read, but be aware that research done by other bodies can be contradictory, eg trail may not be as necessary as we believe (I like plenty though).
For plain explanations, anything written by Phil E Irving is good, but is motorbike related, as is anything by Tony Foale. However empirical experiments I have done tally with what they say, so I'm happy to accept them as authorities.
Personally, I have always been happy riding bikes designed by Brant (now back at On-One again). They have all handled better than my riding capabilities.
Just say Brant, Brant, Brant on this forum and he magically appears. 🙂
As mikewsmith recommends, ride a few bikes and be realistic about your capabilities and intended riding. If you want to race then a bike intended for the sharp end is fine, but if you simply want to cruise around with the occasional burst of speed, there's far more suitable mounts.
Edit: Bicycles and Tricycles by Archibald Sharp os also quite a good read, but it's from 1896 🙂
If there are no negatives to a steeper seat tube angle, why aren't we all on 90' angled seat tubes?
I "think" steeper suits winch and plummet riding, keeps the front end down on steep ascents and adds reach (if other figures kept the same) not so suited to off roading in Norfolk. Just my thoughts, nothing evidenced.
Idiot's guide?
Longer = better.
Lower = better.
Slacker = better.
In reality I wouldn't worry about it, take a couple of test rides and if something mysteriously feels better it could be worth looking at the geometry charts and comparing them. If several aspects of the geometry are different, you won't know if it's one or more of them or just the full combination. If they all feel OK to you, it doesn't matter.
Sure is!
https://www.bikeradar.com/mtb/gear/article/the-ultimate-guide-to-bike-geometry-and-handling-52778/
andyrm that's the article I was going to post a link to as I don't think it falls into the "written by idiots spouting advertorial and no understanding of why certain geometry decisions are made" category.
It’s funny you say all that epicyclo as the two books I turn to and have read cover to cover more times than I care to remember are Bicycling Science and Bicycles And Tricycles.
Some people just jump on bikes & ride them. I rode a demo 5 in 2011 & bought one without checking the geometry. ( I know the head angle is 65deg but haven't a clue what the other measurements are.
My mate demo'd a Bronson & said 'that'll do me', he has no idea what the geometry is cos it doesn't matter. it fits & he likes it. (mind you he's only been riding 28 years so what does he know?)
[very steep seat angles] can play havoc with your pedalling if you can’t get seat back far enough. Won’t affect everyone the same but some bikes annoy the hell out of me as it puts me in a terrible position for seated pedalling, great on steep scrabble uphills but for gradual fire road climbs, long flat mile munching and flat swoopy bits it’s horrible (to me)
This. At some point you end up too far forward relative to the bottom bracket. At the extreme you can find yourself getting kicked off the pedals on the flat when you hit a bump.
mark90
andyrm that’s the article I was going to post a link to as I don’t think it falls into the “written by idiots spouting advertorial and no understanding of why certain geometry decisions are made” category.
Yes, it's definitely not in that category (just in case anyone thought I meant that one). That's a good link. 🙂
“I rode a demo 5 in 2011 & bought one without checking the geometry. ( I know the head angle is 65deg“
It isn’t, it’s 67 deg. Even the current Orange 5 is only at 66 deg.
That Bike Radar article is good - Seb Stott is one of the few current MTB journalists who knows what he’s talking about regarding tech stuff and can ride a bike well.