MTB geometry questions
Discussion
I'm looking at geo figures for a few different bikes just idly pondering what my next bike might be (as you do on a quiet thursday night).
I'm comparing my size L Specialized Epic to the Giant Trance 29er - manufacturer size recommendation puts my 1.76m between M and L for the Epic (L felt better so I bought that size), and would suggest M for the Trance.
The Epic has 456mm reach with a 90mm stem as standard, but I'm running an 80mm (with 760mm bars up from the original 720) and prefer that.
The Trance is 442mm reach / 40mm stem in M or 462 / 50 in L
I'm wondering for comparison if quoted reach includes or excludes the stem?
Ie is my 'effective reach' 456+80= 536 (compared to the Trance at 482 (M) or 512 (L) effective), or does the 456 include the stem so mine with a 10mm shorter stem is effectively 446?
https://geometrygeeks.bike/compare/specialized-epi...
Curious as I've read comments about modern trail bikes having longer reach as part of a more agressive geometry, and the Epic is an XC bike - but actually has very little difference in reach, and longer 'effective reach' if the stem is added? Obviously the much slacker front end of the Trance will be a big factor in handling too, the rear geo looks surprisingly similar.
I'm comparing my size L Specialized Epic to the Giant Trance 29er - manufacturer size recommendation puts my 1.76m between M and L for the Epic (L felt better so I bought that size), and would suggest M for the Trance.
The Epic has 456mm reach with a 90mm stem as standard, but I'm running an 80mm (with 760mm bars up from the original 720) and prefer that.
The Trance is 442mm reach / 40mm stem in M or 462 / 50 in L
I'm wondering for comparison if quoted reach includes or excludes the stem?
Ie is my 'effective reach' 456+80= 536 (compared to the Trance at 482 (M) or 512 (L) effective), or does the 456 include the stem so mine with a 10mm shorter stem is effectively 446?
https://geometrygeeks.bike/compare/specialized-epi...
Curious as I've read comments about modern trail bikes having longer reach as part of a more agressive geometry, and the Epic is an XC bike - but actually has very little difference in reach, and longer 'effective reach' if the stem is added? Obviously the much slacker front end of the Trance will be a big factor in handling too, the rear geo looks surprisingly similar.
It has now occurred to me that I could answer the reach/stem question by just walking downstairs to the garage with a tape measure and an improvised plumb-bob. [ETA: the 456mm reach is to the headset and doesn't include the stem]
Still interested to discuss MTB geo with more experienced bike-geeks though.
I tried a 60mm stem on the Epic and liked it going downhill, but lost a fair bit of confidence in the front wheel on climbs so I've gone back to the 80mm for now.
Pondering that it might be worth trying the 60mm again, but mounted lower (below some spacers instead of above them) to help keep weight on the front end while climbing, but get the advantage of being further back on steep descents. Does that make sense or have I got the wrong end of the stick?
I'm sure my riding ability is still the biggest limitation at the moment, but tinkering to try and improve the setup is fun.
Still interested to discuss MTB geo with more experienced bike-geeks though.
I tried a 60mm stem on the Epic and liked it going downhill, but lost a fair bit of confidence in the front wheel on climbs so I've gone back to the 80mm for now.
Pondering that it might be worth trying the 60mm again, but mounted lower (below some spacers instead of above them) to help keep weight on the front end while climbing, but get the advantage of being further back on steep descents. Does that make sense or have I got the wrong end of the stick?
I'm sure my riding ability is still the biggest limitation at the moment, but tinkering to try and improve the setup is fun.
Edited by GravelBen on Thursday 1st August 11:47
Just been through a similar choice, L or XL on a hardtail. (I'm 6ft 2" so roughly between both for sizing)
Test rode both and went for the XL based on;
- Stack height is lower meaning I feel slightly less over the front wheel for the correct saddle height giving me more confidence downhill or drop off's
- I changed the stem from a 90mm to 70mm and this made a huge difference to the riding position (much bigger than expected)
Test rode both and went for the XL based on;
- Stack height is lower meaning I feel slightly less over the front wheel for the correct saddle height giving me more confidence downhill or drop off's
- I changed the stem from a 90mm to 70mm and this made a huge difference to the riding position (much bigger than expected)
Bear in mind the reach is taken from the bottom bracket centreline to the point where the axis of the steerer tube intersects the top of the headset, so doesn't include the stem. The Reach+stack thing was devised as a way of working out what TT/tri bikes could be made to fit an athlete's riding position given the limited adjustment you get on those sort of bikes, it's been adopted for road and MTBs now too.
Lot of new geo MTBs have a longer reach, but have steeper seat post angles to counteract this, so the distance from the saddle to the bars is about the same. Idea is to get the riders weight more central on the bike, as the front wheel gets pushed forwards by shallower head tube angles (for stability).
Also bear in mind that when you move spacers around on the stem, you're affecting the reach, as the head tube is angled. So lowering the stem actually increases reach slightly.
Lot of new geo MTBs have a longer reach, but have steeper seat post angles to counteract this, so the distance from the saddle to the bars is about the same. Idea is to get the riders weight more central on the bike, as the front wheel gets pushed forwards by shallower head tube angles (for stability).
Also bear in mind that when you move spacers around on the stem, you're affecting the reach, as the head tube is angled. So lowering the stem actually increases reach slightly.
Edited by lufbramatt on Thursday 1st August 12:06
Edited by lufbramatt on Thursday 1st August 12:12
Yep as you’ve figured out yourself, reach doesn’t include stem.
SA = seat-tube angle
ETT = effective/horizontal top-tube
I’m glad of the move to using reach and stack (and wheelbase) to define MTB size. Previously used ETT is a poor measure comparatively due to dependence on SA.
I’ve never managed to testride bikes in my size due to needing XLs, but on paper the main measures I take into account when comparing are:
• Reach
• HA
• SA
• Wheelbase
• Stack
• And to some extent seat-tube length to ensure I can still fit a decent length dropper post, although the days of super-tall teat-tubes seem to have passed.
I’m also very glad of the recent trends for longer reach and steeper SA. Being tall (187cm/6’1ish) I love having bikes I sit ‘in’ rather than ‘on’. And with 29ers I finally look in proportion to the bike! Steeper SA allows longer reach and wheelbase for more ‘cockpit’ space while standing/descending, without being too stretched out while seated. Plus steeper SA places you in a more efficient pedalling position, feeling less like you’re sitting behind the pedals when seated. Plus having weight more centred on bike helps keep front end down when climbing.
As tufbramatt says, moving stem spacers also affects reach too. In a similar fashion, wider bars increase ‘effective reach’ as your body is pulled more forwards.
Neither of my 2 current MTBs are particularly radical in geometry. Not huge differences on paper but to ride they feel quite different with my Carbine climbing as well as my Talboy despite the extra weight and travel – I guess the steeper SA helps as I feel a little bit ‘over the back’ on my Tallboy.
SC Tallboy, XL, 475 reach, 617 stack, 68 HA, 73 SA, 1195 wheelbase
Intense Carbine, XL, 471 reach, 641 stack, 65.5 HA, 74 SA, 1253 wheelbase
However, I owned an XL Nukeproof Mega 290 for a year and it was mahoosive: 515 reach, 632 stack, 66 HA, 75 SA, 1283 wheelbase. The reach and wheelbase were quite extreme (still are!) but it didn’t feel too long due to the steep SA. I did have to move my weight around a lot more to make it go where I wanted it to (guilty of not getting enough weight over the front wheel), so it was certainly more difficult to ride than my current Carbine. It made up for it by how stable it was at steep on rough stuff – it simply blasted through stuff.
I’m curious to try something crazy like one of the Pole bikes or a Geometron, just to see what they feel like.
SA = seat-tube angle
ETT = effective/horizontal top-tube
I’m glad of the move to using reach and stack (and wheelbase) to define MTB size. Previously used ETT is a poor measure comparatively due to dependence on SA.
I’ve never managed to testride bikes in my size due to needing XLs, but on paper the main measures I take into account when comparing are:
• Reach
• HA
• SA
• Wheelbase
• Stack
• And to some extent seat-tube length to ensure I can still fit a decent length dropper post, although the days of super-tall teat-tubes seem to have passed.
I’m also very glad of the recent trends for longer reach and steeper SA. Being tall (187cm/6’1ish) I love having bikes I sit ‘in’ rather than ‘on’. And with 29ers I finally look in proportion to the bike! Steeper SA allows longer reach and wheelbase for more ‘cockpit’ space while standing/descending, without being too stretched out while seated. Plus steeper SA places you in a more efficient pedalling position, feeling less like you’re sitting behind the pedals when seated. Plus having weight more centred on bike helps keep front end down when climbing.
As tufbramatt says, moving stem spacers also affects reach too. In a similar fashion, wider bars increase ‘effective reach’ as your body is pulled more forwards.
Neither of my 2 current MTBs are particularly radical in geometry. Not huge differences on paper but to ride they feel quite different with my Carbine climbing as well as my Talboy despite the extra weight and travel – I guess the steeper SA helps as I feel a little bit ‘over the back’ on my Tallboy.
SC Tallboy, XL, 475 reach, 617 stack, 68 HA, 73 SA, 1195 wheelbase
Intense Carbine, XL, 471 reach, 641 stack, 65.5 HA, 74 SA, 1253 wheelbase
However, I owned an XL Nukeproof Mega 290 for a year and it was mahoosive: 515 reach, 632 stack, 66 HA, 75 SA, 1283 wheelbase. The reach and wheelbase were quite extreme (still are!) but it didn’t feel too long due to the steep SA. I did have to move my weight around a lot more to make it go where I wanted it to (guilty of not getting enough weight over the front wheel), so it was certainly more difficult to ride than my current Carbine. It made up for it by how stable it was at steep on rough stuff – it simply blasted through stuff.
I’m curious to try something crazy like one of the Pole bikes or a Geometron, just to see what they feel like.
Good point about stem height also changing reach slightly, hadn't thought about that. A quick calculation says going 10mm lower with spacers would add 3.5mm to the reach, not sure I'd notice a riding position difference that small.
I guess bar sweep is also a factor, because your hands are at the end of the bar not the middle - a string across my bars from end to end crosses the stem about halfway along, so my hands are only about 40mm in front of the headset in practice rather than the 80mm it would be with straight bars on the same stem.
The Epic has a fairly steep seat angle at 74.75°, makes sense for that to be related to the reach.
I guess bar sweep is also a factor, because your hands are at the end of the bar not the middle - a string across my bars from end to end crosses the stem about halfway along, so my hands are only about 40mm in front of the headset in practice rather than the 80mm it would be with straight bars on the same stem.
The Epic has a fairly steep seat angle at 74.75°, makes sense for that to be related to the reach.
Edited by GravelBen on Friday 2nd August 05:52
Found a couple of interesting articles on stems etc, though they don't quite agree with each other on some points.
https://www.pinkbike.com/news/exploring-the-relati...
https://www.pinkbike.com/u/leelikesbikes/blog/dial...
Anyway, after a bit of thought I've gone to the 60mm stem spaced 5mm lower (which puts my hands about 20mm forward of the headset), and moved the seat forward 10mm to help keep my weight forward when climbing (plenty of room to do that without feeling cramped). I calculated that 10mm change at my seat height is worth about 0.8° of effective seat tube angle, enough to be significant.
Had a couple of hours ride today and its feeling good, improved balance and response on the fun stuff and the nose stayed well planted on some short steep climbs.
https://www.pinkbike.com/news/exploring-the-relati...
https://www.pinkbike.com/u/leelikesbikes/blog/dial...
Anyway, after a bit of thought I've gone to the 60mm stem spaced 5mm lower (which puts my hands about 20mm forward of the headset), and moved the seat forward 10mm to help keep my weight forward when climbing (plenty of room to do that without feeling cramped). I calculated that 10mm change at my seat height is worth about 0.8° of effective seat tube angle, enough to be significant.
Had a couple of hours ride today and its feeling good, improved balance and response on the fun stuff and the nose stayed well planted on some short steep climbs.
Edited by GravelBen on Sunday 18th August 08:22
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