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adam_mac84 03-26-14 08:19 PM

Wheelbase and effect on steering
Assuming the same fork rake and same HTA, fork offset and trail. How much do you think a change of 1.5cm will make. I am on the fence between a 58 and 61 CX bike. My current has the wrong geometry for me (73* HTA and 1018 WB, didn't like the front end handling), and I was first drawn to the 58 (72.3HTA), but would also fit on the 61. Same setup, just a bit different setup with the spacers under the stem.

My brain looks at a 1043 WB and thinks it looks so long, (the 58 is 1028). Am I making a mountain out of a molehill?

adam_mac84 03-26-14 08:49 PM

Additional information.. Same CS length. STA 73.3 on smaller frame 72.3 on larger frame. Given my 85cm saddle height... equals 1cm forward push on larger frame

Scooper 03-26-14 09:13 PM

With the same HTA, fork rake and trail, I seriously doubt the 15mm difference in wheelbase will be noticeable in terms of steering. You may notice a slight improvement in vertical compliance, but since the chainstay lengths are the same the lateral stiffness should feel pretty much the same.

David Tollefson 03-27-14 06:16 AM

STA difference is irrelevant, as your saddle position relative to the BB should be static regardless of the frame size. So that means the reach to the top of the headset would be 15mm longer on the larger frame? Then your stem will be 15mm shorter for the same bar reach, reducing "tiller" and will make steering feel quicker in low-speed situations. The longer FC will tend to make the bike feel more stable at speed, though. Noticeable? Depends on whether you're the princess that feels the pea under 20 mattresses.

adam_mac84 03-28-14 05:44 AM

My biggest concern was that on my previous bike, I tended to wash the front end more than I would have liked. Weather due to geometry (73 HTA 45mm offset) or poor skills, there was a noticeable difference when I test rode a few bikes with different head tube angles. (72 vs 73, same 45mm offset). For some reason when I test rode a bike with the 72* HTA, same tires, same pressure, it just seemed like it drove better. I liked the feeling of the wheel ‘flop’ is all I could describe it as. Of course, I am sure it was all placebo, and us cat 3 world champions are certainly going to feel the difference, and it will be the difference between first and last in the cat 3 world championships I am SURE!

Scooper 03-28-14 07:06 AM

Did you use the same stem on both the bike with the 72° HTA and the bike with the 73° HTA? Stem extension length will affect the steering feel as David Tollefson suggests.

Assuming all other factors were the same, with 700-25c wheels and tires the bike with the 72° HTA will have 7mm more trail than the bike with the 73° HTA (63mm vs 56mm). The bike with the 72° HTA and greater trail will have noticeably slower, more stable steering.

fietsbob 03-28-14 10:14 AM

Assuming the same fork rake and same HTA, fork offset and trail. How much do you think a change of 1.5cm will make. I am on the fence between a 58 and 61 CX bike.

If the wheel is moved to where the CofG is behind it more, a bit lighter feel, steering is possible,
but it may take a greater distance than 15mm . and you may shift your body mass forward with it and then it will be un noticeable

Plan Cyclo cross racing or just getting a Cross bike for the wider tire capacity?

adam_mac84 03-28-14 03:18 PM

I race CX on the bike. I have about 35-40 races over 2 seasons on my current bike (Redline Conquest Carbon). I have played a bit with stem length and positioning. I did find that a 110mm stem (versus my normal 120) flipped up slowed the 'pushing' from the front end a bit. The reason that I did some stem changing was because I was trying to see if I had too much weight or too little weight on the front wheel. Still inconclusive. Just different. The bike is scary fast tho!

The reason I started looking into new frames besides the washing out front end was for something with a bit taller stack. I used to race Ridley (Crossbow) in a 61 (58cm carbon size), but felt it was a bit big and slow steering.

Mind you, I was on the Crossbow when racing Cat 3 for 45 minutes. The move to the 60 minute elite race made a difference in how equipment felt. The Redline did not tolerate getting a bit sloppy since it is basically a crit bike with clearance.

I agree with you, David with the princess and the pea idea too. But I have test ridden almost every bike available in my size and always seem to notice a difference in the feel of the 72 and 73* HTA's. But I rarely have had the chance to race those bikes, which is what is intended. My fear is that I will get the 58cm out of convention of 'sizing down' for handling and handcuff myself into a similar situation, versus getting a more standard fit with the 61 and being pissed I got 2 frames that handle like pigs.

Here are a few shots of various setups over the past few years.

Road bike:

Ridley with 72* HTA, bigger bike, 110 stem

Redline 120 stem (current race setup)

Redline 110 stem flipped up (trial setup)

If it matters, I run 85cm BB to top of saddle. 58cm tip to center of bars, and 73 tip to hoods. ~4 or 5cm drop on cx and 6.5 drop on road. 10cm setback

I notice the shots of the turns, captures the feeling that I get. If i try to drive the front end around the corner it feels like I will have the wheel turned and it will just plow through the turn, so it's almost like I have to let the front end get outside (understeer?) and then muscle it around the turn at the end.

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